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RF in the shack, but only on the lower freqs

Discussion in 'General Technical Questions and Answers' started by K2ENF, Oct 23, 2021.

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  1. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    End-fed antennas have an electronic complexity which is belied by their apparent mechanical simplicity. It is true that an intelligently designed and implemented end-fed antenna will perform comparably to a conventional dipole at the same height; however, to use an end-fed antenna effectively with 50 ohm equipment actually requires a much broader understanding of basic antenna principles than using a center-fed resonant dipole with the same rig, in spite of the dipole's greater mechanical complexity.

    If you want to eliminate the "RF in the shack" problem quickly, consider a vertical of some kind for DX operations, or a low horizontal dipole for local/regional ones. And if you can get a 40m dipole 60 ft. high or so, or an 80m 120 ft. high or so, you will have an outstanding signal on those bands for both DX and local/regional work! But, alas, relatively few can get a dipole that high without building a tower; and if you're going to build a tower, you might as well top the whole thing off with a beam, at least on 40m...
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Random length end-fed wires are nothing but a poor cousin to a vertical or inverted-L antenna which requires an extensive ground-plane made of several horizontal wires in the form of radials that are on the order of the same length as the vertical radiator. The sum of the radial length and the radiator length has to be about 0.5wl at the lowest frequency where used, which would be ~130ft on 80m.

    The OP posted a description of a wire that is 42 ft without radials, and is complaining that he has RF in the shack on 80/40m. Note that 42 ft is small fraction of the required 130ft!

    A half-wave dipole at 80m is ~130ft long. If your center-feed it without choking the coax (1:1balun or line-isolator), depending on coax length and station grounding (or lack of it), you can still get RF in the shack. Only if the coax is choked using ferrite, and the if the dipole wire is at least 1/2wl from the transmitter, are you very unlikely to get RF in the shack.

    With an End Fed Half-Wave dipole (also about 130ft of wire) fed through the proper ratio transformer may put RF in the shack, but it is not too likely, just as with the CF dipole. It depends on coax length and station grounding (or lack of it). The solution is the same as with the CF dipole, but in this case, the ferrite coax choke has to placed about 10 to 20ft down the coax, not right next to the transformer.

    A vertical on radials does not automatically eliminate RF in the shack. With elevated radials, the coax leading away from the vertical/radial junction can act as the "longest" radial, diverting current away from the real radials, and exporting RF into the shack, depending on coax length and station grounding (or lack of it). The solution, as before, is to place a ferrite coax common mode choke on the coax. It is most effective to place that choke about as far from the vertical radiator as is the longest radial.
     
    KE0EYJ likes this.
  3. 2E0CIT

    2E0CIT Ham Member QRZ Page

  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    You would do well to abandon the idea that coiling coax can make a useful coax choking system for a multi-band antenna.
     
    KE0EYJ likes this.
  5. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    Yeah, I didn't see that antenna length. That is a problem, and causing your feedline to radiate in a big way. A purpose-built choke might help, but it would be far better just to go with a longer wire (if at all possible).

    I like my antennas like I like my women. I'm sort of a monoband nut. I can only handle things simple, and one at a time. I tend to think that way.
     
  6. K2ENF

    K2ENF XML Subscriber QRZ Page

    Well, if I was still actually running that antenna, you'd have a point, which is why I moved off the non-resonant to the EFHW. Trouble is, as I say, that didn't help.
     
  7. K2ENF

    K2ENF XML Subscriber QRZ Page

    Ha!
    Well, I was curious to see what kind of performance I'd get out of it. I was actually a little surprised... The design goal was to get the standing wave close enough for the internal tuner to bring it into line.... and it did manage to do that, and worked actually fairly well, considering, particularly on say 20-6. But lower than that, the SWR was happy, but oh, boy what a mess of RF in the shack.

    So, I moved to the EFHW, and was annoyed to find it didn't change the situation much, though the standing wave was better on all bands.
     
  8. K2ENF

    K2ENF XML Subscriber QRZ Page

    Well, that's the question now, isn't it? Still working on that part. I've got a few more toroids on order.
     

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