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I am mounting a hex-beam antenna to the peak of my house (assuming that I can find someone with a cherry-picker or a scissor-lift that can help me get it up there). Right at that end of the house is an open vent into my attic, which is easily accessible. In addition to reinforcing the mounting location with 2x4s, I am wondering if I should run the coax from the antenna into the vent, and from there through the wall down into my basement. Back about 6 years ago, my first antenna was fed that way, but this was a G5RV with ladder line; this caused a lot of RF interference in other devices in the house, and I got a lot on receive also (for those who don't know, the feed-line for a G5RV antenna is ladder-line - window-line? - and is a radiating part of the antenna).

In the current case, routing coax inside the walls would be about half the coax length required if I were to route it around the house outside and then through the bulkhead fitting I have in my basement door. But can I expect inferior performance due to send/receive interference? Assuming no common-mode on the coax, is this a reasonable solution?

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    $\begingroup$ This isn't an answer to exactly what you're asking and I don't have relevant expertise, so I'll just leave a comment: you might want to make sure to include lightning protection in your plans. Depending on what turns out to be necessary for safety, you might find that running the coax the longer route is better. Or not — this is a guess. $\endgroup$
    – Kevin Reid AG6YO ♦
    Sep 22 at 0:08
  • $\begingroup$ Funny you should mention that; I had just been thinking about lightning protection. I have just gotten two gas tube discharge surge protectors, and I was wondering if this should be attached near the antenna with a wire directly to a ground rod, or near the radio and grounded to the common bus that I have set up. $\endgroup$
    – Don Levey
    Sep 22 at 15:21

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Three things that matter here:

  1. Lightning! You need to ground the coax as it enters the house.
    This is difficult if it comes in via a vent.

You should probably ground the antenna anyway, but you definitely want to take the coax outside, down to the ground, and ground the outer of the coax ususally with a flanged barrel connector, or a lightning arrestor. The primary thing here is to protect your house, and for that you don't want an ungrounded cable from high above the roof, coming indoors without being properly grounded first.

If you're in the US you probably have a coax cable for TV/internet, coming from the street, overhead. When it arrives at your house, it goes through a special grounding block, with a heavy gauge connection to ground, before entering the house. You want to do the same with the radio coax.

If the coax is not thick enough for the full lightning current, (RG58 and RG213 are too thin) then you should ideally run an appropriate earth wire in parallel with it (1 AWG Copper or 0 AWG Aluminium). This prevents it from becoming an incandescent fire starter when it's struck.

  1. Cable losses - at HF, with a low SWR antenna, the loss in the cable will be insignificant.

  2. Balun - To reduce RF in the house, caused by the coax (or ground lead) radiating, your beam should be fed with a balun. If not an actual transformer, then at least make an RF choke, winding the coax through an appropriate ferrite core. You could do this twice, once at the feedpoint and once a quarter-wave down the cable.

  3. Lightning arrestor
    Finally, if most of the lightning current has been diverted to earth,aA lighting arrestor may help protect your radio from large differential voltages on the cable. It needs to be properly grounded - install it at the entry point, where you connect the coax to ground.

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  • $\begingroup$ Thank you! I do have lightning arrestors, and my initial plan was to pit them at the point where they enter the house, and a (thick) wire down to a grounding rod. I just got a couple of rods and am waiting to pound them in. I have a couple of CMCs, so adding a choke at the antenna isn't a problem. $\endgroup$
    – Don Levey
    Sep 22 at 22:15
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In addition to Tom's excellent answer which includes suggestions for an RF choke at the antenna feed point, be sure to ALSO place an RF choke at the transmission line|radio interface.

The choke at the feed point helps keep your coax from transmitting RFI during TX and the choke at the transceiver helps keep your coax shield from delivering RFI to your ears during RX.

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  • $\begingroup$ Note that some radios already have a choke at the feedpoint, so depending on the radio, this might be redundant. Also, someone told me if you put two baluns on the coax and they happen to be a multiple of a half wavelength apart, the coax can become resonant and this causes its own problems. (However, I'm not sure about that.) $\endgroup$
    – user10489
    Sep 22 at 22:14
  • $\begingroup$ Notes on the choke at the radio; I'll have to check to see if my radio includes one (Yaesu FTdx-3000, in case that matters). $\endgroup$
    – Don Levey
    Sep 22 at 22:16
  • $\begingroup$ @user10489 important to note that not all chokes are baluns, and IMO shouldn't be used interchangeably. I'm not familiar with any radios that include an internal choke (though that doesn't mean they don't exist!), but I would certainly not call an additional choke at the radio redundant without understanding the local noise floor :-D $\endgroup$
    – webmarc
    Sep 25 at 18:03
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    $\begingroup$ I've opened up the radios and seen the chokes inside. Not all have them, but many (most?) do. Good point on the noise floor.... redundant chokes if done right probably can't make things worse. $\endgroup$
    – user10489
    Sep 26 at 4:10

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