Topband: Share of a common mode current experience

Eduardo Araujo er_araujo at
Sun Jun 10 11:38:41 EDT 2007

Hi Jim and all, Yes I agree with you 100% on both

I was testing with a kind of #78 material which is
more effective at that fcy (around 1.5Mhz).

I am using a home made High Pass Filter, with more
60db attenuation at that fcy but it is being bypassed
to some degree by the common mode current. 

With a couple of chokes with more than 2K, the
broadcast signal is reduced by more than 30 db now
because it is being forced to pass thru the filter in
differential mode. 

Now I am working in all cables coming down because as
soon as I connect each other cable not being treated,
the problem reappears.

I have a few questions: I understand why it is
necessary to use a low loss material like #61 at the
antenna feedpoint (QRO) where current could be high
and if so the R part of Z will need to dissipate a lot
of heat, but what confuse me a little bit with #61
material is:

1 – Using a high Z mostly inductive is ok if the
combination of line length, antenna and ground
connection don’t result in a capacity that combined
with that inductance may make things worse.

2 – What could be ok for one band ie: Z combined in
the right direction, it could be bad in another band
where this system resonates and capture signals from
our others antennas or other sources.

3 – Living at 35 mts from ground level as it is my
case, is it possible to estimate/simulate in advance
if it will be better or worse on different bands?

4 – It is very difficult to get high Z with #43
material al low fcies. If I understood correctly your
measurements Jim, we will need 14 turns on 8 cores to
get aprox 2K at 1 Mhz and 4K at 2Mhz.

5 – I thought to use two chokes at the antenna
feedpoint one with #61 and the other with #78 but if
due to combination of Z #61 does things worse, then
the one with #78 will need to dissipate all the heat
by the current (if any of course) 

I am doing some measurements on the cores extracted
from old TV to have an idea of its characteristics,
and when done, I will append it here.

I hope this may be of interest to the community .....
Regards...Edy, LU2DKT

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