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I0OJJ > ANTENN 01.07.10 18:03l 38 Lines 1515 Bytes #-5797 (0) @ WW
BID : 24638I0OJJ
Read: GUEST
Subj: Re: language and antenna gain
Path: DB0FHN<DB0MRW<DK0WUE<IK6ZDE<I4UKI<I0OJJ
Sent: 100701/1517z @:I0OJJ.ILAZ.ITA.EU #:6092 $:24638i0ojj
On Jul 1, 8:16=A0am, "-.-. --.-" <a...@a.com> wrote:
> if a half wave end-fed *monopole* antenna have the following primary
> characteristics (if IIRC):
>
> - High Z at the feedpoint (voltage maximum and current node);
> - very small counterpoise lenght compared to the resonant wavelenght of t=
he
> antenna (typical 0.1-0.2 lambda)
>
> can i mantain the same characteristics shortening the antenna in any way =
??
With proper design you can maintain the resonance characteristic with
that short antenna, but not its characteristics of SWR bandwidth,
exact radiation pattern (directivity), or radiation resistance.
For a single frequency the only important difference will be radiation
resistance, unless the short radiator is used with a virtually perfect
(zero loss) ground plane.
A typical r-f ground loss even in a set of 120 each, 1/4-wave-long
buried radials is on the order of two ohms. So referencing the
example in the link to Kraus that I posted earlier, the radiation
efficiency of that helically-loaded monopole system with a two ohm r-f
ground would be about 0.6/2.6 =3D 23%, approximately.
The loss of a radial system using 0.1-0.2 lambda conductors would be
significantly higher, so the antenna system radiation efficiency then
would be significantly less than 23%.
A naturally-resonant, unloaded monopole about 1/4-wave high has a
radiation resistance of around 34 ohms. When it is used with a two
ohm r-f ground, the radiation efficiency of the antenna system is
about 34/36 =3D 94%.
RF
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