Steve,
Where are you getting losses out of EZNEC? I did re-run the the dipole at 25
and 55. As expected, all sorts of things are different. But where are you
reading losses?
Jon
On Aug 3, 2013, at 12:58 PM, Steve Hunt wrote:
> 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
>>
>>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|