One of the reasons why the feedpoint impedance doesn't necessarily
correlate with the efficiency is that there isn't necessarily
any well defined location that represents a line of demarcation
between the "antenna" proper and the "counterpoise". You can
build a thin dipole, put it in free space, and insert a small
source in series at the center and expect to get 73 ohms without
much chance for trouble. You know exactly where the source is.
You can expect to get half of 73 ohms impedance for a vertical
only if it is over a perfect infinite ground. Otherwise,
the counterpoise is also an antenna of sorts. Where does the
vertical start and the counterpoise end? At your source? That
is an accident of where you chose to put the source. It can affect
drive impedance either by adding loss or by redistributing current.
It may even radiate to some extent. You can't tell how much
of each effect is in play, unless you measure field strength
at a distance. Also, in real situations, the coax may radiate
if you are not careful.
Rick N6RK
David Gilbert wrote:
>
> My first reaction is that 27 ohms in a real life situation with nearby
> trees and buildings is pretty close to a theoretical 36 ohms for a
> monopole in the clear over a perfect ground, and possibly within typical
> measurement accuracy limits anyway, but if you read the link below
> you'll see that feedpoint impedance is not an absolute indicator of
> performance anyway.
>
> http://lists.contesting.com/archives//html/Towertalk/200706/msg00210.html
>
> 73,
> Dave AB7E
>
>
>
>
>
> John Cowan wrote:
>>
>> I keep reading about this "perfect ground of 36 Ohms". I was trying to
>> figure out how having less than 36 Ohms is significant, insignificant,
>> or just an ambiguous reading.
>>
>>
>>
>>
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