For my undergrad EE capstone project I'm designing and building a 2m band analog amateur radio repeater. Our design goal is to build a solar-powered low power repeater (something like 5-20 watts) which can be carried in a backpack and easily deployed. The frequencies should ideally be software programmable.

One big problem we're going to need to overcome is the duplexer on the antenna. Traditional VHF repeaters use huge resonant cavity filters to get something like 70 dB of isolation, but for us this just isn't practical, both due to size and the manual tuning requirement.

What alternatives do we have? I know we could use a circulator and get about 20dB of isolation, but I don't know if that would be sufficient to protect the receiver. We're designing the receiver ourselves. Any suggestions are welcome.

  • 2
    \$\begingroup\$ Man, my undergrad capstone was a battery. I lit up some LEDs... \$\endgroup\$ May 16, 2014 at 2:18
  • \$\begingroup\$ Hope you get some good answers. If you haven't already done so, you may want to ask this over on ham.stackexchange.com. \$\endgroup\$
    – bitsmack
    May 16, 2014 at 19:20
  • \$\begingroup\$ In full duplex mode you'll need a lot of isolation if you have a 5-20 watt transmitter. You have to keep the receiver amps from saturating due to your transmission. Can you use separate TX and RX antennas? \$\endgroup\$
    – user6972
    May 17, 2014 at 17:56
  • \$\begingroup\$ Investigate PIN Diodes . . I remember a Motorola transceiver (Mitrek) used them for switching the antenna \$\endgroup\$
    – Marla
    May 18, 2014 at 21:36
  • \$\begingroup\$ Have you considered using different bands for TX and RX, like satellite repeaters do? \$\endgroup\$
    – Kevin Reid
    May 19, 2014 at 0:37

4 Answers 4


You've got a lot of projects tied up in this project! What is the actual spec? If it's only to make a solar powered repeater, consider loosening some of the other specs. Designing and building a circulator would seem to be good project of its own. The big problem seems to be keeping the transmitter out of the receiver. So you might have to construct a better front end for the receiver. Lowering the output power will directly help the problem. So will using a split wider than 600kHz. And some repeaters work well using separate transmit and receive antennas. Using vertical antennas, and locating the receive antenna above or below the transmitting antenna (where the radiated energy is lowest) will help as well.


What about portable, easily deployable large cavities? Who said that those can only be made out of solid brass?

Do some experiments with mylar, wire net etc. foldable cavity resonators. They might prove to be impossible, or impractical, but this alone is a great project IMHO.


pick 2 non-common simplex frequencies with a 2 or 3 mhz split... It's not a 24-7 repeater and only temporary so it shouldn't be an issue. get a mobile flat-pack duplexer that will do a 2-3 mhz split and use say 15-20 watts or even less. you should be able to package it up pretty small!


Because this an Amateur Radio 2 meter repeater, I assume that you are using the standard 600 KHz frequency split.

I have a suggestion:

1) use separate receive and transmit antennae. Get them as far apart from each other as possible.

2) use a cavity on the receiver only.

If the Rx & Tx antennae are separated, you don't need a cavity on the Tx side. This reduces the size by about half.


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