I finally have a noise-free loop array distribution system up and running. Performance is near perfect and I use it every day.
My six month journey experimenting with microcontrollers, relays and remote switches has brought success. Shown above is a downsized system to control a loop array with multiple dual and single channel receivers.
To eliminate microcontroller RFI on HF, I used better shielding and filtering. But, most important, I dumped the Arduino. Now, everything is controlled by a NodeMCU 12E with a pair of PCF8574A I2C GPIO expander boards. Essentially, my loop array distribution system is an Internet of Things WIFI server on my LAN.
Unfortunately, the cheap and cheerful PCF8574 modules don’t provide enough current to drive more than 3 or 4 relays, so I removed the relay-intensive external antenna switch. Later this summer, I will build in some Darlington array chips to handle more current. In the meantime, I am in business with the wideband loop array. I will also design the controller so I can get rid of all the pin jumpers and just use some ribbon cables.
You can build this loop array distribution system for around US$150, not counting the AAA-1C active loop controllers. All the parts have been described in previous articles in this series.
Just add a piece of half-inch plywood and a 3D printer. You can modify this design to easily switch two antennas between up to six receivers.
Loop Array Distribution System Noise Performance
I find the NodeMCU provides significantly less RFI on HF than any of my Arduino, especially the Mega. Below, you can see my latest testing with a signal generator and spectrum analyzer across 8-12 MHz.
In the top trace, you can see the difference in noise floor with and without the switching board inserted. Basically, the difference is 0 dB, so I have reduced my microcontroller RFI. At the bottom trace, you can see the output switch is providing around 50 dB isolation at 10 MHz, as expected.
Measurements were made with the RSP Spectrum Analyzer and an RSP2 receiver. So, I am happy.