To illustrate Ken's point, here are some measurements I made on the
feedpoint resistance of my 160m 40ft top-loaded vertical as I added
radials. Rrad for this antenna should be about 5 Ohms.
6ft Ground rod only: 33 Ohms, Efficiency = 15%, VSWR = 1.5
Ground rod + 5 radials: 13 Ohms, Efficiency = 38.5%, VSWR = 3.8
Ground rod + 10 radials: 9 Ohms, Efficiency = 55.5%, VSWR = 5.5
Ground rod + 15 radials: 7 Ohms, Efficiency = 71.5%, VSWR = 7.1
I ended up with a 9:1 impedance transformer at the feedpoint to give me
a reasonable match to 50 Ohms. With just the ground rod I would have had
a reasonable match without any transformer, but the efficiency would
have been 15% (-8.2dB).
Estimated ground loss resistance for the 6ft Ground rod on its own was
28 Ohms.
73,
Steve G3TXQ
On 03/12/2010 07:01, Ken Brown wrote:
A quarter wavelength monopole with a really good counterpoise/ ground
radial system has a feed point radiation resistance of around 37 ohms
and very little loss resistance. That will give you an SWR of about
1.4:1. If you use just a ground rod and have about 13 ohms of resistive
loss in the ground system, the series equivalent feed point impedance
will be about 50 ohms, and the SWR will be very near 1:1. Some people
think this is better. It does "tend to make the matching easier."
I don't know what kind of ground you have, or whether your ground system
resistance would be 13 ohms with just a ground rod. The point I am
trying to make is that a really efficient vertical is likely to have a
higher SWR than a partially radiating dummy load. Many hams do choose a
lo
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