Steve S. Bosshard (NU5D) wrote:
>
> Reply:
>
> Please refer to modulation index and Bessel functions. The frequency
> response is 300 to 3000 hz emphasized at 3db per octave. Transmitter
> instantaneous deviation is supposed to be limited to +/- 5 kc deviation
> from center. Significant sidebands in FM extend beyond the deviation
> limit, hence emission designator 20K0F3E that equates to about 16 khz
> occupied plus a guard band on either side.
I'm not going to argue over 1 kHz of bandwidth - especially when you are
using a LARGER BASELINE (5 kHz deviation as opposed to 4.5 kHz). Isn't
it odd that the extra 500 Hz adds up to 1 kHz (your figure). Again, I'll
paraphrase what I said before: just because the TX CAN do +/- 5 kHz
doesn't mean it has to be set for +/- 5 kHz. In fact, if you do this in
the commercial service, you're pushing your legal limits. (and not
following the manufacturer's instructions, in all likelihood).
Doing this in the ham band on a 15 kHz channelized subband is ASKING for
problems. Yes, there is no legal limit in the ham bands. If you want to
go to 7 kHz, it's perfectly legal. But don't be surprised when you get
complaints from your 'neighbors'.
> They share part of the same channel (honest - I would not kid about
> something like this). Has nothing to do with receiver - both
> transmitters share common ground.
It has NOTHING to do with the receiver? How do you think the
interference is being HEARD? You honestly don't think that it's even
POSSIBLE that this problem has to do with a receiver that is TOO WIDE?
And you further don't believe that a receiver that is too wide will
suffer adjacent channel interference problems more than one that is not?
If so, I'll leave you to your own version of reality and not continue
this discussion with you.
Of course the receiver has something to do with it. It's POSSIBLE that
it doesn't, and that the problem is TXs that are simply too wide, but
considering the manufacturer, I highly suspect that the receiver is a
factor. Let's use this logical argument. If a receiver can't be too
wide, why should you have to replace anything in one when converting it
to SNFM? Same difference. You may have one in this case that doesn't
confirm to NFM standards. If so, of course it's going to pick up more
adjacent channel energy than it should. Just as an FM receiver (standard
FM: +/- 15 kHz deviation) is going to pick up at least two NFM channels
on either side of center due the fact that it's wider yet. So, is the
answer to lower the TX deviation or frequency response of the adjacent
channels? OF COURSE NOT! Because the problem will still be there - even
with a dead carrier. The answer is to narrow up the receiver so it
conforms to standards, or even lower if possible. Again, just because a
standard may be 7 kHz modulation acceptance, there is no reason why you
can't drop it down to 6 kHz. Yes, more people with TXs that are too wide
will 'chop out' of the passband, but then you fix those TXs to conform
to standards.
BTW, would it be too much to ask that you set your mail client up to
properly quote messages?
Joe M.
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