En Robert Casey va escriure en Wed, 15 Sep 2004 18:59:23 GMT:

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:

The input impedance will vary depending on the receive frequency.
It's something that isn't much worried about in radio design for
SW.

Yes, I think this must be the case, but even if it varies I think
it will go up/down to double/half or triple/one third, I don't
think (hope not) it goes to 10x / 1/10th. That's why I don't want
to try to perfect-match, just same order of magnitude match

As the radio's AVC circuits will make up for gain losses due
to mismatches. Also realize that the receiver's front end
noise is much less than the amount of noise produced by the
natural and man made environment. That would limit how much DX
you can do. A perfect match of antenna to front end would just
deliver more noise along with signal, and would be indistinguishable
from a mismatch with the AVC making up the missing gain. I'm
talking about around 10dB mismatches here.

Well, the case is that I've been playing, among others, with
untuned shielded loops, and they really help reduce interference
by carefully turning them. Unfortunately they also produce very
low signal level, close to the point of not hearing much
difference in background noise when connecting/disconnecting the
antenna. That't why I'd like to gain those extra 5~10 dB by
mathing the receiver to the feedline.

Aside, the ICF-7600G is possibly the best receiver I have had so
far in sensitivity and selectivity, matching, if not improving,
my TS-570-DG transceiver. It is also great for carryng arround.
(I know I shouldn't make free publicity...)

Thanks for your comments

--
Toni

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