Now re-run EZNEC with the dipole at 25ft and at 55ft over average
ground, and compare the losses reported by EZNEC with your figures;
you'll see something very different!
Orr's "logic" is way too simplistic!
Steve G3TXQ
On 03/08/2013 20:34, k6jek wrote:
This is the quote I've wondered about. It's from Bob Orr, W6SAI's, Antenna
Handbook.
"Antenna radiation resistance is influenced by antenna height. Assume the antenna is an
80-meter dipole. A common height for this antenna is about 25 feet, which is approximately 0.1
wavelength. Theory shows that the radiation resistance for this example should be about 20
ohms. Actual antenna measurements above "average ground" often reveal a radiation
resistance of about 45 to 55 ohms -- a nice match for a coax line!
The extra 25 to 35 ohms is a result of ground loss, and is equivalent to placing a
resistor of this value in series with the antenna feedpoint. In this instance over
half the transmitter power is dissipated in ground loss."
If you follow this logic and model a 75M dipole at various heights over both
perfect and average ground, you come up with some interesting numbers.
25' Z perfect: 19.8; Z average: 45.8. Loss 57% (just as Orr says)
35' Z perfect: 35.5 Z average: 53.5; Loss 34%
45' Z perfect: 51.5 Z average: 63.8; Loss 19%
55' Z perfect: 67.2 Z average: 73.4; Loss 8%
Modeled with EZNEC 5.0. I ignored reactance which seems to be what Bob Orr did.
If you believe this logic it makes a good case for height. But is it right?
Jon
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