A few weeks ago I installed 3 half size point fed pennants (each aimed
approximately 120° apart), and have been wondering if I need to install any
chokes and/or ground rod on the feedline near the feedpoint of this antenna
system to negate any common mode noise.
I then did a lot of research and see that testing for common mode currents is
difficult (W8JI said it's a PITA), but I hate to add extra components without
verifying their need (also want to make sure I don't actually make things
worse). This got me wondering what simple test can I do to see if common mode
currents are really an issue. I'm most concerned about man made noise, etc.
(very happy with my front to back ratio), so here is my idea of a simple test,
and I would like to see what others think of my proposal.
Proposed Test
Step 1) With HF rig in the shack connected to the antenna via the feedline,
measure the noise level (S meter reading just listening to noise).
Step 2) Take HF rig right out to the antenna and connect it to the antenna
using the shortest length of feedline possible (just a few feet) and power the
HF rig with a battery (12 volt SLA battery) so the system is totally isolated
from commercial electrical wiring, ground, etc. and basically no feedline, and
then measure the noise level again.
If the noise level with the radio located in the shack is not higher than the
noise level with the radio located at the antenna then common mode noise is
insignificant (and I can forget about chokes and ground rod). (Note : this
test assumes feedline loss is minimal, but probably a good test to also perform
would be a signal to noise ratio test with the radio at both locations to
compensate for feedline losses).
Note : I guess I could also do an A vs. B test (in place of my above proposed
test) in which I install a ground rod and make a choke, and then I place these
devices in and out of the system to compare noise levels with the rig in the
shack.
I wonder what the topband community has to say about my "Proposed Test".
73's
Don (wd8dsb)
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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