[TowerTalk] One off balun question

[Steve Hunt]

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
>