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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