All,
It's in the "narrow" bandwidth digital modes like PSK31 and PSK63 that the
filters in the NAR position in the Omni-VI + really shine. The receiving
screen for the PSKxx program is a spectrum analyzer, showing signals vs
frequency. I have a 500 hz filter in the NAR 1 position and a 250 Hz
filter in the NAR 2 position, plus 500 and 250 hz filters in the 6.3 Mhz
IF. When you select the 250 Hz filter in the 6.3 Mhz bandpass and select
NAR2, you have two 250 Hz filters in cascade. Chances are you will not
receive anything; you have to rotate the BPT knob as explained below, so
they are both receiving the signal. You can do this on noise, so it is
easy using the screen. When lined up, this is the closest thing to a "brick
wall" filter the Omni-VI+ will produce. There is a difference on strong
nearby adjacent signals, to the extent that with the second 250 filter
sometimes you can copy the signal and without it you can't.
If "the filters that are already there seem to work real nice" then
perhaps you would not want to bother with the added expense for no
improvement. When would there be an improvement? If you have a strong
signal inside the bandpass of the stock 2.4 kHz bandwidth 9 MHz IF filter
(but not in the narrower 6.3 MHz IF filter), that causes degradation of
readability of a weaker signal inside your narrower 6.3 MHz IF filter,
then a narrower (narrower than 2.4 kHz, not necessarily narrower than the
filter in the 6.3 MHz IF) filter in the 9 MHz IF could make an
improvement. This typically happens in contests or DX pileups.
The choice of which filter to use as your optional narrow 9 MHz IF filter
is not quite as simple as just using the narrowest one you can get. Yes
the narrowest one you can get will give you the best performance as far as
picking out a weak signal next to a strong one, however you loose some
flexibility that way. With a wider filter in the 9 MHz IF you can use to
PBT to move the 6.3 MHz filter bandpass around within the 9 MHz filter
bandpass. This allows you to select a signal without changing the pitch as
would happen using the RIT or VFO. If the 9 MHz IF is really narrow, then
the PBT has to be left in the position where both the 9 MHz and 6.3 MHz
filter bandpasses coincide, or you get no signal through at all.
This is why it is nice to have more than one 9 MHz narrow option filter,
and why I sorta wish I had an Omni VI + instead of a plain Omni VI. For
tuning around the 2.4 kHz standard 9 MHz filter is a bit too wide, and the
500 Hz 9 MHz filter in my Omni VI is a bit too narrow. For digging a weak
one out from between strong signals the 500 Hz narrow option 9 MHz IF
filter is real good. I wish I had an inbetween option. No space for one in
my radio. Having space for only one, I think the 500 Hz one is a good choice.
DE N6KB
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