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
top
loaded vertical with 2 elevated radials?
Where to best put the coil? I assume at the feed point but I read
somewhere
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
maximum.
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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