In the case of the YCCC 9-circle array, which I designed, common mode
suppression is deployed at both the antenna ends of the feedlines and at the
combiner where the antenna feedlines come together. The YCCC feedpoint preamp
already includes common mode suppression, so additional feedline chokes should
not be necessary there.
The reason for additional common mode suppression at the combiner is as
follows. The 9-circle system is different than other receiving arrays in that
there is a center element which is usually collocated with the phase combiner
because that is a convenient place to install the combiner. However, the
convergence of all the feedlines at the combiner, very close to the center
element, can create severe feedline-induced pattern distortion unless common
mode suppression is deployed at the ends of each of the feedlines where they
enter the combiner. This is because the outsides of the coax feedline shields
form what looks like a field of wires that intersect near the center of the
array, much like a radial system. The pattern distortion is created by common
mode currents on the feedlines coupling to the center element, even though
there is no direct electrical connection to the center element.
I never gave consideration to this effect until I deployed my first prototype
system in the field and discovered the pattern of the array was not very good
despite VNA measurements on the combiner and preamps that showed accurate gain
and phase. I eventually discovered through antenna modelling that strong
coupling exists between the feedline shields and the center element. The
addition of the feedline chokes at the combiner effectively breaks the
electrical path for current flow on the outside of the shields of the feedlines
where they intersect at the combiner. This almost completely eliminates the
pattern distortion. Once I deployed the feedline chokes, the pattern cleaned
up dramatically, just as the modelling predicted.
The YCCC combiner does not incorporate the common mode suppression internally.
This is best done on the feedlines themselves, installing feedline chokes a few
feet away from the combiner.
73, John W1FV
From: Dennis W0JX [mailto:w0jx@yahoo.com]
Sent: Tuesday, November 30, 2021 12:48 PM
To: topband@contesting.com; John Kaufmann
Subject: Feedline Choke Placement in RX systems
K9YC made an interesting comment in his post, saying that the feedline chokes
would be most effective if placed at the antenna end of the feedline.
W1FV's 9 circle YCCC array has them right at the controller.
The older K7TJR combiner utilized no feedline chokes. However, I believe that
Lee's new design has built in common mode protection on the combiner circuit
board.
So what is the best way? Is placement dependent upon the combiner design?
BTW, K3LR is using 2.4 in #31 toroids with at least 24 turns of RG-179 as
feedline chokes in his 8 circle HiZ array and I believe that there are chokes
on both ends of the antenna feedlines. Of course, K3LR operates in a high
intensity, multi-transmitter environment and may need all that choking.
73 Dennis W0JX
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