Hi Charlie,
Noise is one reason for using chokes at the feedpoint. Another is
crosstalk between stations in a multi-op (or even SO2R) environment. In
our portable setups for CQP and 7QP, we're running K3/KPA500 stations
into very closely spaced antennas. We found that chokes at every
feedpoint make a very significant dent in minimizing inter-station
interference. They also prevent common mode on the feedline from filling
in pattern nulls. The chokes are in addition to W3NQN BPFs and double
chokes on 80M and 40M antennas to kill amplifier harmonics.
73, Jim K9YC
On 7/24/2020 5:50 PM, charlie carroll wrote:
So following up to last week's thread, a couple questions that will
surely get some traffic...
Do we really need common mode chokes on our transmitting antenna? The
station is located on an island in the Caribbean and the nearest AC
power is about 2 miles away at an industrial site generating their own
power. Commercial AC is probably another 2 miles beyond that. We
generate our own power with a couple commercial generators.
From a noise/interferer perspective, our biggest problem was the
on-island broadcast station that created horrendous fundamental overload
problems for our 160/80 receiving circle arrays. About 2 megawatts ERP
from that station was causing some real havoc until we added a filter
before each vertical's preamp and plenty of choking on the feed lines.
I'm not aware of any other noise issues affecting the station on any band.
With that said, assume that we decide to incorporate CM chokes on the
low-band transmitting antennas and the higher frequency yagis that do
double duty. Do we have chokes only at the antenna's feed point and/or
have others scattered along the coax feed line, including right at the
back of the amplifiers?
Thoughts, and any real data would be nice too!
73 charlie, k1xx
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