I also measured the effect of ground radials when I loaded my 70 foot
tower (with 40 meter beam on top) on 160 meters. I used an HP 9753C
network analyzer and a non resonant loop about 6 feet in diameter
located some distance from the antenna. My radials were all about 125
feet long made from #14 solid copper PVC coated (from Home Depot) and
lying right on the ground. My ground it typical of NH. Hardly any
soil on top of ROCK!
Anyhow, I went in steps from 4 to 8 to 16 and finally 32
symmetrically spaced radials around the tower and measured the field
strength referenced to the 4 radials at each step just noted. My
results (as I recall-my notebook is not nearby) were within 1 dB of
what W8JI measured! From 16 to 32 was less than a 1.0 dB change so
there is where I stopped. The ground in the far field here is so poor
it's probably not worth going to more radials.
BTW, in my opinion measuring the impedance changes to check the
effective improvement is not always a reliable way to determine if
you have enough radials. I also checked impedance at each step in the
process and two things happen. First off, as radials were added,
especially in the lower numbers, the matched impedance not only
changed but moved frequency (from the prior match) so I had to
re-match the antenna at each step. This impedance was also measured
on the network analyzer. I am using shunt feeding so it's a little
bit more complicated than if the tower was insulated and isolated
from other appendages. Not enough real estate here for that.
I did a similar measurement of impedance on an 80 meter vertical
(much modified MFJ-1792) using series feed with 16 and then 32
radials. The changes were slight but nevertheless could easily be
seen with good portable measurement gear like the Autek VA-1 or MFJ
259B. When you have short or loaded verticals the losses can really
skyrocket with a poor ground system. Remember that full sized quarter
wavelength verticals are much less affected by ground system losses.
At 01:12 PM 3/16/2008, Jim Brown wrote:
>Certainly a circularly symmetrical radial system is best, but few of us are
>able to realize it. Mine deviates significantly from that ideal, but the
>antenna works well. The best and most readable description I've seen for the
>function of a radial system is by W2DU, as a chapter of his book
>"Reflections." It's on his website. Google on w2du to find it. N6LF has
>written a rather detailed version that's in the Antenna Book, but there are
>typos in the associated graphics that make it difficult to follow.
>The short answer is that the radial system functions as a return for the
>currents and the fields associated with the antenna. The lower the resistance
>of whatever conductors serve this function (including the earth), the closer
>the antenna comes to full theoretical efficiency. The radial system does NOT
>function as a reflector for the signal. The earth at significantly greater
>distance from the antenna serves that function.
>I have seen many references in the literature to the effect that NEC is not
>very good at modeling ground losses and the effect of radial systems. Count
>me among those who 1) consider real measurement of one radial system compared
>to a reference antenna as the only meaningful predictor; 2) consider soil
>conditions at any given location to be an influence on how many radials it
>takes to get there; and 3) consider Tom's advice, and the advice in the
>Antenna Book to be solid as a rock.
>Jim Brown K9YC
>On Sat, 15 Mar 2008 14:23:26 -0400, Robert Carroll wrote:
> >On the other hand if I were
> >only trying to provide a low loss ground return, I might route the radials
> >for convenience in avoid things like a garden, a pool, or some other object.
> >Which view is the correct one?
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