Message-ID: <20090907122350.2jyo85atjk8w8ggg@webmail6.kuleuven.be>
Date: Mon, 07 Sep 2009 12:23:50 +0200
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
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	<4AA4D150.9050402@sighthound.demon.co.uk>
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Chris, John,

indeed, in theory radiation resistance is proportional to the square =20
of the height (for a short vertical antenna).
So "jacking up" the antenna even a few meter is worth the efford.
As a bonus the heigher antenna has a larger capacitance (although this =20
effect is less if you have good topload) and often (but not always) a =20
lower loss resistance (cfr the footprint theory of G3AQC).
I am in the onfortunate situtation when the loss resistance increases =20
with antenna height, because the antenna (topload) is over an open =20
grass area (30 by 45m) surrounded by forrest. As the antenna height =20
increases the footprint increases and it then includes more forrested =20
area. This results in a larger loss resistance. But even in this =20
situation the benefit of a larger radiation resistance overrules the =20
increased loss:
- antenna height 6m =3D 35 Ohm loss
- antenna height 11m =3D 45 Ohm loss (=3D +29%, but Rrad =3D +336%)

73, Rik  ON7YD - OR7T

Quoting John P-G <gm4slv@sighthound.demon.co.uk>:

> Chris wrote:
>> ....and how does 'Jacking up' the antenna increase the ERP?
>> Chris, G4AYT.
>
> Chris,LF,
>
>   Presumably by increasing the height one increases the capacity and the
> the radiation resistance and thus the ERP will rise for a given TX power
> and antenna current? One probably also reduces the loss resistance too,
> due to needing less loading L, so the current might also increase,
> leading to yet more ERP for a fixed TX power.
>
> John
> GM4SLV
>
>
>