Hi everyone,
With Dan´s permission, I have modified ZPlots to add some extra S21
calculations.
Actually, you can upload any s2p file from any VNA and display most modes: Rx,
Xs, Rp, Cp, Logmag dB, mag, phase and Zcm.
This last one, Zcm, is the one to see how chokes perform.
Please, feel free to download and use the tool. I´ve called it; S21 Xtra:
ZPlots S21
Xtra<http://www.puertobalsillas.com/radio/Balun_Guanella_4_1/Zplots_Xtra.xlsm>
Since I only could test it with two VNAs, comments are welcome.
Thanks
73, Maximo
________________________________
De: TowerTalk <towertalk-bounces@contesting.com> en nombre de jimlux
<jimlux@earthlink.net>
Enviado: viernes, 12 de enero de 2018 13:38
Para: towertalk@contesting.com
Asunto: Re: [TowerTalk] S21 to Rp & Cp - Equivalent Circuit
On 1/11/18 10:48 AM, jimlux wrote:
> On 1/11/18 10:01 AM, Máximo EA1DDO_HK1H wrote:
>> Hi Jim, Steve, all,
>>
>>
>> Playing with those formulas I´ve got an inconsistency.
>>
>>
>> To get R and jX from LogMag and Phase I can use two different
>> formulas, one from Steve and one from Jim, but the results are not
>> same, so I don´t know where the mistake is.
>>
>>
>> Example from Excel;
>>
>>
>> LogMag -29.601925
>>
>> Phase -77.10851
>>
>>
>> Steve´s formula:
>>
>> Rx = (100*10^(-LogMag/20))/(SQRT(1+(TAN(Phase/57.29578))^2))-100 =
>> 573.916
>>
>> X = -(Rx+100)*TAN(Phase/57.29578) = 2944.484
>>
>>
>>
>> Jim´s formula:
>>
>>
>> R =
>> COMPLEX(10^((LogMag/20)*(COS((Phase*PI())/180))),10^((LogMag/20)*(SIN((Phase*PI())/180))))
>>
>>
>> then Rx+jX = IMPRODUCT(50,IMDIV(IMSUM(1,R),IMSUB(1,R))) = -49.930
>> +3.606
>>
>>
>> As you can see, 573 is not same as -49 OHm
>>
>> And 2944 is not 3.606
>>
>>
working in Octave (free version of Matlab), which is a bit easier than Excel
>> s11mag = -29.6
s11mag = -29.600
>> s11ph = -77.1
s11ph = -77.100
>>
>> gamma = 10^(s11mag/20)
gamma = 0.033113
>> gamma = gamma*complex(cos(s11ph/180*pi),sin(s11ph/180*pi))
gamma = 0.0073925 - 0.0322774i
>> z = 50* ( 1+gamma)/(1-gamma)
z = 50.6383 - 3.2725i
This is the series form, and close to 50 ohms, as expected (a return
loss of 30dB is a good match)
Convert to parallel form
>> rs = real(z)
rs = 50.638
>> xs = imag(z)
xs = -3.2725
>> comterm = (rs/xs+xs/rs)
comterm = -15.538
>> rp=xs*comterm
rp = 50.850
>> xp = rs*comterm
xp = -786.84
>>
Which matches *neither* of the other versions.
And using a different way to convert series to parallel
>> Q = xs/rs
Q = -0.064626
>> xp = xs*(Q^2+1)/Q^2
xp = -786.84
>> rp=rs/(Q^2+1)
rp = 50.428
>>
Slightly different R - maybe it's a numerical precision thing Q^2 is small
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