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I’m All Choked Up!

23 Sep

Ah finally, a few moments to sit at the radio and see who is about on the 40M band. I sit down and start tuning only to find an old friend at 7022 calling CQ. How cool is this? I switch over to transmit and touch up things on my tuner a bit and get ready to call my old buddy. That is the point at which my plan to relax falls into disarray. No sooner had I tweaked my tuning and settled under my earphones when, much to my chagrin, my XYL sniped about me trashing her favorite TV show with TVI. Next my kid shouts to me to stop  messing with his telephone call. I knew something was awry because my computer speakers let out a loud howl about the same time all hell was breaking loose in the rest of the house. Perhaps I take literary license here a bit in exaggerating the number of RFI complaints for one evening, but some or all of the mentioned complaints have happened to us all at one time or another.

Sound familiar?

It sounded all too familiar to me and has repeated itself any number of times throughout my Amateur career.  In this case nothing had changed and nothing was wrong-technically. Some hams generally operate during times when the chance of interference is lower. Or they resort to only chasing DX as the transmit duty cycle is lower than almost any other kind of activity using Ham Radio. Anyone wanting to participate in contesting, rag chewing or operate in prime TV time needs to address this problem.

Several years ago I had a similar issue. My transmitting antenna was close to my transceiver (about 8 feet over top of it!) and the possibility of removing it to a remote spot was limited at best. I would get into everything in my shack with RFI. It was horrible. And since I am the type of Ham that tends to not operate when I cant show a clean environment within my own home to any prospective neighbor complaining about RFI, I didn’t get to operate much. Needless to say if you can show no issues in your own home first, it goes a long way toward explaining who exactly owns the problem. In any event, that never became a problem for me once I solved my whole house RFI issues. One simple fix did the entire house. Once I implemented transmission line chokes I only had one strange problem that was solved by removing a wire that was not necessary from my home theater.

Many years ago I recall seeing a diagram in the ARRL Antenna Handbook that showed tower guy lines with insulators inserted into them. This was intended to keep guy lines from being resonant or getting excited by the antennas field and skewing the antennas pattern. This same concept applies to feedlines.  Feedlines operating in the near (or reactive field) of the antenna carry RF Currents along the line and back into your shack. This is called Common Mode and on coaxial cables RF currents travel along the outside of the shield. On balanced lines the RF current is carries by in phase currents on both lines.  Wouldn’t it be wonderful to be able to insert insulators into your feedline to stop that RF from coming into your home and making the speakers squawk? You can do this easily and inexpensively. The reference to W1HIS whitepaper is on my Blog along the right margin. I had to do nothing more than add five common mode chokes along my feedline and all my RFI problems were fixed. This with an antenna just feet above my transceiver. Oh Joy! What a happy day in Ham Radio that was for me.

A couple of caveats:

  • Always make your chokes at least 1K ohms. If in doubt follow Chuck’s (W1HIS) models in his whitepaper.
  • Choke at multiple points along your feedline. I have been spacing chokes at 12 foot intervals along the transmission line.

An interesting aside. Chokes are bilateral devices. That is to say they attenuate RF coming from your antenna while transmitting (and getting into computer speakers for example) and they attenuate noise from your appliances (while you are in receive) from propagating up to your antenna from your power lines and then converting into transmission line mode and entering your receiver as additional noise that you would otherwise have to contend with. You get a double advantage for a single cost.

For a very long time I didn’t really know what was causing my constant RFI issues. I had some glimmer of an idea that it was from my antenna. I had no way to measure what was causing my RFI problems in the shack and the rest of my home. I certainly wish I had discovered these devices a lot earlier in my Ham days. I would have had a lot more dark hair today and more QSO’s in my logbook!

Today my antenna is no longer 8 feet above my shack and is no longer a vertical. It is horizontal and lives up at 27 feet directly above my shack. However, I can also run 500 watts with impunity and no TVI or RFI anywhere in my home.

There are other ways to cure or reduce received noise. However, this post is directed at transmit RFI.

Thanks for reading my Blog. Best, Chas W5PG

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Posted by on September 23, 2009 in Radio

 

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