Audio link from studio to transmitter on 2.4Ghz

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EFR
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Audio link from studio to transmitter on 2.4Ghz

Post by EFR » Thu May 23, 2024 3:39 am

Hello! I have been testing diffrent links for my SW transmitter use.

After couple weeks of failures, I started to think that London pirates did use links in 80's and 90's, so why not to ask directly from them.

My main problem is that my transmitter site has no 3G/4G network coverage, and bands between 30-88Mhz and up are so full of MIL, PMR etc communications, same on UHF too because I live near of quite big military airport.

I have tested multiple 2.4Ghz analog and digital audiolinks from Ebay, AliExpress and from Thomann, but all of them have some problems, mainly on audio or spectral purity.

Best one was leaking its 1.2Ghz LO so badly that i was able to hear it much better on TX site than 2.4Ghz signal.

One of things what I want to do, is use vertical or something as link receivers antenna, I dont want yagi pointing to my house from nearby hill, area is very rural, so not many houses around to search.

So, Im here to ask, what does London pirates use nowdays to link studio to transmitter? Where I live, many of operators still carry transmitter and batteries to hill awith MP3 player and go to nearby petrolstation to wait if police goes get that transmitter. I did this from 2008 to 2014, 120cm of snow and carrying transmitter and lorrybattery to forest isnt fun. Nowdays I have solarpanels and 140Ah battery on a hill.

Sorry for my bad english skills, I have never studied it.
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Albert H
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Re: Audio link from studio to transmitter on 2.4Ghz

Post by Albert H » Thu May 23, 2024 3:38 pm

If you're going to use microwave, direction aerials are essential. Many of the London pirates in the '90s used 10 GHz links for short range.

I used 10 GHz in the 80s to get from our studio to a nearby tall building, to feed a 1 Watt UHF (460MHz) link going up to nearby mountains (the Hollywood Hills!). The power level on the microwave link was so low that it was very directional, and couldn't be heard "off the beam". The receiver was a hacked domestic analogue satellite receiver, fed from an LNB, looking along the street to the studio, where the transmitter was a door-opener radar unit fed with a modulated power supply. I expected it to be unstable, and require constant adjustment, but was delighted to discover that the receiver had a broad AFC tracking circuit that was designed to compensate for the thermal drift in the LNB. After initial set-up, it worked continuously for several years!

Your wish to use non-directional receiving aerials is probably not possible, unless you use (perhaps) 60 MHz. The big trick is to use as little power as possible to "make the trip" with a (just) fully-noise-quietening signal. If you do your research carefully, you should be able to find an unused frequency somewhere in Band I.

Another trick I frequently used to get audio from a studio low down in a building to the roof was to use 140 kHz FM impressed on to the mains supply. There is a nice simple circuit for this in the National Semiconductors Audio Designs book. They used an NE566 oscillator feeding a medium-power transistor, inductively coupled into the mains wiring. The receiver was an NE565 PLL IC fed from a buffer amplifier that was also inductively coupled to the mains supply. As long as the transmitter and receiver were on the same ring main, you could get good quality audio from place-to-place in an extremely covert manner!

All that being said, your location isn't ideal. I'd be quite concerned to try and use a shortwave transmitter in close proximity to a military base. Isn't there somewhere else nearby - perhaps up another hill - that you could consider using?
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Re: Audio link from studio to transmitter on 2.4Ghz

Post by EFR » Thu May 23, 2024 3:58 pm

Not that good place near, and next two MIL bases are about hour to east and north, on west there is city.

I like that location because there is no road even near to it, and VW Transporters arent that good in dense forest as my car.

I used on my FM days link on lower edge of band and transmitter on high end of the band.

I just wired an transistor and relay to an STEREO led on an receiver, when there was stereo audio present, relay turned transmitter on.

Later I used simple VOX circuit to control SW transmitter, if I remember right, I had an 15kHz tone on one channel and audio on second.

VOX was before transmitters 6Khz limiter/compressor circuit, so that 15kHz tone didnt pass to the modulator, but if someone tuned to that FM link, they did send email that my audio has problems.
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87to108
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Re: Audio link from studio to transmitter on 2.4Ghz

Post by 87to108 » Wed May 29, 2024 1:57 pm

EFR wrote: Thu May 23, 2024 3:39 am and bands between 30-88Mhz and up are so full of MIL, PMR etc communications,
Very surprising to hear that the whole 30-88MHz is full nowadays where you are!? Are you certain that is the case nowadays or hearsay?
Its almost empty nowadays where I am (low-band PMR gone as mobile phones took over). Low VHF would work well with a non-directional vertical receive of correct ideal length, unlike higher frequencies where you increasingly need directivity to pull in good enough signal.
Also less need for 'line of sight' if surrounding trees/vegetation obscure your receive aerial.

Its likely most of Band I would be safe, but importantly provided 2nd harmonic is well suppressed!
obviously avoid the ham 6m band etc ...

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Re: Audio link from studio to transmitter on 2.4Ghz

Post by radionortheast » Thu May 30, 2024 7:39 pm

I think they might not be in the uk, I hear nothing below 87.5, you can check with a tiny sa to see whats out there, all I see here is fm, dab, carriers appearing in the air-band aircraft now and again strong shortwave signals thats about it. They are good to have they show a lot of spectrum so you can see when something appears, the frequency on mine is not that accurate, if you find something maybe you could home in on it on a little scanner.

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Re: Audio link from studio to transmitter on 2.4Ghz

Post by EFR » Fri May 31, 2024 12:00 am

Yep, Im not from UK, and our country is quite big and empty, and it isnt Russia.

MIL just bought new rigs for that band, I think they spend like more than ten million euros... When I was younger, just after my own military service, I used to listen soldiers training on radio, and quickly ask something like "Where is Your tent, i heard there is vodka" using modifield 80Mhz taxi radio or CB. Listening that aftermath was fun.

Back then when I started to broadcast, some guys used 190Mhz links, police was between 160-180Mhz if i remember right. It was fine as long as cops didnt get too near link transmitter, there was change that they did hear some Pop and rock from tgeir HTs.

One problem with low band links is that they are easy to track. I have to test something on like 600-900Mhz, between digital tv and mobilephone links.
Old TV logperodic antennas are good there, and tested with NanoVNA and short lenght of Aircell 7, some of these more expensive ones have quite good match to 50ohms.
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Re: Audio link from studio to transmitter on 2.4Ghz

Post by Hugh » Wed Jun 05, 2024 1:02 am

I gutted the essentials from a 5.8g analog AV sender and receiver for doing this. I wanted a short range link to do outside broadcasts for FM radio. a 5.8g "Double Bow Tie" antenna each end increased the range of the thing to more than 500m over clear ground. I sent balanced +4dbm stereo, power (+-15, &12v) up 10-15m cat 5 to a head unit in a sealed ~150x100 electrical box. A piece of double sided blank pcb in there formed the antenna ground plane and t'other side for the electronics, the boxes up a 6.5m pole. It works very well, the audio quality is surprisingly good - and that's simply using the L & R analog. Next version I'd put a ~1W booster amp on the Tx, and might get 10Km range.

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