On Wed, 15 Sep 2004 11:08:21 +0200, Toni
wrote:

Hi,

I have a Sony radio for SWL and I have seen that if I parallel a
300 ohm resistor to the antenna input the received signal drops
considerably [1]. I think this means the receiver's input
impedance is much greater than the normal 50/75 ohms. This makes
me wonder two things:

- How can I measure the approximate value of the receivers input
Z? I don't want to know the exact value, just to know if it is in
the 500 / 1K / 10K range. I guess I can do this by paralleling
increasingly valued resistors until there's not much effect on
signal and assume that it about half that resistor's value.

- Having a 50 ohm feedline, would I benefit by using a 4:1 balun
in reverse at the rcvr input? What kind of balun could I use for
that? It would be great if it also provided some improvement of
CMRR for noise picked by the feedline itself.

[1] I use a 100nF capacitor to isolate the 4~5 V DC level at the
antenna input. I guess this DC may be there to feed some kind of
active antenna.

Buy a cheap passive antenna tuner, e.g. the YAESU FRT-7700
or the one from MFJ Enterprises.

Forget that idea with the resistors.

w.