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:


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