[Scan-DC] BC895XLT Question

[Dave Emery]

On Wed, Apr 10, 2002 at 02:16:57PM -0400, William Rossiter III wrote:
> I decided to listen to the 800 MHz band today on my BC895XLT.  My scanner
> does not cover between 868 and 894 MHz because that is a cell phone band.
> But when I listened between 894 and 895, I heard telephone conversations.
> How is this possible if my scanner blocks the cell phone band?  What was it
> that I was listening to?

	In the USA, the 894-896 band is used for the air to ground 
downlink for those seat back phones in airliners.   It is paired
with 849-851 mhz for the ground stations, most of which are located
at or within a couple of miles of big airports (so they provide coverage
on the ground as well as while flying).

	The modulation used here is digital SCPC (4800 baud QAM) in 6 khz
wide channels - each ground station is allocated a 200 khz wide block in
which all its uplinks are located.   There is usually a continuous
control channel on the high  end of the 200 khz wide block - sometimes two.

	This digital system replaced an old analog system which used
SSB AM with a partially suppressed carrier, also in 6 khz spaced
channels.   A few years back there were still a very few aircraft around
with the old equipment, but I think they have been entirely phased out
by now and all traffic is digital.

	What all this means is that almost certainly did not overhear
the setback phones (AirFone (Verizon/GTE) and AT&T being the two big
carriers), but some kind of spurious response from your scanner to
nearby analog cell traffic.  This would be especially true if you heard
the conversations in nbfm mode of the scanner as the old air to ground
traffic before the digital conversion would appear to be in AM or SSB
mode and not intelligable in fm mode (though it might sound vaguely
speechlike but very very distorted).  The digital signals are a distinctive
hissy buzzing noise - don't sound at all like speech.

	Needless to say, depending on how near the nearest 800 mhz cell
site carrying analog traffic is and how old and cheap your scanner is
there may well be images or other more complex spurious responses that
make cell calls seem to appear between 894-895.  Very new scanners are
required to be designed so these spurious responses do not appear except
when signal levels are very very high (eg the antenna is just outside
the window), but some less expensive older scanners have image responses
to cell phone traffic that aren't all that many db less sensitive than
if they really were allowed to tune the cell band directly.


-- 
	Dave Emery N1PRE,  die@die.com  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18