Communicating BY Light > 자유게시판

• 목록
• 답변
• 글쓰기
• 게시판 리스트 옵션
  • 수정
  • 삭제

The gear in the picture above includes, from left to proper: meccano hand-cranked gramophone with acoustic recorder (out of frame) to chop 78 rpm disc information on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio under it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my most important transistor receiver. Small multimeter in front of S-C radio. National Panasonic four-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones connected to it. The curly wire above the transistor radio was our communal non-public telephone line to my associates David and Bruce Bowden, who lived across the nook in Pleasant Road. HMV portable wind-up gramophone sort C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the cellphone line. Various broadcast-band and shortwave DX QSL cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from reminiscence!). In my left hand, I held the oldest disc document then in my collection, London, circa 1903, of a brass band playing the 'Tancredi' overture. On the best, spare elements for our personal neighbourhood line, and an old electrodynamic speaker. This was the only picture of my 'radio shack' taken within the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, five miles East of Melbourne's GPO (Australia). The house was demolished in 1997. Three house units now stand there.

Author's note, 28 April 2005: As this web page offers with my very own modulated gentle experiments I hope readers will tolerate these reminiscences earlier than I reproduce my 1979 article on the subject from 'Amateur Radio' journal:

WHY MODULATE Light?

Today's younger Australians can barely imagine the frustration of teenagers in the so-known as 'radical' 1960s the place electronic communication was concerned. By modern standards, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on rigid submit-colonial British public service lines. Something as innocuous as an intercom line run alongside your back fence to adjacent mates was unlawful - 'a monetary menace to the monopoly of the general public telecommunications network' in the eyes of the PMG. I discovered this after i set up a neighbourhood cellphone community to different local children' locations early in 1967! Music transmission by way of amateur radio had been banned in Australia since 1939, so that the material legally conveyed on any 'wireless' system was restricted, particularly for music-mad teenagers. CB radios, largely of the single channel hand-held variety, were imported in limited portions and bought freely - however paradoxically they could not be legally licensed or used. You needed to be fifteen years previous to take a seat for the novice radio license, then involving a trifecta of exams in full principle, rules, and Morse at 12 words per minute. For the idea, discursive essay answers had been required, and there was no degree of idea examination below the one most commonplace. From a young individual's perspective, you had to drag yourself over a area of broken glass to attain a legal entry level. Even then, you had to wait on your sixteenth birthday to operate ham radio legally in Australia.

If, like me, you were thirteen years outdated in 1967 and had electronic experimenter friends nearby, your aspirations to communicate confronted years of seemingly endless frustration. There were good reasons to investigate 'different' communication technologies - including modulated mild.

Together with that we had the Vietnam War; the imminent threat of the obligatory navy draft; a conservative authorities in power since our birth; and British-styled college uniforms unsuited to our sizzling summers. Australian teenagers of the 1960s had many valid reasons for discontent. The protest motion had its roots in many features of Australian society, not just the political points for which any road march was a handy excuse...

During the subsequent decade Australians saw the appearance of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor government coming to energy (1972); the first Sunbury rock festival, Australia's 'Woodstock' (1972); novice novice radio licensing (circa 1973); neighborhood broadcasting (1974); ethnic, multicultural broadcasting (1975); and the local legalisation of CB radio (1977). Finally, within the 1980s, personal carriers in the telecommunication marketplace had been allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies had been damaged and the authorized restraints on private digital communication eased markedly. With the arrival of the Internet, how might or not it's in any other case at this time?

I'd argue that Australia was quite unlike America in the 1960s, in that conservatism reigned via a lot of the 1960s right here, particularly in our communication laws. Australia's 'liberation' - if you can name it that - largely got here after 1970...

The advent OF Educational Solid-STATE KITSETS

So, how did your common child with greater than average curiosity get into electronics, and extra notably into optical communication, forty years in the past?

Within the mid-1960s, when the price of germanium transistors had fallen to a reasonable stage, digital development kits have been made obtainable to stimulate the minds of future radio hams and experimenters. These kits have been in contrast to the single-mission meeting outfits then available from Heathkit and Lafayette. Their accent was on schooling, providing a set of digital parts to be arranged in numerous configurations on an insulating 'breadboard' with spring clips and connecting wires. With the aid of a guidebook explaining the operation of each association, more than twenty completely different circuits could possibly be constructed, including Morse code oscillators, simple radio receivers, high-gain audio amplifiers and low-power radio transmitters. At this time, we also had the bonus of a neighborhood Melbourne magazine for young experimenters, 'Transistor Kits' revealed by Colin Witchell - more not too long ago of 'Talking Electronics' journal fame - from a tiny shop in Church Street, Brighton. For individuals who have been fascinated, the mysteries of electronics held the important thing to a new world of technical prospects - and many of Colin's projects involved mild detectors in some artistic approach...

An outdated pal from my major school days, Rowland Legg, acquired a Philips twenty-in-one package for the Christmas of 1965, and I was given the same Japanese 'Eleco' equipment on the same day. My parents had apparently noted my early aptitude for building crystal radio receivers, in order that they'd determined to 'push the envelope'. The natural result was that over the following five years Rowland and that i spent many afternoons collectively, unravelling the mysteries of electronic amplification. Whatever time wasn't spent indoors with the kits or on other electronic initiatives, we spent up on our rooves erecting prolonged wire radio antennas, or making an attempt to get them up on progressively higher bushes or supporting masts.

During these antenna-raising periods, Rowland Legg and I found that we may see one another's houses from vantage points on our rooves. In an try and set up a signalling system, we spent many evenings up ladders with kerosene lanterns, utilizing a black card to cut off the light and send messages to each other in Morse code. The standard Morse alphabet seemed moderately advanced, so Row devised one among his own, following a logical mathematical development:

This Also proved to be too complex to memorise and, with gentle flashes, too tough to relate to any typed record! We looked for a more facile technique of communication, with ample security not to attract consideration from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I turned aware of the potential for transmitting speech over modulated light beams once i purchased a duplicate of an historical, leather-based-bound e book 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early affect was a ebook printed in 1921, 'The Boy Electrician', which gave constructional details of selenium mild delicate cells, Tesla coils, audio transmission by multi-flip induction loops (which I constructed) and even the main points of a small X-ray machine (which, thank God, I didn't assemble).

Like many teenagers of the late 1960s, I experimented with modulated light communication using amplifier-driven torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A web page from my bench notebook dated 25 May 1968 - a couple of weeks after my 14th birthday - reveals my earliest plans for an optical system:

On the next weekend, 1st June 1968, a highschool friend named Howard McCallum and that i arrange the planned modulated gentle system with an incandescent torch bulb for transmitting and a CdS LDR in series with a 9 Volt battery and headphones for receiving, collimating a mild beam between the two with magnifying lenses. The outcomes over a distance of four metres were loud, but very distorted with frequency doubling results - a result of our preliminary makes an attempt to transmit with out DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, published the details of his easy 'photophone' within the 'A Reader Built It' page of March 1969's 'Electronics Australia' (pps. 91-93). The venture's simplicity made it an attractive proposition for younger people, and many Australian experimenters of my technology will remember it:

The geographical setting for our own reconstruction of Murrell's device was lower than salubrious. In actual fact, it was probably the most eccentric radio 'shacks' that I've ever encountered. At the underside of my buddy Rowland Legg's back yard, a large wood packing crate initially used for transport a Volkswagen automotive to Australia was arrange by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we called it. To keep out the drafts, its walls were papered with out-dated advertising posters for Melbourne's weekly scandal-rag, 'The reality', obtained from our kindly local newsagent around the corner in Tooronga Road. It was the type of newspaper that nobody would admit to purchasing, though the newsagent assured us of its glorious local circulation. From each angle in Rowland's radio shed, headlines in an unlimited typeface assaulted the eye: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance among these posters was tactfully hidden behind a cupboard door, its wording being one thing like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm still questioning concerning the implications of that!

In these inglorious surroundings, and within the backroom of my house proven in the picture at the top of this internet page, we spent many weekends constructing electronics tasks of every conceivable type. Together, we soon had a model of Murrell's photophone transmitter built into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was ultimately mounted (with yards of PVC tape!) on a hot water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's residence.

However, my very own initial optical comms exams with the Murrell tools have been finished around March 1970 from the the backyard radio shack then occupied by Hughie Paton, VK3ZEP, at 49 Havelock Road, across that home's again backyard, down its again driveway and across Torring Road to the glassed-in front veranda of our family residence (to the left of the front door in the photo beneath) - a distance of about 50 metres. The system provided a hyperlink in one route only. These checks were logged on open reel audio tape, in order that I could test my audio quality without the need for an assistant. My mom sometimes got here onto our veranda to listen to the consequence and talk her response by waving through the home windows. The tape nonetheless signifies the fairly good audio high quality that one might get with a limited depth of modulation. It additionally demonstrates the very high hiss stage of the OCP71, the actual machine used on that day being seen in the photograph on the fitting.

I was out within the open air on the again of the Havelock Road property, throughout Torring Road from my residence, proven above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a 3 volt 300 mA torch globe at its focus. I later found that the high frequency response of the system was inversely proportional to the thermal inertia of the filament. The best excessive frequency response was obtained with lamps of decrease rated most present, a thin filament with thick lead-out wires to dissipate the heat shortly.

The transmitter gear, consisting of a microphone pre-amp, three watt transistorised audio power amp with transformer output, and two measurement D torch cell battery supplies (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. At the receiving finish, a 7.5 cm diameter magnifying lens focussed the picture of the Aldis lamp onto an OCP71. I am unable to remember whether the OCP71 was directly connected to the microphone input of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether or not it was operating into the line enter via the standard two-transistor pre-amp (photoconductive association). I seem to do not forget that each had been tried at different times, with a minimal difference of outcomes. An occasional transient buzz might be heard when widespread Australian blowflies happened to fly by means of the beam, modulating the light with their wing beats. In the latter part of the extract, a mild shower of rain occurred, the droplets falling by means of the beam producing a type of tender 'plip-plap' sound. The impact of waving one's hand by means of the beam was also demonstrated. Whenever the beam was interrupted the hiss stage rose substantially - an impact for which, even now, I have no explanation, except that it clearly had one thing to do with the impedance of the phototransistor source rising as the sunshine input fell.

The audio tape of the check was made on a warm, pretty overcast Saturday afternoon. Within the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, could be heard washing dishes after lunch and planting hop bushes in the garden. The primary World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a new ingredient for some dwelling brew! The son of the household, Brian Harris (1936 - 1992), was briefly VK3ZFH in the late 1950s. Brian showed the creator find out how to tune up a transmitter and read an oscilloscope on the age of 4, in 1958. This set Chris on the lifelong 'downward' path into technical interests... One of Brian's last jobs involved the design of among the communications gear for the Hubble area telescope. Suburban life might be attention-grabbing with neighbours like the Harris household at 49 Havelock Road!

On this ten-minute mp3 extract from the original 45-minute tape, the author on the age of fifty one in 2005 introduces his squeaky-voiced 16-year-outdated self in 1970. Computer nerds did not exist then - WE were RADIO nerds! Anyone interested can obtain the audio log file of this March 1970 take a look at as an mp3 file right here:

(Currently damaged - coming soon) March1970.mp3

One curious side of the germanium phototransistor OCP71 was its extreme infra-red sensitivity. It was potential to transmit audio, as one can hear on the tape above, with the torch bulb operating at a voltage so low that it had no perceptible visual output. The germanium detector may resolve the modulated HEAT from the lamp filament. Its work perform as a photodetector was very low, which meant that it was inherently topic to massive quantities of thermal noise, much more than silicon and really much more than photomultipliers:

On 1 September 1970 we extended the range of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my dwelling at 6 Torring Road, East Hawthorn, where I stood atop a ladder next to our yard bungalow with an an optical unit fabricated from tin cans, like Murrell's within the photograph above. The audio link coated about 600 metres, and i may clearly see the light focussed on the OCP71 in the receiver tube. In truth, I may goal the receiver by having the focussed spot disappear behind the sq. delicate materials inside the phototransistor. The hiss stage from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was insufficient for the vary and the trebles had been restricted by the thermal inertia of the torch bulb, however this was our first actual 'light beam DX'. On the age of 16, the pleasure of hearing these outcomes precipitated my arms to shake, so that my optical receiver's purpose was erratic, however I managed to log the whole contact on an audio tape in two 'bursts', with a pause to cellphone Rowland to tell him that each one was being acquired, midway. Initially, Rowland and his good friend Neil Florence have been merely relaying the 7:30 pm news broadcast from the Melbourne broadcast station 3AK with its items on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission step by step rose out of the noise - although not by a lot! Later, typical teenage music of the day can be heard - The Crystals singing 'And then He Kissed Me' (with Rowland trying to interpolate a 1970-vintage form of 'karaoke') and the file of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... this is being transmitted on the primary day of Spring, the 1st of September 1970 - and if you can't hear this in spite of everything this trouble I'll must kill you!', I may easily have fallen off the ladder in amazement. We'd finally devised a technique of circumventing the illegal usage of a radio transmitter - though to be realistic, with this appalling sign-to-noise ratio the success was only marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've not often skilled more thrill from experimental work than I did on that evening 35 years ago, and i want hardly add that immediately afterwards, adrenalin-charged, I ran non-cease to Rowland's to report our success! The three of us posed for a photo around that time, with me holding a pair of 1920s-vintage headphones which we ceaselessly used. The picture might be titled 'hear evil, SEE evil, DO EVIL' - however largely, our backgrounds were just too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, different Australian experimenters were making an attempt the potential of atmospheric optical communication extra seriously. In Adelaide throughout 1968, my future collaborator Mike Groth (at the moment VK7MJ, then VK5ZMG) and a lab assistant friend, Stewart Powell, constructed a pair of optical communication units in the suburb of Hammersmith using torch globes and OAP12 germanium photodiodes. They'd a most vary of about 1 km, and, as Mike puts it, "awful fidelity". Mike's spare time for optical assessments was restricted on the time by the calls for of doing the third yr of a BSc - with a new spouse.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of many course initiatives in that yr concerned modulated mild. He defined the next events to me in a letter dated 28 February 1988:

"The mission involved the evaluation of modulated light as a technique of transmitting geomagnetic knowledge over short distances to avoid wire hyperlinks to the remote sensors, which tended to introduce hum loops if not rigorously balanced. This was a golden alternative to learn the idea behind optical hyperlinks and search back by the literature at the University Library. It became obvious that it would be no problem to transmit knowledge over a few hundred metres on a transparent night utilizing the new infra-red diodes as sources, however the reliability of the link was unknown, particularly as it was to be used at a discipline station within the Adelaide hills, the place fogs and mists had been frequent. I constructed an infra-purple hyperlink at 930 nm which measured the trail loss over the winter and spring of 1969, and my estimates of the trail losses [in the 1987 'Amateur Radio' article 'Photophones Revisited'] were based on this data.

Optical communications fell into the background for the following 16 years, as I spent 1970 in New Guinea educating and moved to New Zealand in 1971 to do postgraduate work at the University of Otago in Dunedin. I returned to Australia at the tip of 1979, however was reasonably inactive in experimentation until 1985, when i determined to jot down my experiences as a review of the potentialities of optical hyperlinks. The effort took almost 18 months..."

Mike's resultant article published in 1987 with revisions from 2005 could also be discovered at:

Essentially the most powerful newbie atmospheric optical communication exams in Australia around 1970 have been performed by an Adelaide workforce and reported in an extraordinary Tasmanian journal. The Hobart-based 'Electronics Exchange Bulletin' was published round this time by the Tasmanian staff of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary magazine inspired articles by native experimenters on every conceivable subject associated to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two university students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour fuel discharge lamps, attaining atmospheric ranges in excess of 3.5 miles (about 5 km), just outside Adelaide in valleys shielded from town lights.

Though their gear was massively bulky by modern requirements, Burlinson and Averay pursued a novel line by driving their gasoline discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation efficiency issues. In this fashion, the efficiency was similar to that of the celebrated (on the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I was not aware of the studies of those exams until the beginning of 1976, in any other case I might have skipped the following few steps in my very own checks...

OUR Switch TO Gas DISCHARGE LAMPS

A seek for better modulated mild sources than incandescents drew my attention to neon lamps. Gas ionisation is a much quicker course of than the incandescent heating of a filament. In these days, neons had been cheaply out there in any measurement from a pea lamp to a full size "beehive" bulb from one marvellous source. Melbourne experimenters lively in the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the tip of a narrow stairway leading down from the pavement was a slightly grubby Aladdin's cave of tables laden with cable, warfare surplus junk (each struggle except Vietnam), bins of valves and khaki-painted objects of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates filled with technical cast-offs of every description. I acquired several boxes of neons there, along with various different gas discharge devices to try. A few years later, I discovered that a few of these contained traces of radioactive material to help ionisation. Let the purchaser beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, sort B406. The orangey-pink glow around the cathode on these neons is fairly intense, though it does not even approach the intensity of trendy LEDs or lasers. After establishing the neon modulator I was rewarded with near good audio from my OCP71 for the primary time. Fortunately, a parental veto on building mains-fed energy provides ended on my fifteenth birthday (eight March 1969), so the required 300 volt rail was no downside.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the idea for a simple television system in 1970, by including a couple of Nipkow scanning discs to the outfit. This diverted me from the modulated gentle communication checks for a few years as I delved into the entire pre-conflict Baird television texts to bring the pictures to an appropriate commonplace. This introduced me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, call signal VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' web site. They had been the 'ringleaders' of a larrikin group of amateurs working residence constructed AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' beginner radio interests have been distinctive and individualistic. The vast majority of conservative hf (shortwave) operators had a slender and obsessive emphasis on 'communication high quality' modulation, often 300 Hz - 3 KHz, clipped, non-linear and cruddy. Dan and Tony had been both hi-fi enthusiasts. Their transmitters were relatively broadbanded and immeasurably low in distortion, exceeding the audio specifications of many broadcasters. AKG or Western Electric microphones and broadcast-quality audio peak limiters with dual time constants have been involved. The design and building of their modulation transformers and amplitude modulators was nothing wanting an artwork. Their 160 metre receivers, also, employed biased, low distortion envelope detectors working through rigorously designed audio amps into enormous speaker methods with vented enclosures. The content and audio quality of their in-depth conversations on communications know-how made listening to their transmissions an absolute pleasure. Even their standard of audio compression served to convey the listener into the acoustic surroundings of their houses - an underestimated side of creating a practical auditory illusion. Their activity ruffled quite a couple of amateur operators' feathers at the time... which solely increased my youthful admiration for each of them.

I actually met Dan (3UI) - dare I admit it - on the air on thirty first December 1969, as the result of a quick dalliance I had with pirate radio transmissions on 1.Eight MHz. He lived in Hawthorn solely a mile from my residence, near the nook of Glenferrie and Riversdale Roads. Naturally he was amongst the primary to listen to my feeble and unstable transmissions, and he inspired me to experiment additional to realize the information to get the beginner 'ticket'. We were both given a 'cease it or else' ultimatum by an over-zealous radio inspector known to the locals as 'Uncle Ugh' (many will nonetheless know who I mean), but Dan and i became great friends in consequence. Dan was about two years older than I, and was extremely influential on the rapid future course of my life. As the year 1970 progressed, I found myself spending more time with beginner radio associates and less with Rowland and the old style mob. Rowland ultimately carved out a very profitable profession within the Victorian Police Force (ironical, isn't it?), and i imagine he nonetheless does, but his early interest in electronics declined...

In subsequent decades, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for varied frequency segments. In consequence - and I will categorical an opinion here - they've systematically eliminated the authorized foundation for the kind of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' at the moment are operators of commercial 'black box' transceivers which may elegantly present single channel phone high quality (or worse) on every out there band for $4000+, however which regularly can't be correctly adapted to any other mode or type of experiment. From my perspective, the result's that amateur radio has had progressively less attraction as a pastime, and I'm certain I'm not alone in expressing this opinion. For my money, for those who legislate in opposition to experiment you kill the one attraction that newbie radio ever had. So lengthy because the emissions don't unfold beyond the novice band edges, what is the issue? Anyone who pushes the hoary outdated argument that "bandspace is at a premium, so transmissions ought to be of the minimal potential bandwidth" should be deaf and blind to the steadily declining level of novice band usage over the previous fifteen years. Who's to say what novice radio ought to entail, so lengthy as it provides training and encourages experiment? Many people marvel why I've by no means bothered to pursue an beginner radio license. I hope that I've justified my position. Thank God for alternatives with more freedom of content and bandwidth, like gentle beam communication!

Dan (3UI) and that i finally arranged test transmissions of slim band television using mechanical disc scanners over his 160 metre transmitter early in 1972. The following couple of years gave all of us a superb grounding within the principles of gentle detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others in the United Kingdom, we formed the Narrow Band Tv Association, nonetheless in lively operation and now represented on a website:

http://www.nbtv.org

One particularly sort donation to this mechanical Tv scanner challenge was provided by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev labored in telecine at Melbourne's government Tv station, ABV channel 2 in Elsternwick, working an archaic monochrome Marconi 35mm movie scanner. This was originally said to have been designed for the 405 line British service and used at BBC Tv's authentic studio on the Alexandra Palace in London. By 1972 it was solely being used for half-hour a day, at about 4:00 pm, to broadcast 35 mm film episodes of the children's serial "The Cisco Kid" - the only regular program materials they had which nonetheless used that gauge of film. The EMI 6097 photomultiplers on this Marconi telecine had been written off as quickly as they developed spots on their photocathodes - Kev referred to as them 'dynode spots' - however they had been nonetheless fairly serviceable for gentle detection. In 1972 Kevin saved a few of these from the dustbin for us.

Dan (3UI) and i adapted the EMI 6097 photomultiplier for use in our experimental digital camera and for the modulated mild receivers. Its sensitivity was such an enormous quantum leap from the OCP71 that it opened an entire new world of technical potentialities to us. Further photomultipliers have been acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for sluggish scan television scanners in these days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

Throughout the Autumn of 1975, I built a modulated gentle communication hyperlink that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first flooring level, dealing with one another across suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about seven-hundred metres. The modulator from my Baird mechanical television receiver by then used a 6L6 beam pentode in collection with the neon, which was re-mounted at the main focus of a rough 30 cm moulded glass parabolic reflector provided by Tony (3AML). The reflector had initially been utilized in a traffic signal.

This optical transmitter was placed on Paul's balcony in Glenhuntly Road, on the corner of Clarke Avenue. The receiver was positioned within the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a thirteen cm diameter magnifying lens focussing onto a 0.5mm focal airplane aperture, with a 931A photomultiplier catching the transmitted light behind it.

Using Dave's 160 metre amateur transmitter as the return hyperlink - the ultimate in break up frequency operation - Paul managed to hold on a crossband contact by means of the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue sensitive photomultiplier. The neon was pushed beyond its current scores, in order that its bulb was rapidly blackened by cathode sputtering, but no less than it may very well be totally modulated. I was later able to measure the bandwidth and was astounded to find that the neon could be modulated to around 500 KHz, which was a terrific enchancment on the torch bulbs. Distortion was extreme. We had been pushing the modulation fairly onerous, owing to the poor sig/noise ratio. A pattern of the audio log of the contact is at present obtainable on Dave's web site. The outcomes have been sufficiently good for us to realise that we were heading in the right direction for additional improvement. The contact was logged on tape and an extract might be heard on VK3ASE's internet site:

[ Actually, Dave appears to have eliminated this one for now. (Tony, VK3AML) ]

The primary drawback to be overcome was the low characteristic intensity of the neon discharge. We additionally wanted a source with vital blue output, to match the spectral response of the photomultipliers, which might solely detect violet, blue or green light. Development was accelerated by the involvement of another member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the top of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my home, I was lucky in having an elevated position close to the highest of a hill in East Hawthorn, South of Camberwell Junction and fairly close to the excessive copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs round to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong in addition to parts of Malvern, Richmond, Abbotsford and Northcote in an unbroken 120 degree arc. Just on my facet of the Yarra, on the top of a rise in clear view, was VK3ZGJ, together with his shack facing me at the third ground rear of an previous Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was ideal for optical communication exams, exactly two miles (about 3.5 km) on an East-West path.

In the early 1970s, before I met him, John (VK3ZGJ) constructed a sequence modulator for fluorescent lamps, consisting of a number of 807 output valves in parallel, with the fluoro in the anode return. He additionally built a portable mild dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias into a load of 10 megohms or more, and with its low noise preamplifier it had a lot better sensitivity and spectral match to a fluorescent lamp's output than my outdated OCP71. You had to arrange for the picture of the fluoro to focus exactly onto the gap between the conductive combs on the LDR's sensitive floor. This involved peering on the LDR by means of a 'spy-gap' in the optical mounting while deftly manipulating the receiver's alignment.

LDRs have a very slow response, rolling off no less than 6dB per octave above 50 Hz. Treble increase might only partly appropriate the issue, as John found. My photomultipliers provided the answer to John's receiver problems, simply as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled sources over the following five months.

Late in December 1975, John and i received the communication system working between our homes, utilizing a vertically mounted fluorescent lamp on the roof which "broadcast" gentle in all instructions. The modulator consisted of a number of (4, I believe) sort 807 beam pentodes feeding the forty watt fluoro in their anode circuit. A rail voltage of about 600 volts DC was applied to the lamp. We did not use a lamp starter or a ballast choke to operate our fluoros. Instead, we had a novel starting arrangement consisting of a band of aluminium foil wrapped across the glass near the cathode end of the tube, which was related to the secondary of an automotive spark coil. To start out the discharge, you'd apply the 600 volt rail, then energise the ring across the cathode by making use of a battery briefly to the spark coil primary. The excessive-voltage spikes utilized to the glass close to the cathode began a barely perceptible glow discharge inside that end of the tube, which might immediately spread the entire length of the tube beneath the influence of the 600 volts DC rail. Standing present was different by altering the value of the cathode resistor on the sequence 807's. The filaments at either finish of the tube have been by no means heated up with this association, which appeared to extend the working life of the tube.

Our first forty watt 'fluoro' take a look at transmission got here simply after the Christmas of 1975. It was in one path only, with John transmitting and me receiving. 3ZGJ managed to arrange some quite elaborate music packages interrupted by bulletins, considered one of which I recorded on tape. Within the early 1970s a British group, calling themselves "Radio Love" (hey man, real groovy) had proposed a system of local light beam broadcasting. This appears to have been an attempt to circumvent the heavy hand of British officialdom, which at that time had compelled a number of independent broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its preliminary plans, including the diagram below which neatly summarised their concepts:

John 3ZGJ, in imitation of this 'Radio Love', jokingly announced his packages as being transmitted from "Radio Hush". The title was a bit less poofy, but still retained the essential ambiance of the legal fringe-dweller! Later we duplicated the system to offer full duplex communication (simultaneous transmit and obtain) in both instructions. A typical extract from these fluorescent gentle transmissions in the final week of December 1975 could be heard on this tape, recorded from the photomultiplier output at my finish of the link, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers only managed to achieve 15 dB sig/noise over the 3.5 km range with this arrangement. Fluorescent lamps had greater than their fair proportion of problems as a modulated source. Their phosphor coating had time lag, the persistence limiting the upper modulated frequency to about 5 KHz. The time lag was not fixed with the wavelength of the emitted gentle. The purple phosphor elements had very lengthy persistence, while the blue phosphor was much sooner. The system's upper audio frequency restrict subsequently diversified with the spectral response of the detector, but with a blue-delicate photomultiplier it was more than satisfactory for audio.

The fluorescent lamp discharge would wander and 'snake' contained in the tube, especially at change-on, interfering with the modulation. For some reason which we may by no means clarify, the output at the cathode end of the fluoro, and only at the cathode finish, was modulated by an erratic whine at about four hundred Hz, possibly caused by the discharge hopping about from one part of the coiled cathode filament to a different. This at all times set a particular restrict to the sig/noise achievable, even where there was loads of light sign to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an impact undoubtedly attributable to acoustic resonances of the modulated mercury plasma column inside its tubular enclosure. One might certainly hear a faint acoustic radiation from the tube while it was in operation, as these acoustic waves truly penetrated the glass partitions. Certain modulation frequencies, clearly associated to 'organ pipe' resonances throughout the fluoro tube, would cause the mercury discharge to extinguish, or to interrupt up into a sequence of spaced glow discharges alongside the tube. I later found that this drawback had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" within the book 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps thirteen - 14):

"Enclosed arc lamps operated on a.c. power in the audio-frequency range, or on d.c. and modulated by a.c. currents may trigger sound vibrations to be produced throughout the arc chamber. They're attributable to thermally induced variations in fuel stress that outcome from changes in present density within the arc. At certain crucial frequencies, resonance of appreciable intensity is constructed up by reflection from the bulb partitions. The size and form of the bulb, type of gas or vapour filling, temperature and working conditions decide the frequency of the plasma oscillations which can be similar to standing sound waves in the discharge. Ordinarily this phenomenon will not be noticed because lamps are operated on d.c. or low frequency a.c. with sufficient ballast to ensure stable operation. In lengthy tubes the discharges assume a constricted, snakelike look on the crucial frequencies and are caused by sound energy reflected from the ends of the bulb. In a spherical bulb the sound waves unfold to the bulb walls and are then focussed again upon the arc to supply instability on the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.Three cm lengthy with 3 Amp d.c. present applied with a 2 amp a.c. modulation applied. He famous:

"[...] the arc confirmed violent distortions at 2250 Hz but was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and 3 A a.c. modulation at 2500 Hz, the discharge once more confirmed pronounced disturbances, but was stable at 2300 and 2700 Hz. The instability may begin at either electrode, whereupon the discharge constricts into a skinny luminous ribbon with sinusoidal shape, and the voltage increases because of elevated arc length. [Instability at] harmonics of the elemental frequency could also be observed[...]

"[...] In a spherical bulb with electrodes on the centre [like a Xenon arc] acoustical resonance occurs when the bulb diameter is equal to at least one-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will actually 'blow itself out' by its own sound waves if any of the robust resonance frequencies are applied to the lamp for an appreciable time [...]

[...] In a low strain discharge lamp [eg fluorescent], assuming a median gasoline temperature of 250 levels Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's system. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence at the electrodes happens at a half wavelength from the nodes which are at the ends and centre of the lamp."

Owing to the low depth and prolonged source area of the fluorescent lamp, the radiated flux may by no means be properly collimated. These limitations, and the frequency response irregularities clearly indicated that our gentle transmitter needed a change of strategy.

At this level, Rodney Reynolds VK3AAR drew our consideration to EEB's publication of the sooner work of Burlinson and Averay, whose modulator circuit and mercury arc supply was a wholly novel strategy. Their optics, however, had been quite crude, not practically directive sufficient for our work in suburban Melbourne, where avenue lights and illuminated promoting indicators proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We would have liked a higher depth source, no fluorescent coating, and an output wealthy in blue light to match the photomultipliers. From the electrical provider Arthur J Veall in Bridge Road, Richmond, John and i obtained some Philips high stress mercury arc lamps in January 1976, of the type used for factory lighting. Most of those had inconvenient fluorescent coatings, apart from the smaller lamps beneath a hundred watts output, which sadly had frosted glass envelopes. We bought a couple of HP80 mercury lamps rated at 80 watts. Driving these was an actual problem. The fluorescent lamps that we beforehand used were low current, excessive voltage gadgets, modulated simply with normal output valves. By comparison, the high stress mercury arcs ran at medium voltages (30 to 100 volts) but with excessive current (0.5 to three amp standing current), and so they exhibited a severe damaging resistance characteristic - their voltage drop decreased sharply with growing present stream.

Other issues arose owing to the instability of the mercury arc. Its placing voltage diversified widely with ambient temperature, and its standing current assorted with time as heat brought on the mercury to vaporise and the arc strain increased. At its peak, the quartz arc tube had to withstand internal pressures of around 30 atmospheres, so it was a device that you just needed to treat with nice respect!

The slightest overmodulation peak would extinguish the discharge, and the entire device then needed to cool earlier than it could possibly be re-struck at a moderately low voltage. The lamp polarity also needed to be reversed at frequent intervals with a DPDT switch to avoid premature cathode failure by ionic bombardment. The lamps had been designed for AC operation, the place the effects of cathode heating had been shared 50 instances per second by each electrodes. Regardless of utilizing DC bias on the tubes, for which they weren't designed, we managed to get a few hundred hours out of them, which was Ok when the lamps only price $7 apiece. Today they price about ten instances that.

The modulator had to steer the current passing to the arc. Quiescent class-A amplifier circumstances demanded that no less than half of the provision rail ought to be dropped by the series modulator, the remainder being dropped by the mercury arc. The unfavorable resistance of the arc was our stumbling block. As the current by way of the arc increased its voltage drop decreased. This dragged the voltage applied to the sequence management machine up in direction of the rail voltage as current reached a most. With 200 volts-on the rail and a peak present approaching 5 Amps, no transistor generally out there in 1975 could handle the job. They couldn't take the excessive peak currents simultaneously with the excessive emitter-collector voltage that this load with its adverse resistance would current to the output transistor in school A. We had some expensive pyrotechnic shows of 2N3055 and BUX80 transistor failure owing to this secondary breakdown point being exceeded. Like many foolhardly experimenters, we hardly ever used fast-blow fuses in our gear. Youthful arrogance, I assume!

The elegant answer, suggested a few years later by Rod Reynolds (VK3AAR), was to run the control transistor in parallel with the arc, with a single series resistor up to the supply rail from both. In that configuration, the transistor handed maximum voltage at minimum current, and vice versa, so that a much smaller transistor could be used than within the series modulator configuration, the tradeoff being only a decreased overall energy efficiency. However, on the age of 21 in 1975 - and John was 20 - neither of us thought of that...

Instead, we used a category-A vacuum tube amplifier of elephantine proportions in sequence with the arc. The facility supply was capable of 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt rating! For the modulator, 4 hefty 6080 or 6AS7 high current, low gm triode regulators had been placed in parallel with low-value balancing resistors of their cathode returns. The anodes had a nasty habit of glowing a dull pink when the arc was initiated, because it took a few minutes for the mercury discharge to run up to its usual one hundred Volts drop. During that time, the most important a part of the provision rail was applied to the valves, which had been despatched beyond their dissipation limit. Under these situations, one could see alarming high resistance 'spots' scintillating with a shiny and sparkling yellow mild on the large oxidised cathode surfaces of the 6080s. I all the time anticipated a 'bang' but in some way I used to be at all times lucky...

Each 6080 filament consumed 2.5 Amps at 6.3 Volts. The valve heaters alone consumed a complete of sixty three watts! It was all brute drive, ignorance, bulk power and heat! Fan cooling was obligatory. We used a home fan of 1928 vintage with a bum sleeve bearing, which rattled continually during QSO's. Nevertheless the system worked remarkably effectively, and because of Melbourne's many disposals shops, it could possibly be built for a couple of dollars. Not so now! Electronic disposals stores of that kind at the moment are mostly a thing of the previous.

This mercury arc transmitter was rather unstable in operation, prone to permitting the arc to drift into a slow thermal runaway. To appropriate this tendency, one had to trace the current of the arc by manipulating the grid bias of the output tubes - hence the reason for the metering of arc present and voltage in the modulator circuit, above. The temperature of the quartz bulb was the most important variable, as this and the arc's standing present had been carefully related. The bulb temperature managed the mercury vapour stress within the quartz phial, and subsequently the electrical resistance of the arc. With insufficient average present flow, the heat of the arc would be too low to take care of the mercury vapour pressure, causing the voltage throughout the arc to slowly fall, in turn leading to an additional fall of current. If the standing current was too excessive, the arc would develop into so scorching that it not only became dangerous, but was impossible to modulate fully. One couldn't depart the system unattended for more than about 5 minutes. Arc present would range, notably in the time simply after 'change on', because the log document beneath clearly indicates. Warm-up would take around 15 to half-hour earlier than the system lastly settled into static current situations, with the heat pumped into the arc electrically equalling the heat misplaced to the encompassing surroundings. The common standing present diversified with the audio program content material and with the symmetry of the audio waveform utilized.

A typical instance is supplied by the following log that I took of the arc transmitter's electrical standing, measured by the night of Saturday 31st January 1976. Operation on that evening was greatest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a unfavorable peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

80 V @ 0.42 Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only earlier than nightfall. Arc has been on for forty five mins of warm-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright issues!). Ambient temp. 350 C. VK3ZGJ not listening yet. Power and heater transformers are both sizzling to contact, however Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used on this system had been also designed round parts obtainable from disposals sources. Photomultipliers are superbly delicate, and not practically so fragile or tough to arrange as some so-referred to as specialists suggest. They require a easy a thousand volt supply at about 5 or 10 mA - easily organized with an outdated valve radio transformer working right into a voltage doubler. The hand-held photomultiplier receiver was a relatively hazardous gadget whenever you have been manipulating it at evening from the highest of an earthed metal ladder! Particularly so when the 1000 volt provide had eight µF of oil-crammed block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It turned much more dangerous when rain was falling! In my youth I had scant appreciation of my own mortality. Few teenagers ever do!

The outdated disposals photomultipliers typified by the 931A had virtually no response to purple gentle, so they couldn't be used with LEDs or HeNe lasers. More recent photomultipliers with gallium arsenide photocathodes have good purple sensitivity, but at prices exceeding $a thousand they're out of my league.

Overall outcomes with this cumbersome hyperlink system had been remarkably good. Considering the potential theoretical problems of thermal lag and ionisation time, the mercury lamp's restrict of 50% modulation at 10 KHz was quite usable, solely acceptable for direct amplitude modulation at audio frequencies. Piles of sign were obtainable. The output was terrific. At night, with one's eyes dark-adapted, you wanted to guard your eyes with darkish glasses or welding goggles. But information and slender band video modulation revealed wild section shifts above 5KHz. I suspect that the mercury plasma, when thermally modulated at an audio fee, was attempting to increase and shrink towards its quartz tube housing with each modu1ation cycle, interacting with the arc cavity in a complex collection of acoustic resonances. Indeed, one may really hear the modulation coming from the arc, notably at high audio frequencies, by putting your ear down close to the quartz bulb. One undesirable aspect impact was that sure modulation frequencies close to the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds were particularly dangerous. In answer to a query concerning the modulator I remember saying over the hyperlink "it is working completely!" At the other finish, all John heard was "it is working perf - Click". The arc extinguished itself, proper on cue!

Over the 4 months up to April 1976 the system was in operation up to six nights per week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all however the heaviest fogs and rain showers. On highly regarded nights, low frequency noise and quick flutter fade have been evident, but rarely obtrusive. The effect was as a consequence of hot air cells rising from the heat ground into the cooler night sky. This precipitated a heat shimmer, with a consequent scintillation of the received light. We considered the use of FM subcarrier modulation to clip off the scintillation on the obtained mild modulation, but the mercury arc lamp hadn't a sufficiently fast rise time to help this. Subsequently I discovered that the use of longer wavelengths - pink or infra-red mild - drastically cut back scintillation. FM method may be desirable for links of over 5 miles vary, however for shorter distances amplitude modulation's greater available base bandwidth in all probability makes it the preferred mode.

The old mercury arc/ photomultiplier system might have had considerable long range potential however we never had the opportunity to test it. The heavy energy calls for of the terminal equipment demanded a mains supply at both ends of the link. We never successfully tried DX away from our respective houses and two miles (3.5 km) was the greatest distance examined. We could generally achieve forty five dB sig/noise over that distance.

Communication ceased during the day owing to the scattered blue mild of the sky. The photomultipliers had been saturated by it. Interference filters could have been used to extract a dominant spectral emission line of the mercury discharge from the final background radiation, however I used to be unable to obtain one until the 1980s. They're costly and appropriate only for parallel rays of gentle. There are higher ways of attaining good sig/noise through the day, using a monochromatic supply and a wavelength away from the peak output of the solar.

However, I would power up the photomultiplier with a relatively low voltage - about 500 volts - just earlier than sunset, to catch John's first distinct phrases as the ambient light pale. On sizzling evenings, the sign was at all times lower than expected. I could not account for this, until one evening I climbed the ladder to regulate the receiver and noticed an odd smell. The receiver's optical cavity was full of smoke! I had forgotten that John was almost straight West of me, and because the summer season solstice passed, the position at which the solar set on the horizon was shifting North on successive evenings. That night, it was setting immediately behind John's house - so my thirteen cm receiver lens was focussing the solar's image on the again of the receiver housing, which was made from black-painted particle board. Awk! We had been so fortunate not to have set fireplace to everything - but I saved the incident a secret from my of us. Things that my parents didn't know couldn't harm them! With some embarrassment, I fitted the optics with a removable lens cowl.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low power lasers most likely could by no means rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years ago I acquired a pair of 1 metre diameter searchlight mirrors to attempt the concept. The mercury arc might be mounted in one, photomultiplier in the opposite, each monitoring the same cloud. Street lights at this time use the identical kind of mercury discharge as the modulated supply, so that a hundred Hz hum background plus harmonics could be unavoidable in an urban surroundings. I've but to attempt the thought, as direct linking is undoubtedly more reliable and environment friendly, and has subsequently been my most important line of analysis.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as quickly because the optical link between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, several times every week. For all practical purposes, I was on an intercom in 3ZGJ's shack, and there was no law against that. Legally and technically, I was a supervised (ie, pull the plug out if he misbehaves) customer who just occurred to be current close to 3ZGJ's microphone!

Through the early a part of 1976, there were many cross-band contacts on 160 metres with me "on lightbeam" which appeared to add technical novelty to the proceedings. The discussions were completely open to any subject, offered that the due decorum of the medium was maintained. On some occasions, it wasn't! One night time we one way or the other drifted onto the topic of funerals, morticians, Egyptian mummification and the 'expensive departed'. This contact, with many others from that 'light beam hyperlink' period, was logged on audio tape by Tony VK3AML and by other listeners, and extracts could be downloaded from Tony's 'bluehaze' net site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content material assorted from creditable technical depth to immature ratbag ramblings. I by no means saved track of all the log tapes, and that i never know when bits of it would re-emerge on the weekly replays which were maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our problems with the previous fuel discharge lamp system had been more comical than theoretical, and most were fully unforseen. Moths, as an example, were irresistibly drawn to the ultra-violet output of the mercury arc. Bogong moths, massive Emperor Gum moths, Christmas beetles, flying ants - the full field and dice! They swarmed the transmitter arc on scorching nights, attenuating the beam flux and suiciding towards the recent lamp. Every couple of hours I'd have to scrape their smouldering bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the sunshine they mirrored formed a suggestions path into the adjacent, repeatedly running, duplex optical receiver. If I had my transmitter microphone working close to the monitor loudspeaker, the presence of an Emperor Gum moth (which, I ought to explain for non-locals, are the size of a small dinner plate) was signalled by ear-splitting suggestions warbling at their wing-beat frequency of about 15 Hz. This was often followed by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'flip my bloody noise down'! As we incessantly operated between midnight and three a.m., I can hardly blame them. We called the impact "moth-back" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would stop to operate. The lamp itself seemed perfectly Ok, even on shut examination. We lastly found that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias arrange an electroplating motion, dissolving the wires connecting the base with the bulb. The problem was fairly merely solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, backyard foliage and energy lines made steady contact between the terminal stations the topic of persevering with concern - and a few unusual QRM. The state of affairs worsened in windy weather as there was a tall gum tree very shut the the optical path, in a backyard a few half mile distant from 3ZGJ. In a Northerly wind this huge nuisance had branches that may sway back and forth into the beam path, slicing words and phrases out of our conversations. Eventually the rattling factor became such a drag that we made half-severe plans about carrying balaclavas on an illicit midnight raid on its owner's backyard with ropes and a pruning noticed! Another suggestion involved borrowing a surgical reducing laser from Melbourne University's medical school, and trimming the foliage from a distance - the distance of 3ZGJ's shack veranda, actually. A few years afterward, I took a take a look at the identical path with binoculars, and located that the nuisance tree, by then with several others, had grown to fully block the optical path. L.O.S. paths might be impermanent! The rising of tall trees in suburban streets and gardens is now far more fashionable than it was thirty years in the past. It could also be unlucky for optical communication enthusiasts, but it surely gives an aesthetically pleasing manner of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the bright side of life" while I'm typing this)!

The colour of the mercury lamp also provided issues. It looked the identical as some other street mild in Melbourne. One night time, we drove up to 1 Tree Hill in the Dandenong Ranges outside Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance file. At the highest of the old lookout tower there (removed in 1981) we appeared down on the lights of Melbourne, which were as numerous as the grains of sand on a seashore. With John's optical receiver we searched the assorted lights seen in the overall path of Hawthorn for about half an hour, https://ticklingup.com/ vainly making an attempt to listen to audio modulation among the grunts and