[Elecraft] K3 receiver desensing on CW during contest

[Bill W4ZV]

Mike I believe there are two separate parts to your post:

> At 50Hz width it was quite clear that many stations call off frequency. 
Using RIT, I could see that it was typical for them to be 70Hz or more away 
from the DX station but I could not hear them in my 50Hz passband. The real 
problem was when one of them was S9 or greater. They completely swamped 
weaker DX stations. Sound within the passband just blanked out. No band 
noise and no DX station. And there was no recovery between dits and dahs. It 
was actually easier to copy the DX station when people were calling directly 
on top of him. You could at least hear him between the dits.

The K3 will desense for signals about S9+25 that fall *within* the roofing
filter but *outside* the DSP WIDTH passband.  Elecraft made a modification
("hardware AGC" in the mod kits list) about a year ago which improved this
problem and raised the threshold from a little over S9 to S9+25 (or maybe
+30).  I don't know your S/N but if it's ~280 or less you should install
this mod.  In my opinion, the threshold for this mod could still be raised a
little more based on my experience with Orion which has a very similar front
end to the K3.  You didn't mention which filter you were using but even if
you had the 200 Hz filter installed, you could have desense from S9+25
signals about +/- 150 Hz from your center frequency.  There's simply not
much that can be done about this until SHIFT resolution is changed from 50
Hz to 10 Hz.  Then you could possibly use SHIFT to put the strong
interfering signal on the XFIL passband slope.

> For my style of operating this was a disappointment. I tried AGC-F and
> AGC-S 
but I forgot to try AGC off. I also varied the AGC THR and AGC SLP. I tried 
NR off and on. (I was not using NB). I tried PRE and ATT off and on. I tried 
riding the RF gain. I tried shifting the passband but shift doesn't work 
well when the width is only 50Hz. Nothing helped. I also tried opening up 
the passband. That didn't help either but I don't remember the results.

> This was not a one time thing. It happened at least 20 different times on 
different bands. I guess this is perfectly normal. I was just wondering what 
operational trick is available to compensate.

This could be a second issue (i.e. separating multiple strong stations)
which Dave K6LL has identified.  I haven't experienced it as yet because
unfortunately I haven't had many pileups of strong stations calling me! 
However I believe Dave W6NL has determined that AGC THR and AGC SLP settings
of 5 and 5 are best for this separation issue.  I leave my AGC PLS set to
NOR and have not experienced any problems that I felt were related to it.

As an aside, I never use AGC-S for any mode and have AGC-F set to 120. 
Using Extreme Isolation EX-29 headphones, I select AF GAIN LO in the CONFIG
menu, set my front panel AF GAIN to 9 o'clock (NEVER higher than 11
o'clock), set PRE/ATT so I can hear band noise when antennas are selected
and then back off the front panel RF GAIN to a comfortable audio level.  In
the contest this weekend with ATT on my TX array and PRE on my Beverages
(using both antennas for diversity), I typically had RF GAIN for both
Main/Sub set from 2 o'clock to noon.  

The K3 was wonderful on Topband in the contest, especially the first night
when QRN was low enough to use both my directional TX array and Beverages in
diversity.  It appears I set a new USA record with 434 QSOs and 79 countries
for 102,858 points, breaking the old record of 81k set by N4PN at W8JI's
station in 2006. 

73,  Bill

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