[Elecraft] BL2 choking Impedence measurements?

[Ian White GM3SEK]

 W5OV wrote:

>> I don't own an AIM, but I've seen choke measurements made by an
>>AIM that had  serious errors. A good common mode choke should
>>have a high resistive impedance at the frequencies where it is used,
>>and the AIM is best suited  for  measuring impedances relatively close
>>to 50 ohms (that is, within 1:5 or 5:1). It's fundamentally unsuitable for
>>measuring the high impedances of a  good choke.
>> 73, Jim Brown K9YC

>Jim,
>
>Please allow me to add some facts about the AIM4170C to this discussion.
>
>First, the AIM4170C is not designed only for "measuring impedances
>relatively close to 50 ohms".
>
>>From WX0B:
>
>The AIM4170C can measure 50K ohms R and +/- 50K ohms X easily.
>
>The fact is is that the higher the impedance, as you know from your work
>with the Type 31 cores, even 1pf or .5pf of stray capacitance can
>drastically affect the measurement.
>
>So the test fixture must be carefully built and calibrated to get a
>precise measurement. The AIM4170C or even an HP 8753 would have trouble
>with a choking reactance measurement in a 5K+ environment.
>

I don't own an AIM4170 either, but have made many measurements on
feedline chokes with a N2PK VNA and have traded comparison data with an
AIM4170 owner.

The key point that WX0B makes is that the instrument MUST be calibrated
to include the effects of the connecting leads attached to the device
under test (DUT). This is not an option, it's a MUST!

If the instrument is not capable of being calibrated in this way, then
I'd agree with Jim that it is totally useless for measuring high
impedances. This categorically rules out all the simple antenna
analysers like the MFJ-259B, Autek etc.

However, the AIM4170 is a higher-level instrument that is capable of
being re-calibrated to include the effects of an attached test fixture
(which then becomes "part of the instrument"). With care, it seems that
the 4170 can be used to make quite accurate vector impedance
measurements on chokes. The next step, having calibrated the instrument,
is to check how well it measures a test load like a 5-10k chip resistor.
With appropriate precautions, a pre-calibrated N2PK VNA can measure a
10k resistor within a few percent up to at least 30MHz, which is "plenty
good enough for choke work".

Everyone agrees that even quite small variations in the test fixture can
drastically affect some aspects of measurements on feedline chokes,
particularly the resonant frequency. But that is also a criticism of the
choke itself: if its performance depends on a narrow resonance that is
difficult to measure, even under controlled conditions in the lab, then
that same choke will also be extremely sensitive to the way it is
installed in real-life situations and is unlikely to deliver consistent
performance.

For consistent performance in a wide range of installations, a feedline
choke needs to deliver a high *resistive* impedance over a broad
bandwidth. As Jim Brown has so often said, resistive loss in a feedline
choke is *good*, provided the resistance is high enough! This requires
quite careful design based on a specific ferrite core.

Follow the link below for a reasonably short summary, and then go on to
read Jim's much longer treatise:
http://www.ifwtech.co.uk/g3sek/in-prac/inpr1005_ext_v1.pdf





-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek