Topband: Coax Shield vs. Ground & elevated radials

Guy Olinger, K2AV olinger at
Mon Jan 1 18:42:36 EST 2007

SM0MDG: >> To decople my my inverted L I was using a RG213 coil made 
of 18 turns on a
11 cm plastic pipe. Would this arrangement be enough to decouple my 
loaded vertical with 2 elevated radials?

Where to best put the coil? I assume at the feed point but I read 
that in some cases it might not be the best place... <<

This is one of those situations where modeling can help you with what 
to worry about.

Grounding or not grounding either end, or grounding somewhere in the 
middle, and the length/orientation of the feedline, "radials", and 
even parallel conductors all can move the optimum placement of lump 
common mode suppression devices.

A fuzzy general rule is: a common mode suppression device does best if 
placed at a point which would otherwise be a common mode current 

It can be tricky KNOWING where the pre-suppression common mode current 
maxima ACTUALLY lie. There are no utterly dependable simple rules for 
locating them, other than MEASURING them. Various posted/published 
"rules" that DO work are HIGHLY SITUATIONAL. They simply may not work 
if attempted in different circumstances. When reading anecdotal or 
technical descriptions of successful suppressions **pay close 
attention to the details**.

Placing a single suppression device may simply MOVE common mode 
current maxima without much reduction of loss on transmission or 
common mode noise pickup. A pair of suppressors spaced roughly 1/4 
physical wavelength may work where one did not. This is particularly 
true if some part of the system is grounded at the antenna or between 
the antenna and the suppression.

Be prepared to redo antenna matching after successful common mode 
suppression. It is easy to create a model with more common mode 
current on the feedline than desired current on the antenna. Removing 
such a common mode outlet for power can significantly alter the system 
feed impedance.

73, Guy 

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