On 10/22/2012 5:44 AM, Tom W8JI wrote:
If we look at a popular balun book, to explain why a test dipole at
1/4 wave height did not benefit from a balun, the author cooked up a
very strange theory about cable diameter in wavelengths relating to
balance. The author neglected looking at the distance to the ground
point of the shield. Not looking at the system trips a lot of people up.
I agree. One thought as part of that system -- N6LF and others have
noted that currents in a relatively small number of elevated radials can
easily be unbalanced, and that unbalanced currents significantly
increase loss. Rudy also noted that common mode current on the coax and
an earth connection significantly contribute to unbalancing radial
currents. Bill's radials are pretty high, which I would expect to reduce
that sensitivity.
Also, I would be quite surprised if the common mode Vf of any coax was
much less than 0.95. After all, it's a large diameter copper conductor
covered with insulation.
Like others, I consider "coil of coax" chokes with no ferrite core to
make them resonant at the frequency(ies) of interest to be a bad idea
except for systems where the common mode Z of the feedline will be
strongly inductive at all operating frequencies. AND it should be
remembered that a primary reason for using a feedline choke is to
minimize noise pickup on the feedline, and for that function, the higher
the value of resistive impedance, the better.
So while I agree with Tom that a coil of coax would work on this
particular antenna with the ground stake, I would not recommend it as a
general solution.
73, Jim K9YC
73, Jim K9YC
_______________________________________________
Topband reflector - topband@contesting.com
|