Hi Chuck,
>The insertion loss made it unusable on most bands.
According to Inrad's plots for 9 MHz filters
which should be very similar to Ten-Tec's filters:
Filter BW Loss
756 2400 2 dB
753 400 9 dB
760 250 13 dB (similar to #221)
http://www.qth.com/inrad/tentec_menu.htm
(and click "Performance Curves")
If I understand the way the Omni VI works, signals
first go thru the roofing filter, then 9 MHz 2400 Hz, then
through the optional NAR 9 MHz filter before going
though the 6 MHz filters. This means you will see
about 13 dB additional insertion loss when the NAR
filter is selected, so what you are seeing is probably
normal. You should also be seeing about 9 dB
loss with your 753, which may not bother you as
much as 13 dB.
BTW, the insertion loss of the 6- and 8-pole
TT 219 and 217 are why Orion has an added stage
of 12 dB amplification. Of course that created another
problem by degrading IMD performance versus
the stock 1000 Hz 4-pole, which has much lower loss
and thus does not need additional amplification. The
ultimate solution was the Inrad #762, which is essentially
the same 600 Hz filter in Inrad's kit for the Omni (slightly
different center frequency). The #762 is designed to
be placed in the 1000 Hz slot since it does not require
amplification, and it exceeds the 1000 Hz filter's IMD
performance by 6-7 dB at 2 kHz spacings and around
10 dB at 1 kHz spacings, because its actual 6 dB BW is
about 640 Hz versus 1070 Hz for the stock 1000 Hz .
I don't feel a roofing filter more narrow than 600 Hz
makes sense for contests. 600 Hz BW implies someone
would be +/- 300 Hz from your transmit frequency which
not be productive for anyone in a contest. Of course
it is possible to have strong stations spaced closer in a
large simplex pileup!
Bottom line is I doubt there is anything wrong with
your #221 filter...13 dB loss just comes with the territory.
73, Bill W4ZV
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