Sunday, December 21, 2008

RFI/TVI Baseline and Solutions

RFI/TVI baseline:

I had a few hours to myself yesterday, and spent some time collecting a set of TVI/RFI baseline data: what devices in my home are receiving signals from my station, on what frequencies, and at what severity. I should add that some of the audio/video devices in my house have had some RFI mitigation applied already - I did not remove those mitigation devices. Those mitigation devices are:

- I replaced the long audio speaker leads (zip cord) on our home entertainment system and my son's stereo system with some custom-made twisted-pair speaker leads (2 different color wires + 5 minutes with an electric drill = twisted pair). These speaker leads have a 1-inch-long ferrite core around which the pairs are wound:

The cores are some things I had in my junkbox, so they're made of unknown material. I think they probably came from Weird Stuff, so who knows what they're made of, but they seem to have helped. Before replacing the speaker leads, transmitted CW on 10 meters was clearly audible in the speakers of both our home entertainment system and the stereo in my son's room. Seems like a success.

- A Bencher low-pass filter just past the transceiver in my station. I had no real reason to suspect any sort of problem with harmonics, but the filter was $85 and that's cheap peace of mind for me.

The next day, I did a survey with the following parameters (values used for this test):
  • Input parameters:
    • Antenna (G5RVjr - closest antenna to home electronics equipment in our house)
    • Transmitter power (100w)
    • Mode (CW)
    • Devices:
      • Home Entertainment System:
        • Sony Trinitron 27" CRT-style television
        • Pioneer Receiver
        • Sony DVD player
        • TiVo Series 2 DVR
      • JVC CD/Tuner (in my son's room)
The RFI/TVI problems I have are:
  • 160M: not measured (I currently have no antenna that will load on 160)
  • 80M: no RFI/TVI observed
  • 40M:
    • FM tuner audio disrupted by sent CW
    • When watching DVDs, screen flashes
  • 20M: no RFI/TVI observed
  • 15M:
    • FM tuner audio disrupted by sent CW
  • 10M:
    • FM tuner audio disrupted by sent CW
After recording the baseline data in a spreadsheet, I tried the following:
  • Disconnected the external FM antenna from the receiver, and there was no longer any RFI (nor was there any signal from KQED!). The external FM antenna is a 4-element yagi about 2 feet below the feedpoint of the G5RVjr. Assuming this was some sort of a fundamental overload problem, I dug an old 300-ohm high-pass filter out of my junk box and put that between the 75-ohm transformer (the FM antenna is fed with RG6 coax) and the receiver. Problem solved!
  • Set up to duplicate the DVD interference, and discovered:
    • Disconnecting the DVD player from the TV only reduced the interference slightly
    • Disconnecting the RF input from our cable system made the TVI go away completely.
My best guess is that there's some sort of common-mode coupling coming in on the CATV feedline (it runs very close to the G5RVjr's feedline). So I'll try a common-mode choke once I get some ferrite parts big enough. A small snap-on choke didn't help matters.

So, at this point, unless there are some gremlins I haven't found yet, the only problem I'll have is if someone is watching a DVD and I'm transmitting on 40M with the G5RVjr. That's actually fairly unlikely, given that my droopy radial vertical works better on 40M anyway, and it's never caused any TVI I'm aware of.

So I'm pretty close to being able to declare my own house clean, which is a big confidence-builder for going QRV in contests.

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