There are two distinct and different issues here. Take a look at how the
ARRL Antenna Book tackles them quite separately.
1. Antenna efficiency: directly affected by the ground conductivity in
the Reactive Near Field. This is the parameter we are usually improving
by laying down a good ground radial system
2. Elevation angle pattern: determined by soil characteristics in the
Far Field where ground reflections are taking place. Do some simple
geometry and you'll see that at really low elevation angles - 5 degrees
say - the ground reflections from an 80m quarter-wave vertical will
extend out to at least 750ft from the base of the vertical. A typical
ground radial system will do nothing about the conductivity there.
You can check this using EZNECs ability to have two different ground
media. Set up Media 1 as salt water extending for about 50ft around the
vertical, and Media 2 as rocky soil beyond. Look at the elevation
patterns - they will be those which pertain to poor ground. Now
gradually extend Media 1 outwards until you get the elevation pattern to
match that over salt water - that will tell you how far out the
reflections are taking place and how extensive your radial system would
need to be.
Please note - I'm not saying a good ground system doesn't help. I'm
saying that the usual ground radial system predominantly affects antenna
efficiency and not elevation pattern.
Please also note that nothing I have said conflicts with the Exam
Question answers.
73,
Steve G3TXQ
On 05/01/2011 21:44, Richards wrote:
> Er... ah... No joke. Do you doubt the proposition that a
> good ground plane lowers radiation take off angle? If so,
> I would be curious as to your reasons. (Seriously, and with
> no intention to flame or cause an argument, as I have
> made quite an investment in time and resources in vertical
> developing a reasonably decent vertical antenna system
> for my small, suburban back yard. Any info would be
> greatly appreciated. )
>
> FYI ---
>
> ------ Extra Class Exam Questions of interest -----
>
> Question E9A12 - and the answer is that the efficiency
> of a quarter wave grounded vertical antenna can be improved by
> installing a good radial system.
>
> Question E9A13 - answer is - soil conductivity is the most important
> factor in determining ground losses for a ground-mounted vertical
> antenna operating in the 3-30 MHz range.
>
> Question E9C13 - answer is - When a vertically polarized antenna
> is mounted over seawater versus rocky ground, the far-field
> elevation pattern low-angle radiation increases.
>
> Question E9C17 - answer - The main effect of placing a vertical
> antenna over an imperfect ground is that it reduces low-angle
> radiation.
>
> Also, on the audio study guide, Gordo makes some stray comments
> about using 3 inch copper strap for radials on his roof, and
> mentions improving the ground field increases low angle
> radiation - he goes on to say it does not increase signal strength,
> of course, but only that it increases the amount of signal that
> has low take off angle.
>
> This information is consistent with all that I have read on verticals
> in the ARRL Antenna Handbook, and I did LOTS of research before
> installing a large vertical monopole in the back yard.
>
> A huge ground pane does lower take off angle (as NEC modeling
> shows) and also improves antenna efficiency -- I stopped at 65 radials
> but I wish had installed even more just to be sure. Also, my back yard
> soil is very conductive and remains moist even through the summer, so
> the soil, itself, helps me considerably. Rob Sherwood and I exchanged
> some nice email at the time I was doing this homework, and I believe
> he lives over a more dry, rocky soil, and that is much harder to work
> over. I also corresponded with the infamous Rudy Severns N6LF
> and his findings are consistent with this conclusion. NEC modeling
> produces consistent results.
>
> N'est ce pas? Happy trails OM.
>
> =============== JHR ============================
>
>
>
> I had
>
> On 1/5/2011 1:01 PM, Steve Hunt wrote:
>> I presume the "wink" indicates that is a joke !
>>
>> The only thing that would improve the elevation pattern would be
>> improved ground conditions in the Fresnel Zone where the ground
>> reflections are taking place. That would take some awfully long radials
>> - certainly well beyond my property boundary - and an awful lot of them
>> to achieve a copper density that would affect the conductivity so far out.
>>
>> 73,
>> Steve G3TXQ
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