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RF in the shack?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1QWI, Nov 7, 2020.

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  1. N1QWI

    N1QWI Ham Member QRZ Page

    For my set up, I use a long wire end-fed antenna in an inverted L configuration. This antenna has a 9:1 UnUn at the feedpoint which is about 10 feet off the ground. The Apex is about 30 feet higher and then the other leg runs across my yard for a total wire length of 88 feet. This is the HFEdz antenna.

    Approximately 150 feet of coaxial cable runs to the radio from the antenna and at the point where the coaxial cable meets the ground from the lowest part of the inverted L antenna, I have attached the shield to a ground rod driven 10 feet into the ground.

    This multi wave antenna tunes very well on 80 through 10 m with my Yaesu FT 450 and the MAT-30 tuner.

    yet, I recently did an experiment with a cheap “EMF hunter“ made by OSUN and noted that there are electromagnetic fields detected from my microphone and Morse code key. When I hook up a dummy load, these fields are not there.

    I am wondering if I am getting RF in the shack that should not be there? Interestingly, the “detector“ does not pick it up farther from where I generally have the Morse code key or microphone but it is very clearly around the area where I put my hand when I hold either object.

    should I be concerned? Maybe I need more grounding? I have not yet hooked up a ground rod for my equipment and this is next on the list. Of course I will have to bonded to the ground that the coacts is hooked up to at the antenna.

    Thanks,
    Daniel N1QWI
     
    Last edited: Nov 7, 2020
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    A single ground rod is likely to have an effective "resistance" to earth of 30 to several hundred Ohms. This means that the current below the feedpoint divides at the earth connection (10ft below the feedpoint), some of it flowing into the earth, but most of it will continue along the coax shield toward the rig...

    It is not surprising that a "EM-field" detector would respond if close to the rig or coax. We would have to know more about the detector works and how it is calibrated to determine what that indication means... Frankly, I don't think that instrument is useful except to feed paranoia...

    Your type of antenna is notorious for bringing RF into the shack which causes audible interference with computer speakers, computer crashes, random outputs from mice and keyboards, distorted transmitted audio, RF-caused burns and tingles if you touch parts of the equipment while keyed up, etc. If any of these things are happening, then you have something that needs fixing. Placing a coax-choke (aka common-mode choke, line isolator, 1:1 current-mode unun) in-series with the coax close to the transmitter end is what it usually takes to mitigate those problems. Adding a proper RF ground under the antenna feedpoint would also solve those problems...

    Your antenna is basically a Marconi vertical with a missing ground-plane (the single ground rod is inadequate as an RF ground). The coax shield between the ground rod and the transmitter becomes the missing "radials". This is unlike EF, OCF or CF Hertzian dipoles which are self-contained, and unlike a Marconi, work independent of a ground connection provided that the coax is adequately choked close to the antenna feedpoint.
     
    WB5YUZ, KM4FVI, K0UO and 3 others like this.
  3. K0OKS

    K0OKS Ham Member QRZ Page

    Is it not better to put the choke around where the coax comes in the house, rather than at the transmitter? The common mode current is coming in using the coax as a radiator in the house, no?
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Yes. If you have an earth-grounded entry panel at the shack wall, it is toss up if the choke should be on the rig side or on the antenna side. Since you can build a choke for $10 worth of parts, try both.... :)
     
    2E0CIT, KA0HCP and N1QWI like this.
  5. SP9HZX

    SP9HZX Ham Member QRZ Page

    The chokes (ugly baluns) very close to antennas and ferrite rings on cables in shack should help
     
  6. N1IPU

    N1IPU Ham Member QRZ Page

    I just hung a EFHW. I am using a choke 3' back from the balun consisting of eight turns of RG58 through an FT240-43 ferrite toroid. Build one and make sure its a short distance from the balun. That antenna is notorious for sensing RF back even with a ground. No accounting for soil conditions so the ground isn't always going to save you.
     
  7. WA0CBW

    WA0CBW Ham Member QRZ Page

    Although it probably will not change the RF in the shack the NEC requires ALL external ground rods to be bonded (connected) to the main electrical service ground rod(s).
    Bill
     
    K0UO and N1QWI like this.
  8. KK9W

    KK9W Ham Member QRZ Page

    You do NOT want a ground rod for antenna systems & lightning protection connected to your electrical panel for obvious reasons.

    There is a different NEC code section for tower grounding than residential electrical.
     
    K0UO likes this.
  9. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Regardless of whether you have a ground near the antenna feedpoint or soil contact point of the coax, you still should have a unun/choke/isolator at the shack entrance point (not at the radio) for best Common Mode rejection. Keep the CM out of the shack.

    And yes, best practice is to bond antenna grounds to the house ground. (Let the NEC wars begin!).
     
    AK5B, K0UO and WA0CBW like this.
  10. WA0CBW

    WA0CBW Ham Member QRZ Page

    Sorry, I missed that there was a tower involved.
    Bill
     
    KA0HCP likes this.

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