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There are many times when I could use an ad-hoc repeater especially for ARES use. The recent catastrophes in New York (where I live) have exaggerated this need so I set about building a repeater controller that would be easy to deploy as a moments notice.

The design rules for this repeater were as follows:

  1. Must be easy to use
  2. Must be able to accept any radio equipment without modification to either radio or controller
  3. Must run from 12VDC
  4. Must have multiple inputs allowing for repeater "chaining"
  5. Must get up the nose of all those whom insist that a repeater must be made from Motorola gear
  6. Must be CHEAP!!

So I set about collecting the bits for this project. I already had the metal enclosure to house it in and most of the switches and sockets etc (I keep a well stocked junk box) so all I needed now was the electronics.

Naively thinking that I could just build a controller and then use it I looked on the net for the cheapest controller kit I could find. This turned out to be the ICS Basic controller as pictured on the right. This controller comes pre-built so all I have to do is wire it to the relevant parts.

So I installed the controller into the box and that's when I encountered my first problem. The repeater controller requires a number of different inputs from the radio most of which required surgery to the radio. Objective #2 blown away!! After a good dose of RTFM I discovered that the controller (like all controllers) needs to know when the receiving radio isn't getting a signal any more. In other words, it needs to know when you have stopped talking to the repeater. There was I thinking that I could just wire up the earphone to the controller and it would "know".

The technical term for the signal I needed from the receiver is Carrier Activated Switch or Carrier Operated Relay. "No big deal" I thought, "I'll build a Voice Activated Switch (VOX) circuit". Famous last words. I searched the net and found an easy circuit which when built added more problems to the design.

Rather than fight with it I scoured eBay and found a 440MHz repeater receiver with a COS/COR output. I installed it into the box. Now, whenever a bit of noise came along and opened the receiver, the repeater would transmit the noise. I needed some sort of access tone. These tones are commonly known as Private Line (PL) tones or CTCSS tones. It was unsafe to assume I would get this from the receiver as many radios can transmit access tones but not decode them. This was the case with the eBay 440MHz RX.

qt-1.jpg (54030 bytes) So what was needed was some sort of external tone decoder circuit that was able to not only decode the tone but also activate the PL facility on the controller. Another search of the net produced the QT-1 PL Tone Encoder/Decoder from Ramsey Electronics.

The decoder can decode all the normal PL tones. I have it wired so that it can signal the controller telling it that the radio is receiving something with a valid PL tone. This along with the CAS/COR signal form the 440MHz receiver then makes the controller fire up the transmitter. The decoder then strips the tone from the received audio and presents it clean so that I can use it to feed the controller's audio path. . Stripping the tone is very useful. I can now use the transmitters tone generator to send a tone different to the one I received; a useful feature although not widely used.

OK, so now I have a controler that can be triggered by a sub-audio tone. All I have to do now is add a control receiver so that I can shut down the system remotely should something funny be going on. Back to eBay!

I found a 220MHz repeater receiver identical to the 440MHz one I bought earlier. In it went and was duly wired up to the control inputs on the controller board. Hmmmm; just blown away rule number 4. I gave this some more thought and searching of the net and turned up an audio mixer design. I could now feed many radio's into the controllers audio path. As long as the radio's had some COR/COS I could handle that with a diode "OR" gate.

I built the mixer circuit with facilities for 3 audio inputs. This not only got around the impedance problem but also added a little bit of preamp to the audio too. The output of the mixer is fed to the control input of the repeater. One drawback of this "feature" is that all the inputs can be heard at the same time. mixer circuit.jpg (15835 bytes)
tnc wiring.jpg (25937 bytes) What I needed now was some sort of standards based radio interface so that I could add any radio to the controller by simply plugging it into the back. Again, I poked around on the net but could only turn up the RAyNet wiring standard. When I was with them back in the UK nobody I knew used it as it was too complicated and the plugs were too expensive.

I settled upon the PacCom standard which can be found on most TNC2 clone TNC's. This has everything I need to work the radio's that will be connected. As the TNC2 clone is such a popular TNC its highly likely that someone will already have this cable in their junk box. The diagram on the right shows 2 different cables. I actually combined the 2 cables into one thus eliminating the "which cable is this" problem.

Now that I have a wiring standard sorted out it occured to me that I might want the ablility to either control the repeater on another frequency or add another receiver to it. To do this I needed to accept audio from at least one other radio. I added a row of headphone sockets to the box and then wired them up together. This didn't work quite the way I had hoped. I found that the different impedances on the different radio's had an effect on each others volume. I needed an audio mixer. Again, a search of the Internet produced a mixer circuit that was cheap and easy to build.

Thats pretty much it. having wired all the bits together into a nice metal box I got at Dayton earlier this year I tested it and it all worked fine. I have got 2 of these units built now and plan on installing one of them as a 10M repeater here in New York City. The other I plan to use as an ad-hoc repeater for ARES use. It might just fill in that RF black hole that always seems to exist right where you need to talk to.

Click here for a diagram.

Further to this project I have created some audio files for use with other repeater systems. Click here to download them.

& Mark A Phillips, G7LTT 1997/8/9/2000/01/02
This page was last updated Friday October 08, 2004 22:14 Eastern Daylight Time