On Wed, Jan 21, 2015 at 12:55 AM, Dr. William J. Schmidt, II <
b...@wjschmidt.com> wrote:
> These are roofing filters, so you will only notice a difference in
> adjacent frequency rejection of loud signals... so if you live out in the
> country and don't have any loud stations near you, it’s not likely you will
> ever perceive a difference between 5 pole and 8 pole filters.
Will beg to differ here. It certainly is not just the loud local at issue.
Another is the dB between the very weak desired IN-channel signal and the
very loud close-in UN-desired station that is just barely up or down from
you.
If I had a dollar bill for every time I was trying to copy a very weak 40m,
in-the-noise EU QRP station running a noodle antenna on the floor of his
basement, with a 10 kW mildly clicky Italian station up 300 Hz with a three
element beam up 180 feet aimed at the US, I could buy a lot of new
equipment.
Of course, we were using a five element full sized quad on 40m, but the K3
showing 45 over 9 for his signal seemed high. Between DX contests I did a
complete alignment and calibration. The next time I ran into that station
he was only (?) 40 over 9. Or maybe that was propagation. The ambient noise
was S4. That's S4 in the 350 Hz 8 pole filter ("250"). So I'll give the
argument the benefit of the doubt here call the QRP station S4, although
probably he was only S3 at best. That's a 65 dB difference (probably 70)
between the wanted and unwanted. What the 5 poles do NOT have, and the 8
poles DO have, is sharply steep skirts.
The problem is to drop 65-70 dB in about 170 Hz. And do it without tuning
off your run frequency. Knowing those kinds of situations, the other ops
would not let me put a rig with 5 pole filters on the line. The 8 pole 350
with the DSP at 350 drops 65 dB off center zero in 250 Hz. With the NB set
for medium key click cancel, that takes out my 40 over 9 up frequency
neighbor and most people calling him and allows me to strain to copy the
noodle QRP station with nothing but ambient noise as the enemy.
Do I need that in mild casual operating? Heck no. But it serves me in many
more situations than just when I have another station within a quarter mile.
If I took my K3 out to the contest station with 5 pole filters for CW
running bandwidth, the other ops wouldn't let me put it on the line. It
would be an emergency spare only.
As for SSB contesting, I use the 8 pole 1.8 roofers, and it just barely
uncovers some signals from interference up and down.
73, Guy.
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