Topband: Thanks for the listening and share of a common mode current experience

Eduardo Araujo er_araujo at
Fri Jun 8 18:21:06 EDT 2007

Many thanks for those to try to listen to my TX test
during last week-end. Obviously I choose the wrong
one, Propagation was awful even being accompanied by
LU2DVI/H who has a wonderful reception condition.

Now as promised, I would like to share with you a nice
experience that may be useful to some.

I live in an apartment which is at 11th floor (aprox
35 mts from ground) and all my coaxial and rotor
control cables are 40 mts long.

I have a broadcast at about 10 or 15km operating at
1.030 with 50Kw and 3 elements pointing to where I

Now if I touch my 160 mts inverted V coax with my
ground reference, ie the water plumbing system, with
an small 12 v lamp in series, it turns to full bright
with the signal of the broadcast. A home made
calibrated RF current meter indicates 220 mA.

Now the test, I inserted a common mode RF choke which
has 275 ohms at 1.8 but only 173 ohms at 1.1 Mhz
(mostly resistive).

With the choke in series at my shack, the lamp bright
is reduced to ¼ or 1/8 hard to tell, and the current
meter indicates 70mA.

Then I decided to move the choke at the antenna feed
point (40 mts coax) where I have a voltage type balun
(pls don’t blame for it, I will replace it soon), very
interestingly the bright of the lamp at the shack
stayed practically at maximum and the current meter
drops by 10mA only.

A few conclusions:
1 – If we can’t measure it is hard to know in advance
where the choke/balun will be more effective.
2 – Even if we are at the right point 200 or 300 ohms
may not be good enough.
3 – Probably depending on the case, 1K may be not
enough if we are at the wrong point.
4 – When we have problems with noise or RFI, and can’t
measure or simulate the scenario, (is it possible?) 
It would be better to place a couple of chokes ¼ wave
apart with more than 1K at the problematic fcy.

Don’t forget to read K9YC and W1HIS papers, both are a
must on this subject, and as W8JI said many times this
is a difficult topic.

’73 to all......... Eddy, LU2DKT

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