By the way, I don't want selectivity ahead of a mixer...I just don't
want it back after passing through 10 stages with semiconductors.
Quite often one or more of those additional stages will not be
designed correctly, and even the best designs add accumulated
crud.
> I'de just like to know where Tom Finds all that QRM. I would think the
> 160m cw test is as bad as it gets. Maybe not. I spent about 4 hours in
> that test and just set my filter on the TS-870 for 50hz to 100hz
> bandwidth. I had 40-50db over 9 sigs on both sides of me and no QRM
> problems.
It all depends on the background noise level, and that of course
also relates to antenna directivity and what is in front of the
antenna, as well as how many stations are on around your
frequency.
On 160 meters, my normal winter nighttime background noise is
about -130dBm or so, and typically the strongest signals are -
50dBm with occasional signals reaching -35dBm or stronger. I find
similar situations on 80 and 40 meters.
What most people also seem to miss is with multiple signals, each
signal can be considerably weaker and the accumulation of signal
power from many moderate level signals can cause IM or blocking
problems. The IM effect is like listening to a crowded CW band with
wide selectivity in the AM mode. You hear brief bloops and bleeps
that really are not there, or have a weak signal pumping in and out
of the noise with the accumulated power of many stations sending
CW that just at brief instances all adds up to sufficient power to
block the receiver.
Of course this problem can be totally masked by noise, so it does
not affect everyone. It mostly is a problem when you use narrow
selectivity (because noise power drops proportionally with narrower
selectivity), have a directional antenna beaming past strong signals
towards weaker ones, and operate crowded bands.
For example, the FT-1000D is almost useless here when
stock....yet many people only experience occasional problems.
Most often, people often don't even recognize problems.
73, Tom W8JI
W8JI@contesting.com
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