I know that we can minimize RFI / noise pickup by minimizing the common
mode currents on our feed lines. And, that we want the choke to reduce
common mode current to near zero.
I have seen ***different types*** of common mode current chokes:
-- ferrite bead balun (e.g. W2DU, W0IYH)
-- wound-coax ferrite chokes (e.g. K9YC)
-- broadband 1:1 current baluns (Balun Designs)
-- binocular core coax common-mode chokes (e.g. W1HIS)
There are also suggestions of ***where*** along the feed line baluns should
placed:
-- at the antenna feedpoint
-- at the amplifier output
-- an "at least one other in-between." (W1HIS)
There are RF current probes used for measuring common mode currents.
QUESTIONS:
+++ How do we, as a practical matter, measure common mode currents to
confirm that what we are doing is effective?
+++ Is there more than one method for measuring common mode currents, i.e.
on the actual antennas we use?
+++ Has anyone measured feed line common mode current before and after
inserting balun? what results?
+++ for placing common-mode baluns "in-between" antenna feed point and
amplifier output - - is there an empirical method for determining where
this should be? (assume a mono band antenna)
+++ And, most importantly, has anyone correlated common mode current levels
with the noise floor levels, etc.? (e.g. A-B testing)
73, Alan
K0AV
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