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I have a commercial UHF/VHF Yagi designed for TV reception, also used for 2 meter reception, that has been installed for years but seems lately there's increasing amounts of signal disruption.

I have a grounding block on the antenna coax near entrance to the structure as recommended. The ground wire for the block runs to the service panel, where it is connected to a ground point for the structure. I estimate this ground wire run is at least 30 feet. The mast is also grounded by a separate ground wire going to same service panel.

Is this grounding block with the long ground wire affecting my signal? I suppose all sorts of noise is on the structure ground system along with the 30 feet of wire acting as another antenna. My understanding is my antenna has a built-in balun, but I don't know the specifics of what kind of balun this is (if it matters). Do I need to worry about common mode currents on the outer shield of the coax, or does it not matter because of the balun? If I do need to worry, could a ferrite bead on the coax at some point help? If yes, what would the recommended placement be in relation to antenna feedpoint and the grounding block?

This is for the Winegard HD7694P antenna with a preamp installed on the mast. (Yes, it occurs to me the preamp could also be going bad, but that's not my question here.)

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Is this grounding block with the long ground wire affecting my signal? I suppose all sorts of noise is on the structure ground system along with the 30 feet of wire acting as another antenna.

It is possible, yes, but not more significant than, say, the length of cable running to the power input of your radio, which is also a noise pickup path.

(When you hear about the importance of short ground wires, that is about antennas that need “counterpoise” connections — that use the earth or a piece of metal as part of the antenna system. Your Yagi antenna does not, so the routing of ground wires is only relevant to common-mode noise pickup.)

Do I need to worry about common mode currents on the outer shield of the coax, or does it not matter because of the balun?

The balun helps with the proper operation of the antenna, and you certainly wouldn't want to remove it, but it is irrelevant to how your receiver reacts to common-mode noise picked up on the feed line between the balun and the receiver.

could a ferrite bead on the coax at some point help?

Yes, it might. Note that winding multiple turns of the coax through a suitable ferrite toroid has a much larger effect than a ferrite bead which only accepts a single wire (a “half turn” of coil). It's just like building an inductor or transformer, except that it's an intentionally lossy inductor whose core absorbs the unwanted RF.

The only reason to use a ferrite bead is if you need very little effect or if coiling the line would be infeasibly bulky.

If yes, what would the recommended placement be in relation to antenna feedpoint and the grounding block?

It needs to be located between the noise source and the receiver. So the best placement depends on where the noise is coming from. Personally, if I were throwing ferrite at the problem, I would try one or both of two placements:

  • near the receiver's input
  • on the other end of the same piece of coax, just before it enters the grounding block

There's no reason other than cost and complexity not to install both and leave them — extra chokes won't hurt your reception.

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WB4IVG Laurin here. Ham and RF Engineer of 50 odd years. As for your ground block, very definitely it can and will cause Interference if it becomes impregnated with moisture, especially when it has both RF and DC current running through it. In fact once it presents a substantial short it will also destroy the coaxial cable especially if it is foam type. Just for good measure I would clean all connections and coat them with nolox or similar substance even Vaseline would be better than nothing. As cheap as they are I would totally replace the ground block lightning 🌩 arrestor. That is what the ground blockbas you call it really is. Infact once the lightning 🌩 acts it should be replaced and BTW when one has fired and stopped a lightning 🌩 surge it will often make the exact thing you describe as Interference. 73

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  • $\begingroup$ Very interesting. It occurred to me that the preamp has gone bad, but it didn't occur to me to consider the lightning arrestor since it's passive. As soon as I read your comment about a possible short, I measured the resistance at the end of my coax, between shield and center conductor, and it's reading 670 ohms. I don't know if that's a normal reading with the preeamp mounted at the mast? I will take your advice about replacing the arrestor and protecting the connectors from moisture the next time I can get up on the roof. I have boots on the connectors, but not nolox. $\endgroup$
    – acker9
    Oct 10, 2023 at 23:01

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