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Message-ID: <C6C2DA0F57874523A1E0929328C3DD7E@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <500EB973.8080005@iup.uni-heidelberg.de> <3E9653200305422E8F12BCF9447F314F@JimPC> <500FEC24.9060600@iup.uni-heidelberg.de>
In-Reply-To: <500FEC24.9060600@iup.uni-heidelberg.de>
Date: Wed, 25 Jul 2012 23:51:25 +0100
MIME-Version: 1.0
Subject: Re: LF: Re: Pretests for the 630m band dipole
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Dear Stefan, LF Group,

> Well i want that the antenna works as good as possible. If is imagine the 
> transmitter and battery and all the TX stuff as very compact and floating, 
> then does the position of the feed point make a difference to the 
> radiation pattern? For example, i simply use the MFJ analyzer as the TX, 
> floating in the air. Then i think there should be no difference in the 
> currents. Or am i wrong?

You are not wrong... but the difficulty is that, although the voltage 
between the two terminals of the antenna feed point will be quite low 
(assuming the antenna works as a resonant dipole), the voltage and impedance 
between the feed point and ground will be rather high. This means impedance 
measurement is likely to be affected by the operator touching the controls, 
etc., and if you run a significant amount of TX power into the antenna, 
handling the TX would become quite uncomfortable!

>> I am sceptical about how this will actually work in practice.
> Why does EZNEC not show "problems" in such an easy antenna configuration?
>
I don't know for certain, but I suspect NEC does not handle a "real ground" 
very accurately in regard to its effect on feed-point impedance - in 
situations like this, where the presence of a poorly conducting ground can 
be expected to have a major influence on antenna performance, NEC may give 
misleading results.

Cheers, Jim Moritz
73 de M0BMU