[TowerTalk] Measuring Common Mode Choke with Anritsu VNA

Máximo EA1DDO_HK1H ea1ddo at hotmail.com
Fri Jan 5 10:02:36 EST 2018

Hi all,

Regarding the S11 Vs S21 for Zmag measurements, I've found an interesting article from Keysight;


Two common methods used to perform impedance measurements with a
vector network analyzer are the reflection method using one port and the shunt/
series method using two ports. Figure 6 shows the relationships between the
impedance and S parameter for each of the two measuring methods. When
the impedance measured (Zx) is in the vicinity of the characteristic impedance
(Zo = 50 Ω), the reflection method provides high-sensitivity measurements
because the vector voltage ratio significantly changes with a small change in
the impedance. However, when the impedance measured is not in the same
vicinity as the characteristic impedance, the reflection method is not suitable
due to trace noise.
The two port method allows you measure impedance over a wide impedance
range using shunt/series connections. With the two port method, you can use
either S11 or S21 to perform measurements; however, S11 has a more limited
noise floor than S21, and it is highly susceptible to VSWR and other factors that
exist between the fixture connector and substrate. Because of this, generally
S21 is preferred. The S21 shunt connection is suitable for 50 Ω or lower impedances
while the S21 series connection is suitable for 50 Ω or higher impedances.
Because EMC components span a wide range of impedances depending on their
purposes, their impedances must be measured with an adequate combination of
S21 shunt/series connections.

73, Maximo

De: TowerTalk <towertalk-bounces at contesting.com> en nombre de Steve Hunt <steve at karinya.net>
Enviado: jueves, 4 de enero de 2018 12:43
Para: towertalk at contesting.com
Asunto: Re: [TowerTalk] Measuring Common Mode Choke with Anritsu VNA

Hi Paul,

No, I no longer use the 50 Ohm termination at the jig. I originally
included it because I was using long RG174 patch leads between the VNA
and the jig and wanted to minimise coax loss at the higher frequencies.
For the last year or so I've been using the jig arrangement shown here
without any termination:
G3TXQ Common Mode Chokes - Karinya<http://www.karinya.net/g3txq/chokes/#measurement>
Amateur Radio (G3TXQ) - Common-mode chokes. The following chart presents the results of impedance measurements made on a variety of common-mode choke implementations ...

Steve G3TXQ

On 04/01/2018 12:17, Paul Christensen wrote:
> Steve,
> Over the last week, I've been collecting articles and data on various S11
> versus S21 methodologies for use with RF choke CM measurements.   In your
> S21 measurements, are you still using a resistive 50-ohm source generator
> termination on your test jig? (See quote below from Owen Duffy, Ex-VK1OD)
> If so, is it because the generator does not see Z=50+j0 that's present at
> the VNA's detector port owing to the fact that a series complex Z (the RF
> choke) is placed between ports?  Is the termination necessary to obtain a
> reasonably accurate complex Z and phase result as complex Z increases?


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