[TowerTalk] AES SK

[Jim Brown]

It's good that you raised this, Jim. Fair-Rite is yet another example of 
a great small business that was owned and run by engineers who happened 
to be married. He was the Chem E, she was the EE. I met them in their 
booth at an IEEE EMC engineering conference in Chicago in 2005. Not long 
after that they sold the business and retired. That's probably when mfg 
moved off shore. Several years ago, I heard from a local EE working mfg 
that his company was having serious QC issues with their #61 cores of 
the same sort you described.

My measurements of coax chokes were mostly done in 2007, the bifilar 
chokes in 2009-10. Measurements that produced the families of data for 
1-14 turns of the five different materials were done in a well known lab 
in 2002-3 by my collaborator who has chosen to remain anonymous to avoid 
"issues" at work.

I would NOT, however, solely blame QC for the problem with getting 
consistent measurements on chokes, simply because the circuit Q of 
practical chokes is quite low, typically around 0.5. Rather, I think 
much of it is a measurement problem. It is VERY well known that 
reflection-based impedance measurements have increasingly poor accuracy 
for values of Z that vary by more than about 5:1 from the system 
impedance of the measurement system (usually 50 ohms). This is because 
the equation for Z involves the sum and difference of S11 and 1, so very 
small errors in S11 result in large errors in Z.

This error is in addition to the stray C of the measurement fixture, 
which can cause significant errors in the resonance of the choke. This 
is significant with #31 and #43 chokes that are resonant above about 10 
MHz, and huge errors in higher Q materials like #61. In both cases, the 
actual resonance of the choke is higher than the measured value.

73, Jim K9YC

On Fri,7/8/2016 3:12 PM, Jim Thomson wrote:
> Well  you could work for Fair rite as a type 31 sales manager.  Or better yet,
> visit their new plant in China, where they make all these products, and figure out
> why they have such extremes and variations in their  type 31 cores  since the chinese
> plant opened.   N3RR  bought over  700 of em, 2.4  inch od cores, and found they
> are all over the map, and even sent samples to Fair rite.   Bill ended up devising  a
> simple 1 turn test, then graded all  700 of em into various sub groups.   No wonder the
> initial CMC results  were not repeatable.