Peter,

with the AIM 4170 you've got one of the best impedance measuring tools
which are affordable for amateurs.
I can this tell you from my own hands-on experience, also in comparison with
a professional albeit vintage HP VNA from 100kHz to 110Mz.
Meanwhile I'm using here also the "two port brother" of AIM,the VNA 2180.
Regarding common mode "chokes",the AIM will show you, that when  you use high µ 
cores
(zinc-mangan), the "choke" converts to a more or less highly resistive impedance
with a small capacitive reactance above its self-resonant frequency.

The point is that this frequency is surprisingly low,often below 100kHz,of 
course
strongly depending on the properties of the used ferrite mix and the number of 
turns.
Generally one can distinguish between three types of common mode "chokes".

1.) "Chokes" made of lossy very high µ cores or beads which represent a 
resistive impedance
     from MW to 30MHz.Their advantage is that they are broadbanded due to their
    resisitive character and therefore have no series (or parallel which would 
not hurt)
    resonances in this frequency area.
2.)True chokes made of say µ125 material.You can design them such that the self 
resonant
  frequency is at about 15MHz so that the impedance drops gradually to the 
lower (inductive)
and higher (capacitive) end of the HF spectrum.
3.) Tuned "chokes" i.e. parallel resonant circuits.They offer the highest 
possible impedance
  but are confined to one band.
So one can make his choice which solution fits best for the intended 
application.
Anyway, the AIM always tells you over frequency what you are doing.

73
Clemens
DL4RAJ

To: <[email protected]>
Sent: Thursday, December 03, 2009 12:12 PM
Subject: LF: LF, reducing common-mode noise

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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