ground radials

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2ZDE, May 24, 2011.

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: L-rfparts
ad: Subscribe
ad: l-gcopper
  1. G7DIE

    G7DIE Ham Member QRZ Page

    You're in a good location in Brighton, you should be knocking them for six, that said, timing is very important.

    All the best

    Stephen
     
  2. W8JI

    W8JI Ham Member QRZ Page

    Fewer radials will concentrate the fields in the fictitious media that represents earth below the wire. This is especially true with close spacing between earth and radials and limited segments.

    NEC and modeling is extremely good if we allow for everything that should be included, but there can be large errors when we get a conductor very close to earth.

    Nearly all errors are due to operator errors, not the program itself. I would rely 100% on a proper model if it was more than .1 wave above earth.

    When I have compared antennas to models, they have been virtually perfect. But I don't rely on things being excessively earth dependent.
     
  3. KD0CAC

    KD0CAC Ham Member QRZ Page

    Tom ,
    Not that I know as much as I would like about the subject .
    What occurred to thinking about the number of radials and spacing off ground , was like building a capacitor , the analogy to me seems like designing a variable air cap , like in a tune / amp ?
    Would some of the rules interchange between radials for a vertical & variable air cap , or some overlap ?
    John
     
  4. W8JI

    W8JI Ham Member QRZ Page

    There are multiple things going on with radials John, so it is difficult to have one broad general statement that covers every effect.


    In shortest form though:

    1.) A radial or counterpoise system gives an end-fed antenna something to "push against". Without a low RF impedance ground, it is like trying to stand on marbles and push a car. The antenna base resistance is the thing we are pushing into, so the lower the base impedance the lower the counterpoise impedance has to be to prevent common mode on the feedline. Even a 1/2 wave vertical requires a counterpoise of some reasonable type, and shorter verticals require better grounds because they have lower resonant base impedances. People often screw this up in models because they use perfect lossless ground independent sources at the antenna, instead of modeling a real system with coax and grounded equipment. Cebik almost never included feedlines and realistic sources, but then nearly all people do that. Very few include feedlines. When we make that error, we can design antennas that work nothing like the real world...but this is a modeling error and not a fault of the program.

    2.) Radials "shield" the earth from the strong induction and radiation fields near the radiating element. They ideally should distribute fields evenly over a very large area, so E^2/R and I^2 R losses are lowest. The more concentrated the fields, the greater the loss becomes for a given soil.

    3.) Radials can also radiate significantly when there are less than eight or ten radials effective on each band. Even a four-radial groundplane has significant radiation from the radials, and if a coax shield is connected to the groundplane (or a tower or mast) the whole business radiates as a unit.


    About the only thing radials do not do is affect radiation angle....unless they are radiating like antennas. If they are radiating like antennas because we have too few of them, then all sorts of weird things can happen.

    What we do not want is depend on coupling to the lossy dirt, unless we absolutely have no other option. The very extensive RCA study back around the 1930's proved the fallacies of sparse single wire resonant counterpoise type grounds commonly used before that time.

    73 Tom
     
  5. KD0CAC

    KD0CAC Ham Member QRZ Page

    Thanks again .
    I got a little more understanding , I was aware of the antenna working/pushing against concept .
    It was the spacing of elements , radials from ground that I thought about the relationship to capacitors , being mostly self-taught [ learning from others & my own experience ] I rely on analogies a lot , like using water / plumbing to look at electrical AC / DC .
    Now trying to get my older mind to rap around RF .
    73
     
  6. W0BTU

    W0BTU Ham Member QRZ Page

    Great explanation, Tom! :)

    I have always understood that radials have a field around them which extends out and away for a finite distance. However, I didn't think four (or even two) radials radiated (that is, significantly contributed to the pattern from the vertical portion of a ground plane) IF they are perfectly balanced.

    If balanced radials radiated, wouldn't that show up as high-angle radiation in a model?

    Maybe if I added a feedline to my ground plane model, I would see the light.
     
    Last edited: May 29, 2011
  7. G7DIE

    G7DIE Ham Member QRZ Page

    If I treat every day as a learning opportunity, I should learn a great deal in a lifetime, somehow I don't think a lifetime is enough in order to learn that which people give for free, Tom I really appreciate you taking the time to put in simple terms some of the principles at work with regard to antennas, ground, and feedlines.

    Thanks
     
  8. W7RUE

    W7RUE Ham Member QRZ Page

    I have owned a CHA-250B Comet vertical for just over two years now. At the time I was a new ham (and still am for that matter!). I purchased it because I live in a no-antenna CC&R neighborhood and wanted to play HF without being bothered by the neighborhood Yard Nazis.

    I have it up on a 35' pole and camoflaged next the greenbelt so it is not noticable from the road. I was very disappointed because I was only getting local and weak contacts so I ran about 250' feet of wire radials of various lenghts under it with a connection to the base mounting U-bolt. The manufacturer states in the literature that radials will only hurt it's ablity to work effectively. I didn't seem to make any difference either way. Since then, I have moved on to more effective antennas and I use it only occationally as a listening antenna and/or for local low power contacts.

    Rue - W7Rue
     
Thread Status:
Not open for further replies.

Share This Page