Rx Antenna Switch – Interference

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.

IMAG0219This 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!

2 thoughts on “Rx Antenna Switch – Interference”

  1. Hi Andy!

    Just this week I finalized my antenna-installation with Denkovi 4-relay-USB-card – and now today I found your page!!! Not good news
    for me, but thanks for sharing this info and your experiments!!!! My problem is that I can not test my installation yet.

    I try to use Denkovi 4-relay USB-card to switch 4-different antennas for Medium Wave DX-listening. We have 3 longwires (800 m each)
    with coax-feeds. The installation date will be 1-3.11.2013 in the noise-fre country-side in Mid-west Finland. I can not pre-test my box
    before because my rx-equipment (Perseus) is elsewhere now.

    I have not done it like you have. I change my antenna-relays by Denkovi-relays (I have photo of it), so the RF is not going through
    Denkovi-relays, only minus of 12V antenna relays. (So, I can try to move Denkovi-card inside PC forexample). However, I am now
    very concerned of this possiblity to have interference.

    On what frequencies the interference was worst? Signal peaks in every 1 kHz??? What noise-level above rf-noise??
    Even more noise when software started???

    We need extremely silent band to MW so any leak from usb-card is not tolerable.

    Harri Kujala
    SW Finland

    1. Harri, thanks for dropping me a line. I wouldn’t give up hope on your installation – mine is the absolute worst case because I’ve stripped the RF lines of all their shielding when they run through this box, as close as they can get to the USB device. Since posting this I’ve also overcome some noise problems with a USB-serial converter and my Elecraft K2 by adding ferrites and changing USB port on the computers that were in that setup so I feel a bit more confident about tackling USB noise.

      The most important thing is that you’re using the correct approach of coaxial relays for your signals lines. If you keep the USB connection between Denkovi board and PC as short as possible and add ferrites at both ends of that cable, then separate the coaxial relay and RF signal lines as far from the PC/Denkovi as possible. The noise I had was pretty bad, I can’t remember the levels but remember here my signal lines were unshielded and trapped in a metal box on the same PCB as the interference source! I know it was bad but, like I said, it was a very rough quick experiment to see if I could get away with it. I’m sure when I get a coaxial relay it’ll be OK.

      You’re setup sounds superb! I found your homepage online – VERY interesting stuff. Looking forward to hearing how you get on.


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