Inverted L - feed-point distance above gnd?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K7ZRZ, Apr 3, 2010.

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  1. K7ZRZ

    K7ZRZ Ham Member QRZ Page

    I've been erecting and using inverted L - random-length wire antennas for some time now. I move monthly from one State Park here in Oregon to another, serving as volunteer campground host in my retirement.

    One aspect of the antenna that I can't seem to find any specifications about is the physical distance of the anchor point (feed-point) of the antenna with respect to ground - the physical dirt used to hold the antenna wire down.

    A friend of mine (who is also about to erect his own version of an inverted L) suggested that there might be interactions and feed-point impedance variances with differing feed-point distances above the earth.

    I have been using a big plastic tent stake, pounded about all the way in, and wrapping/anchoring the down-wire (feed-point end) around the stake - making the wire virtually touch the ground. I'm using insulated wire, so there is no electrical contact with the wet ground. My feed-line transformer - a 4:1 longwire/vertical UNUN - is lying on the ground right next to the stake and ground rod/radial plate.

    So, is there any advisement to raise that whole attachment/feed-point off the ground - for impedance issues?

    The antennas always seem to perform well, considering the nature of the beasts and my transient nature. But I'd like to have an educated opinion about it all. Thanks much.
     
  2. M0JMO

    M0JMO Ham Member QRZ Page

    I've always fed mine at, or within a foot of ground level.
     
  3. K4SAV

    K4SAV Ham Member QRZ Page

    Feeding the inverted L higher up on the vertical part will change the feedpoint impedance. The impedance could either go up or down depending on the length of the L. An antenna a little shorter than a quarter wavelength will go up in impedance, and an antenna a little longer than quarter wavelength will go down in impedance, as the feedpoint is moved above the radials. That trend may not be true for a very long antenna.

    A feed point above the point where the radials are attached will cause the wire from that point to the radials to be not at ground potential and a choke should be used on the feedline to keep the common mode currents off the feedline.

    Jerry, K4SAV

    Correction - That trend is not totally true. It depends on exactly how long the antenna is above a quarter wavelength compared to exactly how far you move above the radials. You could easily move above the point of maximum current on the antenna with only a small distance change, if the antenna is just a small amount longer than a quarter wavelength.
     
    Last edited: Apr 3, 2010
  4. K7ZRZ

    K7ZRZ Ham Member QRZ Page

    Thanks, Jerry, and also to Josh for the comment. The antenna TOTAL wire length (including the little bit past the anchor point that connects to the feedline transformer) is 190 feet. I didn't plan it that way. It's just how much wire it took to go up to the first tree-branch attachment point and then over to the far tree that I slung the string into for the wire far end.

    I'll have to try to think through some of what you said I do have, as mentioned that feedline transformer, which as I understand, also serves as a feedline choke. Also I have 14 ferrite clamp-on blocks around the coax, all lined up right at the end of the coax as it goes into the UNUN, plus I have a toroidal feedline choke here in the shack. I don't have a watt of stray RF - fortunately.

    Grounding is a bit of a tricky wicket here in the motorhome, but the grounding of all the equipment here at the station to the chassis and frame of the motorhome - plus the ground rod and radial field seems to have avoided that situation.

    Thanks for the comments. I do have an antenna analyzer, and I ought to get it connected to see what I have, impedance-wise. Me being able to load the antenna with my tuner, and being heard where I want to be heard is what's important to me. The rest is just a curiosity, I guess.
     
    Last edited: Apr 3, 2010
  5. K8JD

    K8JD Ham Member QRZ Page

    The only time I used an inv-L antenna was at a campsite locationj on 40M, had a quarterwave long piece of wire and a weight to throw it up onto a branch as high as I could manage and then swung the pull string over another branch to take up the slack. It was half vertical and about half horizontal.
    I had another few lengths of wire that were originally going to be the other half of a dipole but nowhere to run them to. They became my ground counterpoise.
    When I fired up the TS520 that evening A few of my contacts were in Europe ! Not bad for a picnic table open air shack, bent wire vertical , no UN-UN or tuner, 100 W and no miles of radials buried in the ground !:D
     
  6. K7ZRZ

    K7ZRZ Ham Member QRZ Page

    JD,

    For putting up an antenna in a hurry, I think there's nothing easier than the "L." I have a webpage that I put together about how I have discovered the most effective means for me to do so. http://bingoldsby.com/AntennaTutorial/

    I think I have come to my senses over my original curiosity of the feed point proximity to ground... in that the feed-point of both the radiator and the radial/ground system emanate from the same point - and that there would be little difference in raising the feedline transformer (UNUN) off the ground, as the radials would then also be raised... but keeping the relative position of radiator/radials (essentially, ground level) exactly the same. (Hope that makes sense to someone else).

    A bigger problem has been the structural integrity of the antenna as the wind blows and the trees sway. Because I need the antenna to stay up for a month or two at a time, I often have to deal with adverse weather conditions. I have found 200 lb. "woved" (braided) kite line, which is definately strong enough to hold the wire up. Now I need to experiment with stronger wire. (Had the #18 annealed/tinned wire I was using yesterday break with the wind out here on the Oregon coast.) I will be ordering some copper clad steel stranded coated wire in #18 shortly and giving that a try.

    It's all part of the game.
     
  7. AI3V

    AI3V Ham Member QRZ Page

    Or, you can continue to guess, and add things at random...:p

    Rege
     
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