Emergency Repeater

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Recently Randy K7GE asked me to explain how to build the emergency 2 meter repeater I talked about at the March District 5 ARES/RACES meeting in Roseburg.  Here is my answer to him:

Hi Randy.

Here is how you do it. You need two dual-band rigs (2M/440) with cross-band repeater capability and two vertical dual-band antennas.

You set one dual-band rig to the 2-meter transmit frequency and some 440 simplex frequency, and you set the other dual-band rigEmergency Repeater to the 2 meter receive frequency and the same 440 simplex frequency. You then position the rigs some distance apart on a hilltop or on 2 separate hilltops. (should be at least 1/4 mile apart from my experiments). Then you set both dual-band rigs to cross-band repeat. That's all there is to it. Also, I would suggest that you put both rigs on low power (5 watts or so) because this system keeps both radios transmitting all the time when someone is using them.

Here is how it works: Let's say we want to build a repeater to operate on 147.32+ on 2 meters, and we will use 445.1 as the 440 band simplex frequency. Rig A is set to 147.32 simplex (the 2 meter transmit frequency) and 445.1 simplex on 440 band. Rig B is set to 147.92 simplex (the 2 meter receive frequency) and 445.1 simplex on the 440 band.

Then you set your HTs to 147.32+ . When someone with an HT transmits to the repeater, they transmit on 147.92 right? (Plus offset, remember?) So Rig B picks it up on 147.92 and retransmits it on 445.1. Rig A receives it on 445.1 and retransmits it on 147.32, which is the frequency everyone else is listening on.

That's it. Give it a try. We did. We used a DJ-580 HT for the 2-meter receive side and a DJ-605 base station rig for the 2-meter transmit side. It worked great!

73 de Al KK7XO

Additional Notes:

  • The reason the antennas are separated by 1/4 mile is so the 2-meter Tx signal does not de-sense the 2-meter Rx radio.  Since Tx and Rx frequencies are typically separated by only 600 KHz, you need to either use duplexers or spatial separation to attenuate the 2-meter Tx signal at the 2-meter Rx radio enough to allow its receiver to function. 

  • Instead of positioning the antennas 1/4 mile apart, you may want to experiment with separating the antennas vertically if possible.  If a tower is available, position one antenna directly under the other separated by at least 20 feet.  Vertical separation results in greater attenuation than horizontal separation so you don't need to separate as far.

  • You could try using a rubber-duck for the Tx antenna, Rx antenna, or both.  This will get you additional attenuation.  Remember, you don't need to transmit any farther than you can hear.  If your job is localized to a remote area such as in Search and Rescue, you may be able to attenuate your transmit signal and still reach all necessary areas.  The more you can attenuate, the closer you can space the antennas to each other.

  • There are other techniques that can be employed to provide sufficient attenuation to allow the antennas to be deployed closer together.  Try separating them vertically by only a few feet or so, but use one or more mobile duplexer cavities to notch out the Tx frequency on the receive side.  These cavities are small, smaller than a 1 lb. coffee can, and can be had at hamfests fairly cheep.  I picked up a set for the 2 meter band last October in Rickreall for $50.  It has 6 cavities!

 
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Last modified: 04/30/09