While I’m in the process of setting up receivers and antennas in the loft, I’m becoming a dab hand at locating sources of interference. Unfortunately the latest gremlin is my first attempt at a USB-controlled receive antenna switch…
I’ve acquired a USB-controlled Denkovi 8 channel relay board to implement some control of things in the loft (primarily receive antennas but also thinking of other things that may want remote on/off functionality). For the first stab at using this as an antenna switch I’ve simply wired receive antenna signal lines straight through the switch relays with no attention paid to maintaining impedance. I’m also concious that with no real RF ground arranged in the loft space yet, ganging all the grounds (DC supply return and coax shields) together may not be the best idea, however I’ve put this configuration together in the spirit of try-it-and-see-what-happens.
This first setup has shown that the Denkovi relay board is a good source of interference to the signal lines running through the box. The two obvious problems are the introduction of 1 kHz-spaced lines over a large portion of the HF spectrum when the USB lead is plugged in and an increase in the noise floor by about 10dB when the software establishes the USB connection.
Thoughts now are to acquire a pair of coaxial relays (one for HF, one for V/UHF) and use the relay board to provide USB control of those while keeping its USB lead as short as possible and paying attention to placement and shielding of the control box. Also, now that the main components of the loft receiver setup have come together, it’s time to tidy things up a bit and have a think about an effective RF ground as the tinkering to date has shown that there is much improvement to be made in this area!