Topband: Feedline Choke Placement in RX systems

[John Kaufmann]

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 at yahoo.com] 
Sent: Tuesday, November 30, 2021 12:48 PM
To: topband at 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