A couple of weeks ago I asked how can one measure the mu of ferrite
rings. I received very good information via this reflector for which I
would like to thank those who contributed.
I was experimenting with various common-mode chokes, particularly on how
to measure their characteristics using the AIM 4170.
I was advised that the capacitance of the measuring setup might make the
measurements invalid. I introduced various values of capacitance to
measure these effects and by extrapolation concluded that my measurement
method was acceptable.
Inserting large values of capacitance across the common mode choke
destroyed its characteristic as a choke and turned it into a selective
tuned circuit. This arrangement appears to reduce common-mode noise at
the frequency to which it is tuned but is only effective at that
frequency. The main advantage is that you don't need such large values
of inductance.
Regards
Peter, G3LDO
Paul-Henrik wrote:
Hello Stefan, LF
I can not overemphasize the possible need to keep the feedline "clean" and free
of RFI that may otherwise make it's way to any active whip antenna.
My first attempts at deploying the PA0RDT Mini-Whip here were unsuccessful and
disappointing due to the fact that some of the RFI I tried to avoid was
conducted from the shack out to the antenna along the coax. This may among
other things depend on what kind of radio the antenna is connected to.
If in doubt, my advice is to use one common mode choke where the coax leaves the
house and to ground the coax braid just behind it using an independent
earth-stake and then further a second common-mode choke just before the
feedline goes vertically up to the antenna.
For common mode chokes I use large high-mu ferrite rings that can take 20 turns
of RG-58 or up to 45 turns of RG-174 but smaller rings can be used with RG-174.
There is a picture here of the first test-choke I used for tracking down my
noise: http://peditio.net/utility/Attachments/choke_sml.jpg
This may be "overkill" in most installations but I'm sure some users can gain
from these extra measures just as I did.
BR
Paul-Henrik, OH1LSQ
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