On Wed, 2008-04-30 at 19:17 +0000, Marinus Loewensteijn wrote:
> Recognizing a wealth of expertise on this mailing list I like to invite your
> input on something that has kept me occupied for the past few days.
>
> I like to add a CW filter to my Omni-C. TenTec has sold out of the 217 (500
> Hz) and still has the 218 (1.8 kHz) and 219 (250hz). In planning is a
> replacement manufactured by Inrad.
>
> The consensus seems to be that the 500 Hz is more frequently used than the
> 250 Hz.
I think that's true. When tuning the band with even a good 500 Hz filter
you have to tune SLOWLY else you pass signals between words if not
between letters. And you find many holes. Both of those are much more
pronounced phenomena with a 250 Hz filter. Trouble is when you find a
hole on a really busy band with a 250 Hz filter, the users along side
may be using 500 Hz or wider filters and they get annoyed by your CQ in
what you found as a hole but its in the pass bands of receivers in QSOs
on both sides of that hole.
I have a 200 Hz for my Collins S-Line that I don't remember ever using.
I used a 270 Hz Kenwood that rang excessively and lead to wearing out my
ears in 3 or 4 hours of continuous contest operation. I've had a 400 Hz
Inrad in a Kenwood that I could run at least 6 hours or midnight to dawn
during the US FD before loosing my ability to copy. I have a 500 Hz
Tentec in my Corsair II and a 500 Hz Collins mechanical filter in my
FT-857D. I find the 500 Hz filter is effective at improving signal to
noise of weak signals, but the DSP filters in the 857 ineffective at
that task. But I run it in SSB bandwidth as often as not letting my ears
supply the final selectivity. The 857 can be set up to transmit CW while
in a SSB mode which is very handy at VHF when contacts involving weak
signals may be mixed mode.
>
> Have been looking at a few alternative options from Inrad, the 8 pole 1000 Hz
> #766, the 8 pole 400 hz # 759 and the 4 pole 600 Hz #763. Looking at the
> curves on the old Inrad website I see that the 1000 Hz filter has approx 4dB
> insertion loss, the 400 Hz approx 9 dB and the 600 Hz about 2.5 dB.
>
> The Omni-C has a filter board that can take the 1.8 kHz and the 500 Hz
> filter. This board has an amplifier to offset the filter losses. The 1.8 kHz
> filter position has a 1 dB amplification and the 500 Hz filter position has a
> 7 dB amplification.
>
> (The 250 Hz filter has an approx 14 dB loss)
>
> I am thinking about using the 4 pole 600 Hz filter: I feel it will provide
> better IMD / dynamic range performance because it can go in the 1.8 kHz slot.
> This should go well with the 500 Hz audio filter which is built in.
>
The 4 pole filter will have broad skirts, it may need the help of the
audio filter in crowded band conditions. There's always a trade off.
> Am I missing something / have an error in my reasoning?
There are many trade offs in filter selection and use. Sometimes the
trades depend on the density of signals that you wish to separate and
then for that occasion a narrower or wider filter is more appropriate.
The many choices for filters exist because of that. Of course some would
say the solution is a software defined radio where the user can have
literally any desired filter bandwidth from a few Hz to 20 KHz. But then
the user can be overwhelmed with too many choices and never come to a
conclusion on which is best.
I think you will find most of the time that the 400, 500, or 600 Hz
filter will do you quite well and if you had them all three in the radio
you would not find more than twice a year that you could switch between
them and find one significantly better.
>
> Many thanks, 73, Marinus, ZL2ML
>
73, Jerry, K0CQ
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