Well, it’s time for me to declare my DIY antenna switch disappointment. I can’t get rid of the noise n matter what I do.

Earlier this year, I set out to build a really simple antenna switch using Sainsmart relay modules and Arduino control. It’s been quite an adventure. Also, a disappointment.

Getting the Arduino to control a bunch of relays was simple. Wiring the relays to some BNC connectors to route antennas between my SDR receivers worked great. But along the way, I was plagued by noise (RFI) on the antenna lines. My system performed poorly.

At first, my main problem was Arduino radio frequency interference. I used a combination of I2C GPIO expander boards, sleep modes and limited RS-232 signals to knock this down. At present, the Arduino wakes up for my short serial port commands, then goes back to sleep. This actually works pretty well.

But, even with no Arduino running, the switch is noisy. Above left, you can see the clean signal from my generator going straight into a spectrum analyzer. Any noise is a good 90 dB down from the signal. Above right, you can see the same signal running through my antenna switch disappointment. My relay is closed manually with a power supply, no Arduino involved.

Sadly, the switch itself is a source of very noticeable noise across a broad spectrum.

Relay Antenna Switch Disappointment – Digging Down

The noise is either coming from the Sainsmart Relay Board, or being picked up from nearby, or both. My switching circuit is very simple point to point wiring using #20 AWG and a bit of RG174 coax between modules. There is some grounding and shielding, but not really a proper PCB ground plane.

Port isolation is a meagre 25-30 dB. My switch raises the receiver noise floor by at least 15 dB, more with the Arduino running even in sleep mode.

Perhaps it’s time to instead buy some proper antenna relay PCB kits and give those a try. I notice some on eBay.

One piece of good news. Arduino relay control of my LZ1AQ loop amplifier control boards work fine and does not introduce noise.