[R-390] Input impedance of R390A

[Bob Camp]

Hi,

Ahhhh, but that's the whole point.

One system has a 25 ohm impedance at the input to the radio the other has a
50 ohm impedance.

The first system has half the voltage on it as the second, not due to power
but due the the way the setup is defined. In other words it's not a constant
power question at all.

The noise voltage in the 25 ohm resistance is 0.707 times the noise voltage
in the 50 ohm resistance. Again not constant power as much as that's just
the way the formula works.

The ratio of signal voltage to signal noise is better in the second case.

Of course that says *nothing* about the radio you could or could not build
and how it would perform. All it realy says is that you can get good signal
to noise without matching the input of the radio to the source.

    Enjoy!

        Bob


----- Original Message -----
From: <DJED1@aol.com>
To: <ham@cq.nu>; <wwarren1@nc.rr.com>
Cc: <r-390@mailman.qth.net>
Sent: Saturday, September 07, 2002 1:48 PM
Subject: Re: [R-390] Input impedance of R390A


> Sorry, but you're not quite right.  The best power transfer is always when
> you have a conjugate impedance match.  You might get twice the voltage
with a
> much larger Rx impedance, but the power delivered to the Rx is voltage
> squared over R, where R is the Rx input resistance.  R must be much
greater
> than 50 ohms to double the voltage.  Try a few examples.  The noise power
> from the antenna scales the same as the signal power from the antenna, so
no
> benefit to S/N.  Sometime you can get a better NF by mismatching the
antenna,
> but this depends on the details of the Rx first stage.
> Ed
>