On Wed,11/26/2014 1:50 PM, Wilson wrote:
As for balance, there’s a good bit of impedance in a wavelength or two of
coax shield, so I doubt if the shield end at the top is actually very near ground.
This is mentioned in the list of factors someone gave. No doubt, but
how much does it affect the pattern?
WL,
A good common mode choke prevents the feedline from being part of the
antenna. Does that matter much on transmit? NO, EXCEPT if it puts RF in
your shack. Does it matter on receive? YES! because the signal picked on
the feedline couples to the antenna and fills in the nulls in the
pattern. Goodbye front-to-back and front-to-side rejection. Hello NOISE.
I’m not competent to make analytical arguments about which choke is best, but
rather I maintain that using the ARRL choke and getting the Z of the choke close to
ten times the feedline Z is likely to be all anyone needs.
In the link I posted, I showed that RESISTIVE choking Z on the order
5,000 ohms (10X your "rule of thumb") is a far better design goal, both
for the decoupling concerns (TX and RX) and for power handling. I also
showed why chokes must be resistive, not inductive, which means that
they must be multi-turn chokes on #43 or #31 cores.
73, Jim K9YC
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