[Amps] Understanding S Parameters

[Markku Oksanen]

If my memory serves me correctly:
Is it not so that S11 refers to the input port "input impedance" of a two port network?So, in this case the input impedance is just simply about 1 ohm with small reactance (phase angle -3.6).SWR at the input should be around 50.
Without S12 or S21 you should not be able to know stability?
Markku

> From: pc5m at xs4all.nl
> To: amps at contesting.com
> Date: Tue, 16 Jun 2015 22:45:08 +0200
> Subject: Re: [Amps] Understanding S Parameters
> 
> Hi Drax,
> 
> If S11 > 1 it means it is not unconditional stable, i.e. it might
> oscillate...
> 
> Your formula for input impedance is only valid if s12 or s21 = 0, and I
> think that is not the case.
> 
> Otherwise: Gamma(Inp)=S11+(S12*S21*Gamma(outp))/(1-S22*Gamma(outp))
> 
> 73's Carel.
> 
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Drax Felton
> Sent: dinsdag 16 juni 2015 21:53
> To: amps at contesting.com
> Subject: [Amps] Understanding S Parameters
> 
> I am trying to understand S Parameters for a RD00HHS1 transistor.
> 
> The S11 parameters listed in the datasheet are as follows:
> 10 MHz		1.002 (mag)	-3.6(ang)
> 30 MHz		1.003		-9.9
> 50 MHz		1.005		-16.8
> 100		1.007		-33.5
> 150		0.989		-49.8
> 
> I'm trying to estimate the input impedance at 0.475MHz.  But I am uncertain
> what it means when the mag is greater than 0.999.
> 
> Using the 10 MHz S11 value because it's available I get:
> 
> Gamma = 1.002 = (ZL-50)/(ZL+50) --> ZL= -50050 ohm impedance or a VSWR of
> negative 1001:1.
> 
> What am I doing wrong?  What does this mean?  
> My other attempts at calculating input Z also result in negative impedances
> in linear form like -50.58-1589j My RF books do not seem to say anything
> about this.
> 
> 
> Kb3x
> 
> 
> 
> 
> 
> 
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