[R-390] Input impedance of R390A

[Bob Camp]

Hi,

This gets a little weird but here it goes:

Let's say you have two systems set up -

    System one - a 50 ohm generator running into a 50 ohm matched radio
input

    System two - a 50 ohm generator running into a high impedance radio
input


In both cases the generator is set to the same *indicated* signal level. In
the first case since you are matched half the voltage out of the generator
is dumped by the divider action of the source and the load. In the second
case you do not dump half of the voltage. The result is that you have twice
as much voltage on the input of the radio in system two.

Since signal generators measure a high level voltage (at the input to a big
attenuator) even though the second system has twice the voltage on it the
signal generator meter reads the same both times. Seems a bit odd. The
reason it's odd is that most antenna's work the same way. As a matter of
fact any 50 ohm source works this way provided it's a matched output.

Still with me so far I hope.

Now for the fun - noise in a resistor is a constant power function. The
voltage goes up as the square root of the resistance. The noise voltage on
the second system is 3 db higher than the noise voltage on the first system.
Since the signal voltage is 6 db higher (twice the voltage) the best case
signal to noise ratio is 3 db better on the second system. Of course the
actual signal to noise depends on a lot of things, but the best you can do
is still 3 db better on system 2.

What does this all mean about radios ?  If you have a really hot receiver
the input impedance may not be anything like 50 or 125 ohms. If your antenna
does not behave like a matched source who knows what will happen when you
hook antenna A up to system B.

End of strange but true tale ....

    Enjoy!

        Bob Camp

        KB8TQ





----- Original Message -----
From: "Tom Warren" <wwarren1@nc.rr.com>
To: <r-390@mailman.qth.net>
Sent: Tuesday, September 03, 2002 9:52 PM
Subject: [R-390] Input impedance of R390A


> Has anyone actually measured the input impedance at the balanced feedline
> connector of the R390A?  Yes, I know they specify 125 ohms as the working
> impedance for the antenna/feedline, but that doesn't mean that the actual
> input impedance looking into that port is 125 ohms.  I'm pretty sure they
> don't specify a conjugate match (then meaning that the input impedance is
> 125 ohms).  So the question stands, has anyone measured the input
impedance
> at a number (even one) frequencies.
>
> Curiosity still gets to this cat!!
>
> Tom, W4PG
>
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