Hi Helmut,

En Helmut Wabnig va escriure en Wed, 15 Sep 2004 17:47:43 +0200:

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

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.

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

This is just what I wanted to avoid. I just wanted a
better-than-nothing approach that doesn't require retuning at
every band change. Aside, having no s-meter it would probably be
difficult to calibrate the tuner.

Something different would be a pasive preselector. I will
probably buy or make one, but this is a different beast.

Forget that idea with the resistors.

You will excuse me but I don't see why. It may not be a high-tech
computer-controled impedance analyzer but I just need
order-of-magnitude precision, I don't even need the first digit
to be much accurate.

Thanks for your comments

--
Toni

"Auto" = prefijo griego que significa "no funciona"