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. K2ENF

    K2ENF XML Subscriber QRZ Page

    Hello, all.
    Until recently, I was running a non-resonant antenna about 42 feet long. I noticed with it that I was getting a pretty fair amount of RF back into the sheck to the point where it was really messing with my computers.

    On the idea that the problem was the actual mis-match on he antenna, feeding back to the internal tuner on my 991, I put an 80m end fed up. The match, as you might expect is much better to the point where at tuner isn't even required. But, I' still getting RF back into the shack, particularly on 40, 60, and 80m.

    (yeah, I guess I'd kind of expect some feedback on 60.

    I've put a torroid donut on the line feeding the end fed, with about 10 turns of coax. It helped the upper bands...(10-17) but it doesn't seem to have touched the rf on the lower bands.

    Obviously, I'm missing something. Ideas are appreciated.

    K2ENF
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    How is the end-fed antenna installed? As in, how close is it to your "radio room" and the rest of the house?

    Sometimes RFI as you describe isn't conducted, but radiated; if you're sitting in the antenna near field, which on lower frequencies is hundreds of feet, there's lots of RF floating around every time you transmit.

    But if the problem is really common mode conduction both the design and the placement of a current choke on the coax are important. Using small coax like RG-58C/U and large ferrite cores like FT240-43 it might take "more turns" and even "more cores" to raise the choking impedance high enough to be of benefit.

    Sometimes a "line isolator" which performs dual duty as a current choke and an earth grounding point helps. If so, usually that would be installed at ground level just outside the hamshack with the isolator bonded very closely to an earth ground rod. Some of those have enough ferrite in them to work at 80m, or even down to 160m (takes a lot of ferrite).

    But if the antenna is close to the shack and the problem is radiated and not conducted, then all that often won't help.
     
    N1VAU likes this.
  3. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    2E0CIT likes this.
  4. N1VAU

    N1VAU XML Subscriber QRZ Page

    Balun Designs manufacturers 1:1 chokes with the specifications published for each band. Pick the one that offers the greatest choking impedance on the bands your having trouble with, if your problem is truly common mode.

    I have found the balun designs chokes work much better than the MFJ version.
     
    Last edited: Oct 24, 2021
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    If you run a 42ft wire as an antenna on 80m, there will be huge amounts of CM current on the coax shield because every molecule of metal that the rig, coax, antenna are connected to become part of the radiating antenna. That CM current is looking for a way to flow into earth.

    Once it gets to the chassis of your transmitter, it will flow out the AC line cord, into the wall socket, along the in-wall house wiring to the electric utility grounding system and ultimately to earth. It will also follow audio or USB cables from the transmitter to your computer and out the computer's AC line cord to the AC house wiring, and likely crash the computer in the process.

    The root cause of your problem is your piss poor antenna!
     
    N0TZU likes this.
  6. K8JD

    K8JD Ham Member QRZ Page

    Non Resonant and End Fed Troubles
    I tried an end fed (Zepp) antenna design and it was TERRIBLE for RFI in the shack! :eek:
    I had to re do the whole thing to a centerfed and EDZ length, and then NO PROBLEMS.
    I do not trust End fed antenna designs for RFI free performance now.
     
    KP4SX likes this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Unlike randon length end-fed wires fed through a 50:450 Ohm transformer, EFHW antennas are just fine if made half-wave resonant on their lowest band (67ft on 40m) with a 50:2450 Ohm transformer and a ferrite coax choke on their coax about 0.1wl from the transformer. Lots of folks build these without the choke and report no RF in the shack. The propensity for RF in the shack is a function of a bad coax length and station grounding.

    That might well be the solution to the OP's problem, except that an EFHW for 80m needs to be ~130ft long. The main reason he is having trouble is because he is using a 42ft wire on 80m!
     
    W4NNF likes this.
  8. K8JD

    K8JD Ham Member QRZ Page

    I newbies went back to the old reliable centerfed dipoles we would not be reading all the complaints of RFI :D
     
  9. N3QAM

    N3QAM XML Subscriber QRZ Page

    I run two end feds, one at 129 ft 80-10 and another roughly at 260 feet 160-10.
    With 1:1 chokes where the coax enters the house , the only RFI I get is from the powered subwoofers in my media room in the basement when running 300watts or more on 10 meters. And that’s only because the sub amps are not running a 3 wire power wire and the amps are built into the enclosures. 2, 10” subs stacked in a closet.
    A couple toroids on the power and audio leads will cure that.

    I don’t bother because it adds to the stations operating effects by shaking the windows and walls with my modulation.
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    ...but only if they add the required coax choke (line isolator) to their center-fed dipoles. Otherwise, we get to hear about RF in their computers or stereo systems..., or we get to hear about how much RFI they are receiving...;)
     

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