secure link between sites

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silent freq
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secure link between sites

Post by silent freq » Fri Jun 07, 2019 11:39 pm

I'm trying to learn more about how to secure our live studio site in case of a raid.
I've found that pirates in London used microwave transmitters beaming the broadcast up to microwave recievers connected to the main FM transmitter. I can't fint any information on how to build such a system, can anybody help me out by explaining me how to do it, or pointing me to a site where I can learn more?

I'd also like to learn about other options. I've seen someone mentioning other technical solutions like sending over the mains(???) and over the internet, the last one doesn't seem very practical to me, but I'm more than willing to be proven wrong.
I'm hoping for a solution that can do stereo, but am willing to go mono if it's much easier to achive.
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Re: secure link between sites

Post by Albert H » Sat Jun 08, 2019 2:32 am

Microwave was easy when there were plenty of analogue satellite receivers around. Unfortunately, they're getting a bit rare these days. The transmit end was usually a Gunn diode in a waveguide aerial, modulated by varying the DC supply to it. Properly aligned and accurately tuned, it was possible to get a few miles out of these.

My favourite choice was Band IV - up in the UHF TV bands - in the gaps between TV channels. The aerials were ordinary TV aerials which didn't look out of place anywhere. My receivers were TV tuner heads with PLL control of received frequency, feeding a two IC IF / demodulator with a tone decoder for remote switching. The transmitter was a PLL running at ~60MHz and multiplied up to the output frequency, with a PA (using a pair of 2N3866s in grounded base mode) giving about a Watt output. The antenna gain would multiply the ERP up to several Watts in a very narrow beam. The longest range I achieved with one of these (in stereo) was about 23 miles.

The most sophisticated linking system I used was microwave across the street from the studio to a nearby high building, UHF from there to the first hilltop, then a second UHF link across the valley to the next, much higher hilltop. It was stereo, fully noise-quietening, could remotely switch the main transmitter and everything outside the studio was solar powered, with lead-acid batteries (and a complicated battery management circuit at each site) to keep everything working during the night. This system was in California, so there was plenty of sunlight!

I've used links up the mains - great when the studio was in a lower floor of the block that the main transmitter was on. These work by putting an FM carrier at around 130kHz into the mains wiring. A simple PLL receiver at the far end recovers the signal from the mains and gives audio out to feed to the main transmitter. This works really well if done properly, but wiring stuff to the mains needs to be done by someone who really knows what they're doing!
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Re: secure link between sites

Post by ronald001 » Fri Jun 14, 2019 12:28 pm

Microwave is not really secure / untracable, it can quite easily be traced.

A cheap PC, 3G dongle with prepaid card does the trick

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Re: secure link between sites

Post by nrgkits.nz » Fri Jun 14, 2019 11:21 pm

My approach would probably be to use a pair of Ubiquiti power beams, have a raspberry pi at the transmitter end and enable wpa2-psk encryption plus frequency hopping on the power beams. The beam width is also very narrow which would make tracing the link more difficult.

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Re: secure link between sites

Post by Albert H » Sat Jun 15, 2019 3:02 am

Microwave - done properly - is virtually untraceable. Unfortunately, most of the clowns building this gear had no real clue.

Practically speaking, UHF in the TV bands is easiest to set up, difficult to trace, and can work over long distances with quality that's indistinguishable from a piece of wire! If it's done in a couple of "hops", it becomes even more difficult to trace.
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Re: secure link between sites

Post by silent freq » Sun Jun 16, 2019 7:41 pm

Albert H wrote:
Sat Jun 15, 2019 3:02 am
Microwave - done properly - is virtually untraceable. Unfortunately, most of the clowns building this gear had no real clue.

Practically speaking, UHF in the TV bands is easiest to set up, difficult to trace, and can work over long distances with quality that's indistinguishable from a piece of wire! If it's done in a couple of "hops", it becomes even more difficult to trace.
Is there of the shelf systems or kits to be bought for UHF in the TV bands? This seems like a perfect solution to my problem.

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Re: secure link between sites

Post by Albert H » Mon Jun 17, 2019 12:27 am

There are very expensive commercial systems (like the Marti OB gear), but some of the better rig builders will be able to sort out a system for you.

Be certain exactly what you want: RDS? Mono or stereo? Remote switching facilities? How far do you want the link to go?

Some of the Dutch chaps on this board will have the ability to build suitable equipment.
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Re: secure link between sites

Post by NOYB » Mon Jun 17, 2019 3:57 pm

Up-vote for nrgkits and Ubiquiti or other similar digital links if you have £100 or so and some old PC hardware to throw at it.

Alternatively a cheap PAYG 3G or 4G SIM might be another way - gives "plausible deniability" that some absolute rogue is picking up your legal internet stream and relaying it over FM!

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Re: secure link between sites

Post by SamTheDog » Mon Jun 17, 2019 4:31 pm

ronald001 wrote:
Fri Jun 14, 2019 12:28 pm
Microwave is not really secure / untracable, it can quite easily be traced.

A cheap PC, 3G dongle with prepaid card does the trick
I use 10GHz still! It all depends on what you use... most station I knew in the past either used a VHF or Uhf link, or if they were serious used microwave. Although most used a simple gunn-diode oscillator driving a simple horn...

I use a small dish at the TX end too, this has a much higher gain, and gives a far narrower beam. the advantages are....
Longer link distances. Miles! Trickier to trace with simple hardware.

But the disadvantages are trickier to set-up and align. Instead of "Gaffer Taping" the solfan head high-up on a window. It needs to go on a camera tripod or similar, and you may need binoculars or telescopic sight to get it aligned!

In the past whenI set-up a studio only a couple of miles away from the O/P site, I linked away from the O/P and then back again both TXs used parabolic dish reflectors driven by a feed-horn, and the RX sites used a simple horn on the LNB.
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Re: secure link between sites

Post by SamTheDog » Mon Jun 17, 2019 4:55 pm

Image
This is one of my Microwave transmitters. It's a wideband gunn oscillator producing between 5 and 12 mW
(depending on what frequency I tune it to)
link2.jpg
this is the control-unit. There is a modulated regulator in the box, and an RDS encoder.
the controls are audio-level, RDS data level , on a multi-turn. and GUNN-Volts on the second multi-turn.
link3.jpg
this is the micrometer head at the end of the gunn-diode cavity. Adjusting this and the GUNN-Volts
control sets the operating frequency of the transmitter.
Beamwidth of this kit is VERY-NARROW ! I use a telescopic rifle-sight to align it!
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And many moons ago.. WNKR on FM and Shortwave.
info@torradio org / qq22.net / TorRadio.ORG

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Re: secure link between sites

Post by SamTheDog » Mon Aug 12, 2019 1:03 pm

There is a load of ancient Marti and other kit on the internet at the moment.. Loads of it is the 1980's RPT / RR stuff.
This kit is OK. BUT it is crystal-controlled and pretty ancient. If you get a full setup all on the same frequency it MAY be useable. if not you will have a big pile of old poo sitting around waiting for new crystals to be ordered and fitted.

Even if you do get a matched TX and RX you may find that the frequency it's running on may be not useable in your area. Just saying, so keep an eye out.... I have a mid / late 1980s marti RPT25 and RR50 here. it all works a treat BUT it's from a local station so not really that practical to use as a link to an output site around here......

I still think that 10GHz offers far more for the money. unless you are doing outside-broadcasts under 10 miles from your studio. when this kit works OK...
Sam The Dog.....

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And many moons ago.. WNKR on FM and Shortwave.
info@torradio org / qq22.net / TorRadio.ORG

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Re: secure link between sites

Post by Albert H » Tue Aug 13, 2019 4:14 am

I still prefer UHF through TV aerials!
"Why is my rig humming?"
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