DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
  s=s1024; d=btopenworld.com;
  h=DKIM-Signature:X-Yahoo-Newman-Property:X-YMail-OSG:X-Yahoo-SMTP:Received:Message-ID:From:To:References:In-Reply-To:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE;
  b=r2pGl8ODgOzYjdvqoVkaMyNyP6PwOjEbfT6eQp0wjDf4ucdb5/cWuM/76Ob1iOBKUeenGsxJ1SkIQJT4OEZW3THKNTIr7IyqSUWaFgEk2IMw6mIJognjU9kDO6/U3eSMmBumvQhRswLdeNR3/ASlZPsyig7NZ0UESAwxNnwqrjA=  ;
Message-ID: <3E9653200305422E8F12BCF9447F314F@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <500EB973.8080005@iup.uni-heidelberg.de>
In-Reply-To: <500EB973.8080005@iup.uni-heidelberg.de>
Date: Wed, 25 Jul 2012 03:28:16 +0100
MIME-Version: 1.0
Subject: LF: Re: Pretests for the 630m band dipole
Content-Type: text/plain;
	format=flowed;
	charset="ISO-8859-15";
	reply-type=response
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
X-AOL-SCOLL-AUTHENTICATION: mail_rly_antispam_dkim-m218.1 ; domain : btopenworld.com DKIM : fail  
x-aol-sid: 3039ac1d4119500f5a137e88
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


Dear Stefan, LF Group,

...> there will be a real component of
> Z=R (=resonance)of 46 Ohm. So i would not even need a transformer to match 
> the antenna to the PA.

...However, if you wish the antenna to operate in a "dipole" mode, you will 
need some way of making sure that the currents into the 2 terminals of the 
feed point are equal - otherwise, you will have 2 asymmetrical inverted-L 
vertical antennas back-to-back! You could achieve nearly equal currents by 
ensuring all the system, TX, power supply, etc., was "floating" and 
completely insulated from ground, but the resulting unpredictable RF voltage 
between the equipment and the actual ground might be a problem. So a 
well-insulated transformer would probably be a necessity, even if not 
required for matching.

> 85% means that one wire is "just" 47m. So if someone lives close to a 
> forest, there would be a chance to build such a TX antenna and run the 
> major part of the antenna outside the garden. Just a short rest has to be 
> inside the own property. For a 90% feed, R would be 96 Ohm. This could be 
> matched to 50 Ohm very easily and the shorter part of the antenna would 
> only be 31.5m...

I am sceptical about how this will actually work in practice. The short 
section of the asymmetrical dipole on its own will no doubt behave as a 
normal inverted-L antenna, and will have of the order of a kilohm of 
capacitive reactance, measured between its terminal and a ground connection. 
In order to end up with a resistive overal feed point impedance, the longer 
section will have to have a corresponding large inductive reactance. These 
reactances will be quite large compared to the resistive components - 
intuitively, I have my doubts about whether a long piece of wire trailing 
through the forest will behave as a fairly high-Q inductor... The concept 
must fail at some point, e.g. what happens when you have only 1m of wire in 
the short section of the dipole? Having used NEC antenna simulators in the 
past, I am not at all sure that they handle the effect of a lossy ground 
very accurately.

Cheers, Jim Moritz
73 de M0BMU