2 port automatic antenna switch

Back in the 90s I used a pretty simple 4-port automatic antenna switch built around a logic IC to be QRV with 4 modems and 4 CB radio’s on 4 channels. Although you would expect some drawbacks, this worked pretty well.

Recently I wanted to do some experimenting with SSB packetradio instead of FM. Currently my node and BBS setup are 24/7 QRV with AFSK 1200bps on CB channel 24 (27.235 MHz) FM. Besides that I also wanted to go QRV and experiment on another channel with a switching circuitry just like massively done back in the 90s. But now using a more modern solution with some nice extra features I didn’t had back then.

Goals were:
– Using 2 TNC’s or soundmodems at the same time.
– Connecting 2 radio’s.
– Using only a single antenna.
– Built-in PTT watchdog timer.
– DCD sharing and interlocking.

In a very early stage I decided to write firmware and draw a circuit around an Atmel microcontroller (ATtiny 461). So much more convenient to use those, or modify/tweak things, instead of using a combination of logic and analog ICs for that purpose.

Very early stage of building the control board.

Some hours later the control board almost finished and I started building a RF shielded relay box for switching the 2 radio antenna outputs and the antenna.

Early test fitting the control board and relay box.
Almost there…

At the back connectors are wired and placed. The 25p sub-d connector is for interfacing with 2 TNCs and 2 radio’s.

Back view

Below is the radio and TNC adapter cable. All 4 of the in/output connectors are wired with standard TAPR TNC2 pinout so I’m able to use cable’s I already have and TNC’s can be connected with a standard 5-pin DIN cable.

Radio and TNC adapter cable.

After some testing, needed to make sure it’s functioning ok and all ‘features’ do their job, I’m now QRV with packetradio on both ch. 24 (27.235 MHz) FM and ch. 36 (27.365 MHz) LSB. Using 2 CB radio’s, 1 soundmodem which is built-in NL5VKL’s node setup, 1 external TNC and only a single antenna.

Works like a charm and thanks to the DCD sharing and interlocking features it seems to perform pretty well.

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