Eric/Sinisa,
My calculations agree with Rick's. A constant current
source is not a good model of an antenna. An antenna
is going to have a finite output impedance - presumeably
something close to 50 ohms depending on how well it
is matched. A true constant current source has an infinite
output impedance.
If you don't believe me, put a variable resistor at the output
of that vector impedance meter that is always trying to
source a current of 12uA. Now start with the variable
resistor set to zero and then measure the voltage across
the resistor as you raise the resistance. The voltage across
the resistor will rise linearly with resistance, and so will the
power delivered to the resistor (P = Io^2*Rload, where Io
= 12uA). The voltage and power will continue to rise until
the output stage of the vector impedance meter saturates
against its supply rail, at which point the current source
will no longer be a constant current source. This is hardly
a good model for an real antenna driving a receiver.
For a better simulation of an antenna, shunt a 50 ohm
resistor across the output terminals of the vector impedance
meter, and rerun your tests. In this case you will see that the
maximum power transferred to the variable load resistor
occurs when the variable resistor is set equal to 50 ohms.
You can also repeat the tests on the FT-1K's using this 50
shunt resistor across the current source and an antenna
tuner between the receiver input and the source. In this
case you will find that instead of the large signal variation
you were getting using the 12uA source alone without
the 50 source termination, you will only get the 3dB of
variation predicted by a 5:1 mismatch loss when you adjust
the tuner to transform the input impedance of the receiver
from its worst case value of Zin = 10 + j 16 (5:1 VSWR)
to the perfectly matched case of Zin = 50 ohms (1:1
VSWR).
73 de Mike, W4EF..................
----- Original Message -----
From: "Rick Karlquist" <richard@karlquist.com>
To: <K8LV1@aol.com>
> > These readings illustrate the futility of trying to calibrate the
> > S-meter in absolute units, with over 10-20 db of variation at the
> > input terminals. Another interesting note: I used an HP 4815A Vector
> > 73
> > Eric von Valtier K8LV
>
> There is not 10-20 dB of variation in the S-meter. The
> worst case mismatch you show is 10+j16. Even this 5:1
> SWR only results in a 3 dB mismatch loss, still not much
> in the overall scheme of things. (You can find mismatch
> loss on any decent Smith chart).
>
> Rick N6RK
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