[Elecraft] BL2 choking Impedence measurements?

[Jim Brown]

On Thu, 26 Aug 2010 08:08:16 +0100, Ian White GM3SEK wrote:

>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,

Yes, the AIM is a high quality instrument, but one tool does not fit all 
needs. It is, in essence, measuring S11, which means that very small 
errors in the measurement can cause very large errors in the result. The 
only GOOD way to measure a choke is by measuring S21 -- that is, as the 
series element of a voltage divider. 

The errors I have seen in choke measurements show up as lead capacitance 
that is not precisely cancelled, which results in an incorrect 
determination of the resonant frequency. The error is small for low 
frequency, low-Q chokes, but can get very large at increasing frequency, 
and for higher Q chokes. 

In the materials on my website, there are graphs showing the effect of 
the 0.4 pF stray capacitance of my own test fixture on a very low Q choke 
(Q=0.4) wound to cover a tri-band beam (20-10M). It shifts the resonant 
frequency about 25%. The error is insignificant for #31 chokes wound for 
40M and below. 

The error becomes very bad with a higher Q choke, like one wound with 
Fair-Rite #61 or #67 (Q around 10). The AIM data I've seen shifted the 
resonant frequency of a choke like that by a factor of nearly 2:1. The 
AIM measured the resonance of choke as being around 16 MHz that was 
actually resonant above 30 MHz. And, because the resonance moved and the 
loss of the material varies with frequency, the peak in the choking 
impedance also was in error, but by a smaller amount. 

73, Jim K9YC