Hi Ron:
> Tod I use an AIM-4170 VNA purchase through Array Solutions to do testing
as you have described.
I have the AIM 4170C from W5BIG, but it is not a VNA. I think the VNA
instrument is an AIM 2180 and the AIM 4170 is an antenna Analyzer. I tried
using the AIM 4170 to measure this type of very high impedance choke but
found that my 4170C instrument measured quite low once you got above 5 MHz.
The shunt capacity of the instrument may be the cause of that. That was
mentioned in the K9YC paper. I believe that Jim Brown stated the input shunt
capacity was about 2 pF which he felt made measurements above 5 MHz
inaccurate.
> A few things I have noticed is your table, it looks like a folding type,
if it is, I have the same. One thing I have found is the metal support under
the table will change the Z-Mag or impedance of the choke being tested if
placed over the top of the support.
The table on which I made the measurements is a folding type table. The top
is made of wood composite and there is a narrow metal band for stiffening on
the underside that is located about 2 inches behind the table edge. There
are no metal supports under the table and any metal from the legs is located
well away from the area I used for the measurements. I was concerned about
that as well and positioned things on the surface of the table so that the
effect of the metal band would be minimal. There is no connection to ground
made by the metal band so the capacitive effect should be quite small. I
looked again at the picture of the jig that I had put on the web site and
saw that I had used an older version of the jig. I took two more pictures
and replaced the old one. The orientation of things was done with the
thought that I wanted to eliminate or at least reduce the effect of stray
capacity. I also made a quick measurement with the jig positioned about 4
inches higher above the table. The measurement was identical to my previous
measurement [happily].
The general effect of stray capacity is to reduce the attenuation values
[essentially reduce the measured Impedance magnitude] . I tried to make all
of the measurements in the same way which would help to make them internally
consistent when comparing the results between chokes. The individual results
may have a consistent experimental error when compared to measurements made
by a very accurate instrument. [I am not sure just what instrument that
might be but I suspect that I could not afford one].
I endorse the idea of critiquing experimental data and the way in which they
were collected. That is imperative if we want to improve our measurement
results and understanding of things. Thank you for your comments.
I would welcome someone else making similar measurements to see if they can
get the same results. When time permits I will make a few measurements using
the AIM4170 and the component jig that is a part of W5BIG's AIM4170 manual.
My previous effort to make such measurement left me with opinion that the
measured results understated the impedance magnitude. When I measured a 5000
ohm resistor with the 4170 I felt that the result was close to what I would
expect. That suggests that the resistance measurement is correct or at least
close. That leaves me with the conclusion that the reactance measurements
might be incorrect. It may be that the AIM 4170 jig is not a good choice
for trying to measure a common mode choke such as those I have fabricated.
73, Tod, K0TO
_____
From: Douglas, Ron [mailto:Ron.Douglas@sumcousa.com]
Sent: Monday, August 29, 2011 11:24 AM
To: tod@k0to.us
Subject: Testing chokes with 31 material
Todd I use an AIM-4170 VNA purchase through Array Solutions to do testing as
you have described.
A few things I have noticed is your table, it looks like a folding type, if
it is, I have the same. One thing I have found is the metal support under
the table will change the Z-Mag or impedance of the choke being tested if
placed over the top of the support.
Bob at Balun Design has many of the 4170 VNA plots on his website, he has
helped us design our own choke for mobiles and our home antenna package.
Please look at his site, he builds a very nice product.
I have enjoyed reading all of your data and have a few things I would like
to talk to you about if you have the time.
Please use the scorpionantennas@cox.net for any and all replies.
This e-mail is my real job e-mail address.
I am off this wed through sat if I could give you call one of these days.
Best Regards
Ron Douglass NI7J
scorpionantennas.com
scorpionantennas@cox.net
cell 623.326.8780
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