Feedline isolation and grounding for temporary/portable setups

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3WD, Jun 11, 2021.

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  1. W3WD

    W3WD Ham Member QRZ Page

    Sorry to keep pestering you fine folks with basic questions, but I'm trying to do this right the first time, without damaging equipment or personnel.

    "Do this right" is an oxymoron because my setup is temporary/indoor-portable, which means I'll be plugging it into a wall outlet for a couple hours while operating and then tearing it down.

    Fair weather only, as I will not have "proper" NEC-compliant grounding or lightning protection as one would for a permanent setup.

    My plan was to lay the coax on the grass running directly choke to choke (see attached diagram) and operate-- and then I made the mistake of talking to one more ham, who again mentioned the proverbial "ground the coax before it enters the shack" and also made reference to static & static draining, which I hadn't heard about or considered.

    In humid summer weather without convective activity, rain, or excessive wind (or blowing desert sand!)-- is static accumulation on your vertical antenna and/or coax feedline a serious issue? For a temporary/portable setup? Enough to affect or damage your radio?

    I thought I was pretty well covered for common-mode with a good choke on each end-- so can I simply run coax between the chokes, fire this thing up and run it?

    What I understand I should NOT do is, for example, pound a separate (unbonded to house ground) rod into the earth and run the coax through a PolyPhaser or similar-- which should(?) take care of lightning and also lesser static/accumulating voltage. Tempting but apparently a no-no. Also not particularly relevant, as I'm not operating in bad weather.

    Essentially what I'm looking for is a few of you experienced operators to take a look at the picture below and-- with the understanding that this is temporary/indoor-portable and therefore NOT kosher as far as NEC-- tell me that on a bright sunny day, barring some strange scenario, I'm unlikely to have problems with RF, have problems with static accumulation, or get zapped or zap any equipment.

    Put another way: On a bright sunny day, for a couple of hours, would you operate on this setup?

    I don't mean reckless or rolling the dice-- I mean confidence that you'd taken steps to ensure a reasonable degree of safety and effectiveness given the temporary nature of the config.

    If not, please tell me what you need to see in order to be comfortable.

    Antenna is a military kit (fiberglass sections) and tuner is an AH-4.


    Attached Files:

  2. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    As long as the vertical is away from any power lines then I think it should be fine, essentially zero risk of static, lightning, etc, if you operate it temporarily and only in good weather, as you said. Have fun!

    If it becomes more than temporary, then it sounds like you know what you need to do. (Check out the sticky in this forum for a list of grounding resources if you need them.)
    W3WD likes this.
  3. WE4E

    WE4E Premium Subscriber QRZ Page

    All day long. Your lightning risk is way down in the noise floor.

    I would also figure out how to run a handful of radials. The ground stake just isn't going to cut it, really, and your coax will become your radial. Well, if you didn't choke it, that is. Run some radials and you won't need the chokes, and the antenna will work infinitely better. On ground, they don't have to be tuned. I have a portable setup very similar to what you show, and I use a dozen 33' wires, packaged in four bundles of three wires. Might take all of ten minutes to set up and doesn't take up as much space as it sounds like. Pix on my QRZ page.
    W3WD, AK5B and WA7ARK like this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    The efficiency of a 1/4wl Marconi monopole sitting on a ground stake (tuner or no, no radials) is terrible. More than 70% of your transmitter power is dissipated as heat in the earth immediately surrounding the stake, assuming a 3-4ft stake and average dirt. Another 5 to 10% is lost in the tuner. The feedpoint impedance of this is low and near resonant (jX small), so the tuner can be bypassed or is just loafing. This is a good reason for avoiding a 1/4wl vertical if you have a poor ground system; with the single short ground stake, it is very poor!

    The efficiency of a (just a bit shorter than) 1/2wl Marconi monopole sitting on a the same ground stake (tuner or no) is much higher. Less than 10% of your transmitter power is dissipated as heat in the earth immediately surrounding the stake. Since it is difficult to deal with a 60+ft vertical (for 40metres), you can put the wire into an inverted-L shape, and go up ~30ft, turn the corner, and go horizontal or even slope down for another ~30ft. The goal here is to start with wire length that is about 0.44 wl, (61ft at 7.15MHz) so that the antenna is close to, but just below its anti-resonant frequency. That makes the feedpoint impedance <1000 Ohms +j1200 Ohms, which the tuner will tune ok.

    Even better, if the wire is 61ft, the tuner will have an easy time of it on 80m, 60m, 40m, 30m and 20m... With a 100W transmitter, the losses in the ground stake will be 73, 47, 10, 50, 47W, respectively. The loss is lowest on 40m, and highest on 80m.

    It all depends which band you are going to spend most time on. That is the one where the wire length should be chosen to be ~0.44 wl.
    AK5B and W3WD like this.
  5. W3WD

    W3WD Ham Member QRZ Page

    Thanks all and much appreciated (and TZU for putting together that excellent grounding resources thread)-- I neglected to mention that I'm currently running 30 radials (18 AWG, insulated) cut at 5m length-- 6 bunches of 5. Will test it out and can certainly add more or adjust the length. I understand that more short radials are better than fewer longer ones.

    Also understand that I could elevate and tune the 4 radials (on fly fishing reels) that come with the kit and get decent performance that way too.

    I plan to run 1/4 wave for both 20m and 40m by adjusting the height of the antenna (adding/removing 4' sections), so 16' (-ish) for 20m and 32' (-ish) for 40m. Initial plan was to run the full height for both bands, but I believe folks suggested that 32' on 20m-- though 1/2 wave-- is non-resonant (or even anti-resonant) and would give the tuner a tough time (or even burn it up).

    Thanks for the insights!

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