Up one level
Back to Home
The $100 GMRS Repeater
By Jim Sharp
HTML'd for the repeater-builder web site by Kevin Custer W3KKC
You may read this dated HTML presentation, but it is advised
you download the latest representation of this article which is available as
two PDF files. The article is available from the first link, and the board
layout from the second:
Once you have decided you want to put up a GMRS repeater you go to a local radio shop and they lend new meaning to the phrase "sticker shock". A new repeater complete runs from $1000 to $8000. Then, the other solution is go buy a used system somewhere and hope its reliable and try not to pay to much for it. Here is choice number three. Build a complete, reliable, high performance, 20 watt repeater for $550 or less. The repeater will be composed five components; the radio itself, a 12 volt power supply, a tone board or repeater controller, a RF preamp and a duplexer. The radio is a EF Johnson PPL6060 mobile radio (very popular UHF mobile a few years ago) and you can find them used for as little as $25. The power supply can be found used for a few dollars or purchase a new Astron RS7A for $48 from Tessco (call 1-800-472-7373 and request a catalog). If you plan on adding anything else you may want to get a larger supply. If you want CTCSS (also known as PL, Channel Guard or Call Guard) in and out order a TS-64 (Tessco, or from Communications Specialists directly) for $58 (if you want tone in only use the board that came in the PPL). For a controller, it's your choice or none. The duplexer is your choice but you can get out for as little as $291 for a Sinclair MR-356 from Tessco (or you can use two antennas). The optional preamp is a P450VDG available from Advanced Receiver Research (they are on the web) for $80. If you use two antennas, a low cost, used power supply, a Com Spec tone board and no preamp you can get going for about $100. The choices on options is governed by your budget but the real "trick" is the 20 watt duplex radio for $25 or so.
The following is a description of how to modify the radio. You will need a radio (try to get a later version as it was rated at 20 watts out instead of 15), a manual, a set of crystals for the GMRS repeater frequency and a technician to do the modifications.
1. First, crystal, checkout, and tune up the radio simplex and be sure to set audio deviation and tone deviation (they interact). I suggest you use the Com Spec TS-64 because it will encode while it is decoding (the EFJ board will not and you could use it to decode but the station will transmit without tone).
2. Begin by bringing out the rx antenna port. Build a 10 inch pigtail out of RG-142A with what ever type of connector you want to use. I use a crimp BNC and then a BNC to UHF adapter. Remove C-12 (33 pF) and remove the trace on the bottom of the PC from the C-12 eyelet to where it connects to the first helical filter (about 1/2 inch). Connect the center conductor of the RG-142 to the point where the first helical filter is. Solder the braid to the ground plane. Get some 0.010 brass shim stock and make a small cover for this connection. Also, construct a small shield for the area of Q-1 (the first RF amp). This will be a 1 inch square of shim stock with the edges turned down about 1/16 inch on 3 sides. The better these two shields fit and RF "seal" the less desense you will encounter later. I usually file a small square notch in the rear case (near the power connections) to run the RX pigtail out. Then you can re-install the bottom case and the pigtail will hang out the rear of the radio. The original antenna connection becomes the transmitter port. Between the radio ports and the duplexer (or separate antennas) use RG-142A and quality connectors with as short as possible cable lengths.
3. Provide a duplex PTT connection. Locate R-402 and lift the end that goes to the microphone socket connection and install a 1N914 diode in series with it with the cathode toward the mike socket connection and the anode connected to the loose end of R-402 (1K). Disconnect the PTT lead from the PCB (comes from pin 2 of the mike connector) and connect it to the junction of the diode and R-402. This is now a duplex PTT.
4. Remove Rx muting by removing CR-203 and CR-206. You are finished with the modifications required to duplex the radio. You may now wire in a controller or complete the "repeater modifications".
5. To wire in a controller follow its instructions. Rx audio is picked up at the junction of C-277 and R-253 (this is NOT squelch muted audio but has tone removed). Discriminator audio is available at the junction of R-244 and C-270. Rx COS is positive-going at pin 8 of IC-202B. TOS is available at pin 2 of the call-guard board (this is negative-going at this point). PTT is the mike pin 2 that you moved in step 3. TX audio is on pin 1 of that connector (remove R-319 if installed, it's there to provide +9V for the pre amplified base mike). Mike pin 4 must be grounded for the tone decoder to operate properly. If you use the EFJ tone board ground that pin and turn the TX tone control on the board all the way down (the TX tone is unusable).
6. Install a 10 K pot from the RX audio to ground and connect the wiper to the TX audio point (set this pot for +/-4 KC TX audio deviation with a +/- 4 KC modulated 1 kc. tone into the receiver). Connect a 15K resister to the COS point and connect the other end of it to the base of GP NPN transistor. Ground the emitter of the transistor and connect the collector to the duplex PTT. If you are building a low-cost carrier operated repeater without a controller you are finished. If you want tone control then continue. Consult the tone board instructions for the references below. Install the above 10K pot but connect the wiper to the FILTER IN. Connect the FILTER OUT to the TX audio in. On the tone board itself, remove jumper 1 and replace it with a germanium diode with the cathode toward the collector of the first transistor. Install jumper 2 and connect the collector of an extra external GP NPN transistor to it. Ground the emitter of this extra transistor and connect the base back the COS point through a one transistor inverter. This is required to mute audio and control the transmitter with COS and TOS. Connect OUT 1 through a series 1N914 diode to the PTT point. Of course, connect the ground to a ground trace and + to the rear of the power switch. DISCR IN connects to the discriminator point described above. If you want a repeater hang-timer, change the 10 uf cap in the tone detection circuit on the tone board circuit to 100 uf.
7. I have built several dozen of these repeaters and they have always performed perfectly. I have one that has been in continuous duty for 10 years with a community repeater panel on it. And I have used this design to build several full duplex mobiles for telephone interconnected systems on commercial repeaters. I've even mounted one of these in a weather-proof box and tower-mounted it with two antennas and ran it for a year or so (saves on cable costs). If you have a problem or some confusion or would like for me to do the modifications (for a reasonable charge) e-mail me at jsharp2316 //at// msn //dot// com or call me at 830-612-2648 evenings (US Central time) or weekends.
Back to the top of the page
Up one level
Back to Home
Copyright © July 28 2000 Jim Sharp
HTML © Copyright July 28 2000 by Kevin Custer W3KKC
Updated March-28-2005, J.S. and K.C.
All Rights Reserved.
This page originally posted on 30-Jul-2000
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.