Hi Ray,
Here's a sketch of the test set-up I use with my VNA2180:
http://www.karinya.net/g3txq/temp/s21/vna.png
The choke under test fits between terminals A and B.
If you compare it with Fig 40a (Page 31) of Jim's paper, you'll see it's
essentially the same arrangement that he uses: the VNA port A becomes
Jim's "RF generator", and VNA Port B becomes his RF Voltmeter/Spectrum
analyser. This set up typically adds an equivalent parallel capacitance
of about 0.2pF to the choke measurement.
With the "Generator/Voltmeter" method you can only easily measure |S21|
and therefore the choke Z; but of course the VNA will also give you the
vital S21 phase information and the choke R+jX. Plus, the VNA carries
out the frequency scan automatically and can be calibrated to take out
the effect of the interconnecting leads.
The 50 Ohm resistor is there to provide a reasonable termination for
Port A no matter what the choke impedance. Its effect is to modify the
Port A source impedance to 25 Ohms, and that is taken into account in
the calculations.
If the measured S21 amplitude and phase are Amp and Ph respectively, the
choke R and X are given by:
R=(50*10^(-Amp/20))/(SQRT(1+(TAN(Ph))^2))-75
X=-(R+75)*TAN(Ph)
I dump the S21 data from the VNA into a spreadsheet which does all the
calculations and produces a chart of choke Z and R; it can be just a
matter of seconds from pressing the VNA "scan" button to viewing a
chart. Here's an example - this one's for 9 turns 0f RG316 on three
stacked FT240-52 cores:
http://www.karinya.net/g3txq/temp/52x3_9.png
Hope that helps,
Steve G3TXQ
On 01/11/2014 16:10, Ray, W4BYG wrote:
Steve and Jim,
Both of you gentlemen are recognized as very knowledgeable on the
subject of chokes.
So for those of us looking for testing guidance in brief, what is a
good way to determine the resulting impedance of a given choke at a
frequency(s) of interest, given a MiniVNA or whatever method you suggest?
I have studied both of your online compositions in detail and am left
a bit confused by all the dialog. Can you summarize for us?
Ray, W4BYG
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