My antenna switch adventures have provided a valuable learning opportunity. Especially to reduce noise in RF circuits under Arduino control.

Over the past few months, I have been trying to implement a remote dual channel antenna switch using popular Arduino relay modules. My idea, shown above left, was to route a pair antennas to each of four diversity receivers. You can see the basic idea above left (marked “A”). These relay modules are readily available, cheap and easily controlled with an Arduino.

My disappointment came through noise, or radio frequency interference. Antennas switched fine, but the system introduced around 30 dB of noise, drowning out the weaker signals. About half of the noise came from the Arduino. I learned to mitigate this by using a variety of methods, described previously.

But even when running this board under manual control (no Arduino) lots of noise remained. Just attaching control wires to the Sainsmart relay module created noise at the output, typically 15-20 dB, more on some frequencies.

So, my friend Jim VE6JF took some pity on me and developed a homebrew antenna switch, shown above right (marked “B”). His prototype uses the same Songle 5V SPDT relays, but mounted on a proper circuit. Three big differences. First, a ground plane. Second, bypass capacitors on the control lines. And third, using multiple relays in a signal path to increase port isolation.

Would this work any better?

Antenna Switch Adventures – Doing it Properly

You bet it will. Take a look.

Here you can see the results of my relay board switch (A) versus Jim’s proper switch (B). His Rigol spectrum analyzer is sweeping 2 to 20 MHz, with a -80 dBm (S8) test signal at 15 MHz. There is no RFI created through the control lines. I suspect the ground plane and bypass capacitors make a big difference. The Arduino relay board has neither.

So my antenna switch adventures have taught me the importance of proper design. And, since Jim plans to use some Arduino control and relays in his homebrew receiver design, he learned some valuable lessons which he can apply to his work.