On 20 Aug 2011 08:26:15 GMT, Rob wrote:

Jeff Liebermann wrote:
TNC. There's also the stuck transmitter on commercial and
public safety frequencies. Those are fun and gets us plenty of points
from the various agencies. Extra credit to the local comm shop that
likes to defeat the timeout timer, for no obvious reason.

It does not seem the public safety radio devices always had this
timeout timer either.

Methinks that just about every radio, by Motorola and Kenwood have a
transmit timeout timer.
http://802.11junk.com/jeffl/pics/drivel/slides/too-many-radios.html
The dip switch programmable radios (SP10) do not have a timer, but
literally every radio that requires a computah to setup has a timer.
Note that I was talking about mobile radios all of which are
programmable, not handhelds.

Before the public safety services here switched to a digital system,
they used NBFM equipment from companies like Motorola, and it was very
common to hear stuck transmitters that blocked an entire repeater channel.
(which often consists of multiple receivers and transmitters over a city area)

That's a programming issue. The timer is off by default and must be
enabled and set for a specific number of seconds. 60 seconds is the
typical timeout.

Digital radio comes in many flavors. One of the big advantages of
digital is that the repeater and channel can handle more than one
transmission at a time (either by TDM or FDM). Therefore a stuck
digital transmitter does not timeout or disable the repeater. In
addition, many such radios are full duplex, which allows the
dispatcher or comm shop to remotely disable the transmitter and yell
at the user. Whether the PSAP is setup and trained to do this is
debatable.

Police used mobile transceivers in the cars on VHF and portables on UHF
so it usually was possible to alert the users on the other channel and
hope the stuck PTT key was found that way, but on major events like
queen's day it often happened that the portables channel was jammed for
half an hour or so before the culprit was found.

Yep. Simply identifying the stuck transmitter is often insufficient.
I've heard stuck transmitters in vehicles rolling down the freeway for
an hour with a stuck transmitter. It was obvious who it was but
nothing short of an RF seeking missile was going to get their
attention. In one case, someone noticed that they were listening to
specific local radio station. The dispatcher called the radio station
and had them make a special announcement to the driver to unstick the
transmitter. It worked, instantly.

Of course the equipment was aging by that time, it probably was from
the eighties. Now they are all on TETRA and AFAIK that system allows
the operator to just tune out stuck transmitters or devices that fell
into wrong hands.

Yep. Nice feature. In the US, the high end radios are P25. Various
other narrow band solutions exist, most of which have remote
programming and remote deactivation. Despite all the government
programs intended to provide interoperability, the manufacturers (and
the FCC) have gone out of their way to introduce proprietary
enhancements and introduce intentional incompatibility.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558