[Elecraft] K3: use of 1.8 filter - clarification

[Bill W4ZV]

Hi Doug,

##### You asked:

Bill...I do have a question or two, with respect to your statement below.

1.  Just how often do you find that you run into a situation such as
you describe below?  I don't mean simply, "during every a contest," I
mean more like how often during a contest?  For how many Q's?

##### Fortunately not very often.  I operate 10m almost exclusively in SSB
contests.  If a hippo comes near my watering hole, I first make my presence
known (ask him to QSY, etc).  But rather than pointlessly (pun intended)
slugging it out, I'll move (lots of room on 10m).  Of course moving is not
an option on many bands (e.g. 20m) in SSB contests, otherwise you'll not
make many QSOs at all.

2.  If the really close and really strong signal is pumping your AGC,
why not just turn off the AGC and ride the RF?  This is not a
rhetorical question, because I know you do this.  So I'd like to learn
why you would, in some situations, prefer to not do that.  I would
love to learn a new trick!

##### Remember that you CANNOT turn the hardware AGC off because it's always
on to protect the ADC in the DSP stage.  HAGC will still cause pumping even
if the DSP AGC is turned off.  Of course you can use an attenuator but that
also has consequences for copying weaker signals on your frequency.

3.  And just out of curiosity (on SSB), how long would you tolerate
being so close to someone who would pump your AGC?  Of course, for me
(QRP), the answer is zero seconds.  :-)

##### It depends.  If I think someone is intentionally trying to steal my
run frequency, I may stay longer than if I think it's accidental.  In the
recent CQ WW, I had one station try this twice but they left after ~5
minutes each time.  The top operators NEVER do this but I keep a mental list
of some folks who routinely try it and sometimes return the favor.  ;-)  It
often helps to point one antenna in my 3-stack toward someone who
"accidentally" lands on my run frequency, so they can hear me better.  :-) 
Fortunately this does not happen often on 10m (unlike more crowded bands).

73,  Bill

W4ZV said:
I agree IMD reduction is not a big issue on SSB but blocking dynamic
range (BDR) is the primary reason I sometimes need a narrower filter.
If you have an S9+25 SSB signal CQing next to you (sometimes the case
in  crowded contests), a narrower filter will prevent it from
de-sensing your receiver.  If part of the interfering signal falls
inside your 2.7k filter, signals on your frequency will be gain
modulated by his signal (commonly called   AGC pumping).  In this case
a narrower filter will prevent the problem.  Of course when this
happens, we still have to contend with his transmitted "splatter"
(typical SSB TXs run -35 dB IMD which would result in ~S7 splatter
from an S9+25 signal), but at least you can still copy signals at S7
or above (and lower when he's not transmitting).

73,  Bill




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