I said I'd report back....
XYL and I were out with the EX500s today. I copied the channel we
normally use for simplex and changed nothing but made it a narrower
channel.
Results. Noise squelch seems sloppier (normally I use DPL or PL, so
that really isn't to big of a deal), audio fidelity is reduced. We
weren't far enough apart to really test range. I think the audio
quality was still pretty good, but when do do "ok, go back to channel
3 now..." It's quite clear the narrow sounds quite noticeably better
than the narrower.
73 DE N0MJS
P.S. I also wonder about the frequency accuracy of radios going to the
super-narrow band. I've looked at a lot of ham rigs on my service
monitor. They are usually worse than the commercial radios in this
area. Isn't that going to have a more pronounced effect?
On Oct 3, 2009, at 4:22 PM, John Sehring wrote:
I think it's worth repeating (no pun intended!):
0. In a narrower band FM system, with only the carrier present, you
may well get a bit more ultimate quieting sensitivity (but not
necessarily better SINAD) as the receiver's IF bandpass
(selectivity) is narrower, letting less noise thru. However, the
question is: how much of that slightly increased sensivity is
actually useable?
1. Reducing FM deviation to less than about 5 kHz results in less
power in the sidebands, which sidebands convey the intelligence (the
carrier is just there to enable the usual demodulation (detection)
process). As the detector needs the sideband energy, even granting
(1) above, you'll have less recovered audio available. The signal's
spectrum then begins to resemble that of an equivalently-modulated
AM signal; the major difference is that with an FM signal, the
carrier is 90 degrees out of phase with the sidebands, whereas with
AM, carrier and sidebands are in phase.
2. Reducing FM deviation (and narrowing IF bandpass) allows more
distortion in receivers at low (fringe) signal levels, so it's less
able to deal with things like multipath propagation, AM noise, FM
noise (yes, there is such a thing), and co-channel interference.
Signal to noise ratio is thus reduced.
3. Squelch action becomes sloppier because the demodulated audio
spectrum which is used for noise-operated squelch is quite a bit
less when using narrower band FM. Rule of thumb for the squelch
detector's bandpass: it extends from A) just above the voice audio
band, say, 4 kHz, to B) about one-half the IF bandwidth. The latter
is distinctly less, so the squelch sensing bandpass is less making
squelch action less responsive.
If you use an audio spectrum analyzer to look at a demodulated FM
signal, you can see the spectral differences between 75 (FM
broadcast), 25 (NTSC TV sound), 15, 5, and 2.5 kHz deviated signals,
esp. as the signal strengths are reduced to zero.
--John
