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Re: Topband: RF choke woes??

To: "John Tait" <bravo@iol.ie>,"Jimmy Burrell" <jimmy@burrellshome.com>, <topband@contesting.com>
Subject: Re: Topband: RF choke woes??
From: "Tom Rauch" <w8ji@contesting.com>
Reply-to: Tom Rauch <w8ji@contesting.com>
Date: Wed, 9 May 2007 08:43:40 -0400
List-post: <mailto:topband@contesting.com>
>> Just built a wire vertical dipole (Lazy H style) 
>> yesterday for 80m
>> (very similar to the one shown on EI7BA's web site 
>> here ).   Since I
>> am feeding the antenna asymeterically I knew an RF Choke 
>> would be in
>> order.  EI7BA recommends an FT240-61 with 12 turns of 
>> RG-58.  Not
>> having the recommend ferrite at hand, I proceeded to 
>> homebrew a choke
>> out of a large diameter (~7") plastic coffee can and 20 
>> turns of
>> closely spaced RG-58 well secured.


Baluns and chokes are a big complex issue. The problem is 
the uncontrolled environment. A feedline exiting a HV point 
or paralleling an antenna conductor is a worse case 
situation.

To test for sensitivity you can add a source set to zero 
current  (or a load set to very high impedance) in an EZNEC 
model. Place it on a wire between ground and the antenna 
feedline connection point to estimate the common mode 
excitation. Move it to each side of the feedpoint and you 
know how much voltage and voltage phase unbalance you have. 
I'd bet it is a pretty big number with the vertical lazy H.

http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

Also, the common mode impedance of the feeder in combination 
with the choke impedance you add is critical. Certain chokes 
can INCREASE the problem. The most system-critical choke 
would be a very low loss choke, one that is mostly a pure 
reactance. I'd stay away from low loss materials unless you 
know the common mode characteristics of the system at the 
choke insertion point and understand how the choke 
interacts. There is an intense electric field to deal with 
also. A choke could actually increase problems.

Let me give an example where models go wrong. There have 
been two articles on how a Zepp feedline does not radiate 
because models show the currents are pretty well balanced 
and the pattern isn't too distorted. One article was from a 
west coast amateur who marketed modeling programs. The other 
is on a web page of someone who models hundreds of antennas. 
The flaw is both authors assumed the "thing" we connect to 
the feedline is an ideal infinite common mode impedance 
current source. That's an impossible-to-build device. Worse 
yet, when we look at the electric field around the feeder 
the electric field is very intense. We have severe nearfield 
problems with RF coupling even though the currents are 
perfectly balanced in the model, and the whole system falls 
apart.

It's very easy to sit in a comfortable chair and make all 
sorts of antennas that appear to work very well in models 
that are very difficult or impossible to tame in the real 
world. The cookbooks or models should include real world 
feed systems as well as a plot of nearfield field 
intensities. After all, the feedline interface is very often 
the thing that requires all the thought, and it generally is 
given no real-world thought at all!

Remember....balanced currents and minimal pattern distortion 
does NOT mean the feed system is easily repeatable in the 
real world. It doesn't even mean the feedline is working 
properly in the model.

73 Tom








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