[TowerTalk] Quad questions

[Martin Sole]

I'm not familiar with a 'line isolator' but on hearing the term I would
surmise it to be equivalent to a common mode choke. Certainly a common mode
choke at the feed point will provide the required protection against braid
outer current and I understand how that works, I think. So if each element
has a choke balun to prevent any current flowing on the braid outer then
none should appear on the parallel braid of the connected feeds going to the
other elements. On this basis I am happy that connecting the braids at the
switchbox should not have a detrimental effect on the operation of the
antenna.

If I can find a source of #43 rings I have some RG142 which I think should
be okay. Is there a good bead alternative to a #43 ring that will go over
either RG142 or RG213? 

Martin, HS0ZED


-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Brown
Sent: 24 May 2010 22:56
To: TowerTalk at contesting.com
Subject: Re: [TowerTalk] Quad questions

On Mon, 24 May 2010 04:10:10 -0700, Michael Baker wrote:

>One sure way to break any common connection to the other elements is
>to use a line isolator/ 1:1 in line with each elements feed line. There is
>NO common ground connection at that point. You have a balanced antenna
>element to start with and an unbalanced feed line so putting a balun in
each
>line would make sense and with the feed impedance being about 100 ohms a
>matching transformer would be prudent and would save the matching problem
>unless you used a 1/4 wave 75 Ohm stub on each feed element. You would
still
>have an unbalanced stub to a balanced load issue. Interesting conundrum.

The connundrum here is that words and phrases are being tossed around
without 
an understanding of their meaning. 

At least one product sold as a ine isolator is nothing more than a common 
mode choke, also known as a current balun. It is childs play to wind an 
excellent common mode choke for 15-30MHz using nothing more than coax and
one 
or more #43 toroids. Guidelines for doing so are in my tutorial. Such a
choke 
looks like an open circuit to a common mode voltage and the very small loss 
of the coax for signal going through the isolator. The impedance of the
choke 
is the impedance of the coax. 

73,

Jim Brown K9YC 



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