In article ,
"Lenny" wrote:

As the subject says, I'm looking for ways to hear the weak ones that
are getting drowned out by the strong ones in the same frequency
area. I'm using a Kiwa modified R-75 with an Antenna Supermarket
Eavesdropper antenna. I've tried every control on the Icom. With
some stations, I can tune them so that I can understand the voices.
Others, I'm not so lucky. Am I at the limits of what the R-75 can do?

Following what you wrote "I've tried every control on the Icom" means
you tried every blocking function the radio has so yes. It's possible
that you may not have had all controls in a optimum state at the same
time though.

Are there any other mods I can make to it, within reason, to improve
it?

For the radio itself a better filter then the one being used could help.
You need the filter to be sharper and it must also have a greater
ultimate rejection. A improvement in the combination of these two
specifications will improve the radios blocking.

If you have used all the functions the radio has, installed an improved
filter and you still have an adjacent channel problem then you could
use a more directive antenna where the null would be on the offending
station. You could use an antenna to peak the station you want relative
to the offending station. This is not likely as the offender is only 5
KHz away and that would take a very high Q peak tuner and the same
could be said for a tunable notch.

Best bet I think would be a small loop antenna where you would put the
offender in the null. The drawback is the loop null becomes effectively
less deep the higher in frequency you go. It should help 10 MHz.

This will only work if the offender is on a different heading.

The radio does great with pulling in hams, the utility stations I've
tried, stronger World Broadcast stations, and even some of the weaker
World Broadcast stations. But if there's a weak station next to a
stronger one, no dice. Does it take a better receiver, such as the
Drake R8B to isolate out the weaker stations?

The R8B has good to excellent blocking depending on which specification
you look at and the R75 manual does not have those numbers but I expect
that it is similar except for the Drakes sideband selectable sync.

What is the property of the receiver that lets this happen?

Its called poor blocking another word would be poor selectivity.

This type of interference happens when the radios blocking specification
has been exceeded by the combination of amplitude and frequency spacing
of the two signals in question.

Is it extremely sharp cutoffs on the filters? Please explain?

The IF filters make up a part of the radios selectivity and sharper is
better because by definition the response falls off more rapidly but you
can't at the same time ignore the filters ultimate rejection.

Because you are concerned with 5 KHz spacing it really depends mostly
on the If filter characteristics. If you were concerned with larger
spacing then other circuits in the radio such as the roofing filter,
how the conversion is done and how many conversion stages there are in
the chain would come into play.

One more thing, there are also problems on the transmitter end where the
signal can take up more bandwidth then it is supposed to and there can
be other problems such as "splatter" on poorly operated transmitters. A
better radio can't help with problems on the transmitter end.

--
Telamon
Ventura, California