[222mhz] ACSB

[Chris Boone]

> -----Original Message-----
> From: 222mhz-bounces at mailman.qth.net 
> [mailto:222mhz-bounces at mailman.qth.net] On Behalf Of Dr. 
> Gerald N. Johnson
> Sent: Wednesday, November 07, 2007 11:32 AM
> To: 222mhz at mailman.qth.net
> Subject: Re: [222mhz] ACSB

>> Since we hams know how to tune and how to listen to noise and 
> don't so much need the perfect tuning and squelch of the 
> pilot tone working to make SSB sound like WIDE BAND FM, we 
> probably can get along without the ACSB parts and just use 
> some of the pieces for ordinary SSB. And SSB repeaters have 
> been built for nearly as long as FM repeaters, and many flown 
> on Oscars.

Ahhh no, SSB rptrs have not been around as long as FM rptrs...in fact, some
of the earliest relay stations were likely more FM than AM (modulated
oscillators).......SSB did not come into being until after WWII.....and SSB
rptrs were not feasible without a pilot tone...it is very hard to maintain a
correct offset with older gear...today PPL gear can generate such an offset
but for the RPTR, now you have a rcvr and a transmitter that must be somehow
freq locked to one standard...otherwise, the drift and offset will be
different in both and the audio difference will be very noticable..
OSCAR sats do not run SSB "Repeaters" but translators...where the input band
is mixed up or down to another band..thus any mode can be used as there is
no demod/remod of the baseband audio...A Linear Translator has problems on
land based use though..its power output is divided among the users (10watts
out with 10 users = 1 watt per signal!)...also the output signal is
proportional to the input signal....(AGC driven, etc)...thus a user CLOSE to
the LT site will be the strongest and get out the farthest....a weak distant
signal will be weaker through the system and may not be heard on the LT
output in its own area...unlike an FM rptr where the carrier can be! A LT in
space on a OSCAR,etc. has the advantage of EVERYONE being a couple of
hundred miles away...soo everyone is on an equal playing field..but one on a
mountain top or tower is another problem altogether.

A single freq SSB rptr NEEDS a pilot tone to maintain freq stability and
audio quality....and it will require a separate RCVR and XMTR so all signals
can be heard at the same output power (provided the weaker input signals are
AGC up so their audio drives the SSB xmtr to the same PEP output). Constant
carrier rptrs (AM or FM) are much easier to work with in this case.
 
> As we go narrower FM deviations, we might as well go raw SSB 
> though in commercial circles SSB would give way too much 
> range. The narrowness off deviation and channel gives shorter 
> range but gives more channels to be used and allows closer 
> reuse of a frequency and though the narrow FM works poorly 
> compared to the wide FM did when I was 40 years younger (and 
> I've been on 2m FM since 1959, first with a club station and 
> then with my own radio in 1961) the added channels make it 
> sell. It looses range and quieting (and so recovered S/N). 
> Commercial interests are not ham interests and going narrow 
> is far less a benefit on ham bands, but its here as a spill 
> out from commercial practice.
> 
> With 15 KHz deviation we used to work simplex mobile to 
> mobile at distances now worked only with repeaters at least a 
> few hundred feet up.

Wider deviation does lower the "threshold" of signal detection in FM...but
it also gives you weaker S/N. A Modulation index of 1.333 (5 kHz dev/3 khz
audio) is a good spot for excellent S/N and receiver selectivity...going
wider means wider IFs which means worse sensitivity (ever seen a rcvr that
hears wideband FM have a sens of 0.2uV? nope) and selectivity. Wider FM does
not cover any farther than narrow FM....it DOES allow for "weaker" signals
to be copied....but that's it; once above the threshold where the limiters
saturate, it doesn't matter wide or narrow. There are trade offs in every
aspect...and the 5 kHz deviation currently used in NBFM is the best overall.
RF wise, the carrier still covers the same range....wide or narrow FM. SSB
is ok and gives better range but for commercial use, it is too cumbersome to
use. You have to have a RIT control OR supply one automatically (hence the
pilot tone)....most two way users don't even want to deal with squelch..

Chris
WB5ITT