The RFI “Sleuth”

In the last year I made two changes to my antenna farm that introduced RFI issues. Not RFI in the shack, but RFI that I am causing that:

  1. Only on 160M I all of a sudden cause my Trane Heater blower to turn on and off for short periods – just enough to blow cold air through the house and make fan noise
  2. Turn my garage opener on and open the garage door opener when on 20M

Must be Common Mode RFI – because I clearly remember Jim, K9YC discussing this:

At first I thought it was the antennas – that they were too close to the house, but that doesn’t wash because I have had similar antennas on these bands before (in the same locations) – and was running 1500 watts with my ACOM 1500.

There is one thing I have done that must be causing this – I used to have a Palstar AT2K tuner in the shack and I never had these problems. This leads me to believe that my problem is caused by my coax radiating RF because there is no choking in the shack. There is choking on the antennas, which I thought would have prevented this, and I don’t have this issue on my DX Engineering DV-40-P 40M phased array. But that antenna is the farthest from the house.

SO, the best question to ask so you can try to inform a hypothesis:

“What has changed that could be causing this?”

The only thing has been to move the tuner from the shack out to the base of the antenna, and I sold my ACOM 1500 and replaced it with the SPE Expert 1.3K FA.

I have used my home brewed Field Strength meter in the shack looking to see if maybe the SPE amp isn’t as well shielded as the ACOM was, but when I use my KX-Line running 50 watts in JT8, I still turn on the Heater on 160M. The “drop off” away from the amp and any radiation is literally an inch or so – so its not a “leaky” amplifier.

I will set the KX line on during the day and walk along the feedline and see where its radiating. I am guessing that the near field of the coax – near the point of entry where the coax leaves the house and goes to my remote Array Solutions Rat Pack switch does go right next to the wiring that goes to my HVAC Air Conditioner condenser. I will measure, but I do think I will move the coax away from that line. That would be the easiest solution, but it doesn’t solve the 20M issue. The 20M yagi is as low as when I had a Moxon in the same location, but I am 100% sure that moving the yagi and getting it from 27′ right over the garage to almost 50′ – where my N6BT DXU-32 used to be – will solve that problem easily. That RF field will be over the back yard – and I will have no antennas over the house at all.

I also had the KPA-500 and KAT-500 between having the ACOM 1500 + AT2K combo, and before the SPE Amplifier. I had no problems then as well. There would have been no choke in the shack. That would then only leave the tuner at the base of the 160M antenna.

I am betting on using a tuner in the shack where it has a choke at the input – and that having the choke there AND at the antennas prevented excessive feedline radiation. Here is a very good paper on answering the question – tuner in the shack or at the base of the antenna:

So, there is no real serious reason to have the tuner at the base of the antenna, and if it solves the Common Mode RFI problem, then I’ll be a genius – hi hi

I have to admit – I miss using a tuner in the shack – while having the tuner at the base is supposedly a lower loss solution, I do miss the convenience of just having knobs. I do NOT want an auto tuner because I have one in the SPE – but compared to the KAT-500, its range sucks. It really wasn’t worth the extra $$$$

I’ll follow this post up in a couple of days when I have the time to test all of this out.

And I am also going to move and raise the 20M yagi so its no longer over the garage. That one is very easy to solve.

In essence, I will be going back to the way I had things before all of this crap started happening!

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