I recently acquired the coveted 221 filter from a nice ham who had one unused
still in the box from Ten Tec. I installed it in my Omni VI, Opt. 1 with CW
roofing filter mod from INRAD.
It was a big disappointment. The insertion loss made it unusable on most
bands. I promptly reinstalled my INRAD 753 in the 9 MHz position. According
to the specs, the difference should not have been as great as I observed. The
INRAD 753 has a nominal insertion loss of about 9 dB, while the 221 is
specified at 14.0 dB. I estimated a theoretical loss of an additional 2-3 dB
due to the mismatch in center frequencies of the 221 and the roofing filter.
So, there's a difference of 7-8 dB. Yet, the 221 was unusable on 160m, 30m and
40m-- signals were down at least 3 s-units. The receiver seemed starved for
gain. Only on 80m was it useable.
In fact, the difference was so great that I went outside and worked on my
antenna for several hours before concluding it was my filter.
I suppose this particular unit has excessive loss. I'm planning to try it out
again when I receive the new roofing filter from INRAD (the special order one
with 500 Hz offset to match the 221). I expect, though, that the INRAD 753
(400 Hz BW CW filter with 600 Hz offset) will prevail.
This is the second 250 Hz filter I've tried in the 9 MHz position. I should
have listened to Stan at Ten Tec Sales, who (at the time I ordered the Omni VI)
told me I wouldn't like a 250 Hz filter up front.
73 de Chuck NI0C
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