[TowerTalk] another common mode choke question

[Jim Brown]

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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