>> 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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