Hi Ken..mayby a little info to help you.
The pure impedence on a 1/4 vertical is about 37.2 ohms. Thus, if no
radials are used, you should get almost a 1:1 SWR. But guess where
that extra 13 ohms is coming from. Right, ground loss.
I would think you are at a practical level with the number of radials
although you did not mention their length. But, again the rule is a
larger number of short ones is better than a few long ones.
As to your increase in efficiency, the best way to measure it is to
put a RF ampmeter in line with the radials and the shield of the coax.
If you have such, and want to determine how more efficient the antenna
is, P=(I^2)R will give you the answer. Let's say we disconect all
radials and use the ground rod only in series with the ampmeter. And
let's say also that at a certain power level ( and you must keep this
constant ), the meter shows a current flow of 1 amp. From this
R=(I^2)/P. This is an excellent way to determine just what your ground
loss is. Now, let's reconnect 15 radials, keep the power the same, and
see what the current is. Let's say it goes to 2 amps. Now, that is a
doubling of the power consumed in the antenna!! Assume 100 watts in in
both cases, and in case 1, you have .351*100 loss or 35.1 watts,
leaving you with about 65 watts into the antenna. Now, case 2, I has
doubled, so our loss now is 17.6 watts or an increase in EFFICIENCY of
the radiating antenna of exactly 50 %, and about 82.6% of our power is
now going into the antenna.
Incidentally, sadly, but insugnificant, our 1:1 SWR is going to
increase as we approach 37 ohms with a perfect ground. In effect, if
you get the antenna impedence below 40 ohms or so, you are at a dead
end.
73
Ed
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