[Elecraft] Hearing the effect of narrower roofing filter

[Guy, K2AV]

    ..... Maybe
I should have ordered the 200 Hz 5-pole instead, but I got it into my head
that maybe the 8-pole filters were "better" in some way.  I didn't repeat
this mistake when I ordered filters for the subreceiver.

Dick, K6KR

I know I will incur some religious wrath from some.... however here is the
OTHER side of that argument.  It's not that I am denying the issues that are
raised to question getting the two 8 pole CW filters (400 and 300 in
actuality).

I am solving a SINGLE happenstance which occurs over, over, over and over
again in contests, so much so that until the K3 it was in my mind the
LIMITING issue in improving 40m CW DX scores. 

I am at a contest station in eastern NC just off Pamlico Sound. On 40m to
Europe we are using a 5 element wire quad suspended across a 220 foot NE/SW
catenary between two towers. 

Broadcast signals above 7.1 routinely peg meters and light all the lights. 
So do some US stations in the NE, and so do some number of VERY LOUD
European stations (VLS).  WHAT they are doing to be so loud is not part of
my exposition. They just are very loud.

At some point in the contest, one has worked all the VLS, loud, medium and
well-antenna'd QRP signals. What remains is a bewildering and seemingly
bottomless pool of stations that can hear us (QRO on 5 elements) and are
trying to work us on antennas with the gain of a basement floor joist wet
noodle antenna. There are hundreds and hundreds of these. We have some
number of recordings made in Europe of these, and they are quite weak over
there. 

It is guaranteed, only a matter of time, and sometimes immediate, that a VLS
will settle in the next slot above or below. They may be as close as 350 Hz.
The problem now is working the QRP wet noodle station who is into the noise
without hardware AGC pumping or other effects from a VLS INSIDE the roofing
filter that was set to hear stations who will call sometimes +/- 200-250 Hz.  

I need the roofing filter is to get down 30 db as quickly as possible,
without giving up too much of the +/- 250 Hz. The 400 Hz is a good width
even for VLS +/- 500 Hz.  If a VLS squeezes me on one side, I only want to
give up +/- real estate in the roofing filter on that side. The procedure is
to drop to 300 hz roofing and DSP and move the RX center 50 hz away from TX
frequency and the VLS. I give up listening so far on the side of the VLS,
but keep the real estate on the other side.  

It is a matter of the width out to the edge and the largest db drop per 10
hz in the skirts thereafter. The roofing filter keeps the VLS from getting
into the hardware AGC or pushing the DSP to the extreme.  

I have used both of these filters for years in my FT1000MP and am completely
familiar with their shape and use in a contest. 

I have heard it said that the 200 5 pole filter will do better for picking
out signals in a very crowded situation, perhaps so in an extremely tight
situation. But thus far I have been able to go narrow with the 300 8 pole
and it is the DSP handling the work there. 

At some point I would like to be able to tune the center of the DSP CW
selectivity up/down at 25 Hz rate using RIT *WITHOUT* moving the position of
the roofing filter relative to the band.  The point of the roofing filter is
to reject the VLS in the next slot above and below me. Tune the whole thing
up or down to listen to a weak station off-frequency for whatever reason and
you let one of the VLS in under the roof.

Just the view from the other side of the river... :>)


73, Guy. 

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