RF on computer chassis with portable end-fed

Discussion in 'Mobile Radio Systems' started by KJ7WT, Nov 25, 2017.

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  1. KJ7WT

    KJ7WT Subscriber QRZ Page

    We recently made a trip to visit family in Oregon, and I brought my IC-7300, LDG Z11 tuner, and an end-fed 31' wire and 9:1 unun with a 16 ft single radial. I had tried this combo at my home QTH and it seemed to work OK (i.e. it tuned, I heard and worked other stations) but had unpleasant experiences when trying to use it in Portland and later on the Oregon coast. I had my laptop connected for FT-8, and when transmitting, even with lower power, the computer would glitch horribly (40m was the worst), with windows popping up randomly, not responding to the trackpad, etc, and at one point, I touched the metal chassis and got a very unpleasant RF shock. I tried a multi-turn coax "balun" near the 9:1 unun feedpoint in the hope it would reduce the effects, and that did seem to help, but did not eliminate the problem completely. I suspect that I really need a purpose-built balun to choke the feedline RF, but I thought I'd ask here to see if there is something else I could do. The radio was powered from my van battery, but had no direct connection to the van sheet metal other then through the battery "-" wire, and the laptop was running from the internal battery, both were sitting on the van bench seats.
  2. AI3V

    AI3V Ham Member QRZ Page

    My suggestion would be to stop using half of an antenna.

    Build yourself a simple 468/f(MHz) Hertz antenna, that's a center fed half wave dipole in modern terms.

    I would also suggest you type the following into your search engine:

    Hertz antenna
    Marconi antenna

    Note both if them are fed in the center, your antenna should be too.

    AK5B likes this.
  3. KJ7WT

    KJ7WT Subscriber QRZ Page

    Well... If I had the room and the hardware and the height to put up a 1/2 wave 40 meter antenna, I'd do that, but this is a situation where I don't have those advantages. I know that others have used shorter antennas with the 9:1 unun with good success, so I'm hoping one of them will have some more relevant suggestions for me. 73
  4. K0BG

    K0BG Ham Member QRZ Page

    Here is something to remember. RF must return to its source. It will do so over the path of least resistance. In the case of the end fed wire, that is on the OUTSIDE of the coax shield. This current flow is termed COMMON MODE.

    While it is true the antenna comes with a 9:1 balun, unfortunately it is a VOLTAGE balun, not a CURRENT balun, and has virtually no common mode rejection.

    The question remains, why did it work while you were home? Simple answer is, you were lucky! The same scenario applies to any unbalanced antenna, especially off-center fed ones. If they don't have appreciable common mode, it is more luck than anything else.

    What Rege told you is dead on, but even in the case of a balanced antenna, you can still have different currents flowing in each leg (primarily due to capacitive coupling to the surroundings). In this case, a 1:1 CURRENT balun is always a good bet.

    And remember this too. If it sounds to good to be true, it isn't true.
    AK5B and KA0HCP like this.
  5. AI3V

    AI3V Ham Member QRZ Page

    You were 3/4 of the length of a 40m halfwave. Your 31 ft wire and 16 ft radial are 47 feet long, a real 40m halfwave is 66 feet.

    Get yourself another 19 feet of wire and try again, I think you will be surprised, especially if you can get the antenna a wavelength or so away from your rig.

    And as high above the ground as you can, even a dozen feet will be mucho más better than what you have.

    AK5B likes this.
  6. KL7KN

    KL7KN Ham Member QRZ Page

    This may be of some help


    for the CW sub-bands

    from the monograph - http://udel.edu/~mm/ham/randomWire/

    I use a 42 foot random wire. But I also use QRP power levels and purpose built equipment (NEU-PSK) - hope this is of some help.
  7. KJ7WT

    KJ7WT Subscriber QRZ Page

    Oh, wow! I am right in the "don't go there" zone! Yikes! OK, antenna length mod coming up when we get home!
    Thanks for that info!
  8. KL7KN

    KL7KN Ham Member QRZ Page

    David, ham radio is all about learning. The linked article is a real winner for data on these kinds of antennas.

    Let me know if this made a difference.
  9. AI3V

    AI3V Ham Member QRZ Page

    Your not getting it.

    There is no such thing as an end fed antenna.

    All antennas are fed in the middle

    If you do not provide both halves of an antenna, the radio will "try" and find the other half somewhere else.

    In your case the somewhere else is your computer.

    You have deliberately built a antenna that is not center fed.

    Fact a 31 ft wire is very close to 1/2 of a 40m antenna

    Fact it does not need a 9:1 thing, whoever sold you that for a Marconi antenna ripped you off.

    Fact, add another quarter wavelength wire to the other terminal of your transmitter and you will have a whole antenna.

    A simple, 1/2 wavelength Hertz antenna, fed directly with coax will fix your problem.

    Also, do not confuse feeding a antenna in it's center with balanced and unbalanced antennas.

    Both balanced (Hertz) and unbalanced (Marconi) are center fed.

    If you really must $pend money, I suggest some antenna books. :)

    Good luck

  10. AI3V

    AI3V Ham Member QRZ Page

    So you not only don't use a resonant antenna, you also only use half of an antenna.

    That's why you have to use qrp.

    Also a 33 ft wire has a LOW impedance on 40m, whoever published that chart has no clue about antennas.

    AK5B likes this.

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