Re: AA-230 Zoom - measuring chokes
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Roger, Thanks for the info. Since I don¡¯t need to do this often, decided to trust my construction technique and just follow the instructions to build the chokes. 73, N2TK, Tony ?
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From: [email protected] <[email protected]> On Behalf Of Roger Need via groups.io
Sent: Tuesday, October 13, 2020 4:48 PM
To: [email protected]
Subject: Re: [RigExpert Family] AA-230 Zoom - measuring chokes ? On Tue, Oct 13, 2020 at 05:09 AM, N2TK, Tony wrote: Trying to measure the choking impedance of some chokes I wound. Using #8 to
display all parameters. Frequency set to 1800 KHZ.
I am getting strange readings. As an example, chokes wound with a pair of
#75 ferrite cores with 21 turns that should read 13K read Z - 2.3k and R -
292 ohms.
Am I doing something wrong (the usual case) or maybe the AA-230 isn't
designed to do this?
Thanks for any feedback
N2TK, Tony
Tony,
The RigExpert is a one-port VNA that measures the magnitude and phase of the complex reflection coefficient ¦£.??? The relationship between?¦£ and the device-under-test (DUT) complex impedance (R +jX)? is? Z = Zo*(1+¦£)/(1-¦£) where Zo is usually 50 ohms (can be 75, 300, 600 etc.).? When we have a perfect match DUT Z = 50+j0 and the reflection coefficient ¦£ is 0.? When Z is larger or smaller than 50 +j0? ¦£ changes and it is 1 when Z = infinity and -1 when? Z = 0.?? So ¦£ is in the range of -1 to 1 with 0 occurring with a perfect match.
As the impedance increases beyond 50 ohm ¦£ does not change in a linear fashion but changes in increasing smaller increments.? Here are some examples where the load Z is a pure resistance (X =0).
Z - load?? Magnitude of ¦£
----------?? --------------------
50???????????????? 0 ???
75??????????????? .2
100????????????? .333
150????????????? .5
200????????????? .6
250????????????? .666
500????????????? .818
1000??????????? .905
1500??????????? .9354
2000??????????? .9512
3000??????????? .9672
From the above you can see that once you get above 500 ohms small measurement errors of the reflection coefficient will result in large errors in the estimate of Z.?? So it is not possible to accurately measure Z in the thousands of ohms (which is what you want to do if measuring RF chokes)? with a one port VNA like the RigExpert.? It is possible using a 2 port VNA using what is known as the S21 method but this requires a high end VNA in order to get accurate results.?
Roger
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