On Wed,2/3/2016 10:10 AM, Herbert Schoenbohm wrote:
It has come to my attention by a seasoned topbander that I have this
all wrong and the decoupling coil should be close as possible to the
Beverage transformer. He reasoned that the long RG6 cable (some 500
feet long) couple random pickup right back into the Beverage
transformer and could also reduce the side lobe rejection from the
Beverage itself. So where is the best placement for for a feed-line
choke?
He's right -- if you're using only one feedline choke, it should be as
close as practical to the feedpoint (that is, out by the transformer, or
up in the air for a TX antenna). There is also benefit to one or more
additional chokes on a feedline. Two benefits. First, it minimizes
interaction between that feedline and other antennas, fulfilling the
same function as egg insulators breaking up guy wires.
Second, it minimizes feedline current, which, due to imperfections in
the coax shield, can convert noise picked up in the shield to a
differential signal on the coax. The metric for this conversion is
called the transfer impedance of the cable, and is defined as the
differential voltage divided by the shield current. EMC guru Henry Ott
notes that the lower limit for transfer impedance is the resistance of
the shield at the frequency of interest. The uniformity and the
"coverage" of the shield also contributes.
Coax made for CATV is optimized for its performance at VHF-UHF. Shields
are typically thin Al braid plus one or more layers of Al foil. These
are pretty good shields at VHF/UHF, but their relatively high resistance
can seriously degrade shielding at MF and HF.
73, Jim K9YC
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