ground radials

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

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  1. KC2ZDE

    KC2ZDE Ham Member QRZ Page

    Gentlemen and Dear Ladies, I just bought a Comet CHA 250 vertical multi band HF antenna. It says no radials required. However, I've seen some reviews where a couple of guys added radials anyway. Also, I've seen a couple guys add grounding. Two questions, First, where and how exactly should I add radials to this antenna. Attached to the bracket that mounts the antenna to the Mast? And a ground, how would one go about setting something like that up? Thanks All, C.
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    How do you intend to mount and install it?
     
  3. W8JI

    W8JI Ham Member QRZ Page

    Here is how the Comet CHA 250 compared to a 1/4 wave vertical:

    http://g8jnj.webs.com/cometcha250b.htm


    It looks like, with 16 radials, the Comet is anywhere from 30 dB to around 3 dB weaker (depending on band) than a 1/4 wave vertical. CHA 250-1 is the factory CHA250 antenna. The -2 is something the author modified.

    [​IMG]


    I would install the ground system so later on when you decide to get a good antenna, you don't have to redo the ground system. How you install the radials depends on where you place the antenna, but the radials would connect to the mounting bracket.
     
  4. G0VQY

    G0VQY Ham Member QRZ Page

    These type of antennas are not meant to have radials and you won't gain anything by adding them, in fact you will probably degrade the performance, not that the performance is much good in the first place. I think you will realise within a day or two that this antenna is not going to be a pile up "Buster"
     
  5. NN4RH

    NN4RH Premium Subscriber QRZ Page

    Looks to me from the product detail download that it is a single radiating element. So of course it needs some kind of ground or counterpoise. Also product literature calls it a "ground plane" antenna, and the installation instructions say to mount it at least 35 feet off the ground for "maximum performance".

    My guess is that they're relying on the coax feedline shield to supply the "ground". Which means significant common mode currents on the feedline.
     
  6. M3KXZ

    M3KXZ Ham Member QRZ Page

    Assuming the antenna matching unit has some degree of efficiency matching the low impedance, then putting a short antenna like that, when used on 160 or 80, over an extensive radial field is going to make the antenna perform worse, is it not, as the radials become the dominant radiators?

    Fair enough to put a full size 1/4 wave over a decent radial field, but to then compare this with a 7m vertical on bands where 7m is less than 1/20 or 1/10 of a wavelength is all wrong. The gain at low angles of elevation is seriously reduced by using big radials, and it's at low angles where G8JNJ measured the field strength.

    Furthermore, JNJ didn't make a proper comparison with 1/4 wave verticals for 160 and 80..... he "calculated" the relative gain figures based on results of a 10m tall vertical, but gives no details of how these are calculated.
     
    Last edited: May 25, 2011
  7. W8JI

    W8JI Ham Member QRZ Page

    No. I'm confused why anyone would think that.



    That's not true at all either. Gain at low angles is largely unaffected by radials. All they will do is increase efficiency....unless the coax is the antenna instead of the little thing hanging on the end of the coax we call the antenna. :)

    If I go out and remove all my radials from my 200 foot tower, the fields strength at all angles will decrease. If I go out and remove the radials from my 30 foot base loaded 160 meter vertical, the field strength at all angles will decrease even MORE than it did on the taller tower!!!

    The radials never should radiate. They only provide something for the antenna to "push against", like a counterpoise.

    Unless the feedline is radiating, adding radials can only help. The shorter the antenna, the more important radials become. A very short vertical in terms of wavelength requires MORE radials and closer to full size radials than a very tall radiator would.

    Of course if the system is very poor and depends on the coax shield to act as an antenna, any radials or anything that prevents the feedline from radiating will hurt the signal.

    I would expect the Comet antenna, based on what it is dissected, to follow the general trend the G8 station indicated. It should be horrible on 160 gradually moving toward poor on 40 and fair on 17 meters and getting worse again higher.

    Almost anything larger would be better, although the Comet is certainly better than having no antenna at all.

    73 Tom
     
    Last edited: May 26, 2011
  8. M3KXZ

    M3KXZ Ham Member QRZ Page

    Tom, you're right - equal and opposite radials don't radiate. But have a go at modelling a 7m tall antenna, close to ground, over a set of 4m long radials, and then over a set of 20m long radials. Ignore matching for a minute - just assume it can be matched ok - and see which one has greatest gain at low elevation angles on 80m and 150m. I mean low angles - 0 to 10 degrees say.

    For a ground mounted 160m vertical, what would you propose as being full size radials in a typical small garden that's maybe only 9m across?
     
    Last edited: May 26, 2011
  9. W8JI

    W8JI Ham Member QRZ Page

    It has long been known that low angle radiation is formed in the Fresnel region which extends out from 1/2 wave to a few wavelengths from the antenna. Efficiency (and gain) at low angles is progressively determined more and more by distances extending from close to progressively further out from the antenna as lower and lower angles are considered.

    The ground and ground system close to the antenna primarily determines overall efficiency, and to a very minor extent pattern at elevation higher angles.

    As we move further out, the ground (earth) progressively affects lower and lower angles and less and less overall efficiency.

    To improve very low angles, we have to change the ground from the antenna area out to several wavelengths. The height of the radiator, once less than around 3/8th wave tall, has very little to do with any of these effects. The height, when less than 1/4-3/8th wave high, mostly affects efficiency at all angles equally. The shorter and lower loss the antenna and matching system, the more important the radials become.

    That is just how it works.

    This has been known since 1930 or so when RCA did a study of vertical radiators and ground systems. Look up Brown, Lewis, and Epstein and the RCA paper "Ground Systems as a Factor in Antenna Systems Efficiency".

    The Fresnel region interaction started sometime before Christ, I believe (not sure) by someone in what is now India. but was finalized by a fellow around 1827 by a fellow working with very short electromagnetic waves (light).

    http://en.wikipedia.org/wiki/Augustin-Jean_Fresnel
    73 Tom
     
    Last edited: May 26, 2011
  10. M3KXZ

    M3KXZ Ham Member QRZ Page

    Tom, I know about the Fresnel region and that it's been there since Before Christ, so that's fine :) What I was hoping you could clarify is why, when modelling a 7m tall vertical on 0.25 wavelength radials on 160m, the low angle gain is 6 or 7 dB LOWER than the same 7m tall vertical over radials just 4m (0.025 wavelength) long?
     
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