[Trunkcom] 400mhz trunking system help needed

[Brian J Cathcart]

On Thu, 03 Jan 2002 01:14:04 +0000 b52hbuff@att.net writes:
> I'm familiar with programming the Base and Offset for
> a 400MHz system using my 245.  It doesn't ask for
> a 'Offset Channel'.

Correct.  The "Offset Channel" is not used in the 245xlt since it only
allows one set of Base & Offset to be entered.  Motorola VHF and UHF
systems can have up to 3 sets of Base & Offsets, but the majority of them
(in the U.S. anyway) only use one set.  The 780xlt has the ability to
enter up to 3 sets, and thus needs to know what the Offset Channel is for
each set.

So what is the Offset Channel?  It is simply the "starting point" of each
set.  Below is an exerpt from one of my books that explains these systems
and how it relates to the 245xlt.  I have not had time to update it yet
with 780xlt info, but I have thrown a snippet at the end.

=-=-=

For these systems, you must enter a base frequency and the offset.  Why? 
To understand this, it is necessary to understand a little bit about how
the control channel works, and how VHF/UHF systems have to be configured.
  All Motorola systems work by transmitting frequency information over
the control channel.  The radios only need to "listen" to the control
channel to be told what frequencies to use.  This frequency information
is transmitted as 3-digit hexadecimal numbers, which the radios interpret
as frequencies to use.  On 800-MHz systems, these hexadecimal numbers are
001 (851.0125) to 2CF (868.9875); on 800-MHz splinter channelss, these
numbers are 2D0 (866.000) to 3FF (868.975); on 900-MHz systems, these
numbers are 001 (935.0125) to 18E (939.9875).  Each hexadecimal number
always equates to a specific frequency; for example:

- Hex 18D always means 860.9375 on 800-MHz systems, or 939.975 on 900-MHz
systems.

- Hex 144 always means 859.1125 on 800-MHz systems, or 939.0625 on
900-MHz systems.

VHF and UHF systems are different - they use hexadecimal numbers 17C
through 2F7 to represent receive frequencies, but those frequencies can
be anything,.  In other words, hex 17C can be 451.575 in one system and
406.950 in another.   The hexadecimal numbers do not necessarily equate
to the same frequency from system to system.  [NOTE: in case you're
wondering, hex 000 through 17B are transmit frequencies which are not
used by the TT but are displayed on TRUNKER]

The system designers have to program the trunking controller and radios
with the "base" frequency and the "offset".  Now perhaps you're used to
those two terms in another light, that of the "base" being a base station
and and the offset being a repeater offset.  But in Motorola VHF/UHF
trunking, these terms mean something completely different.  The "base" is
the starting frequency of the system, while the "offset" is the step size
in between each frequency (12.5-KHz, 25-KHz, etc).  Thus, if the starting
frequency is 406.950 with a 25-KHz offset, the system would look like
this:

 17C 406.950
 17D 406.975
 17E 407.000
 ... ...
 ... ...
 all the way to....
 2F7 416.425

Or, in another system where the starting frequency is 451.525 and the
offset is 12.5 KHz, it would look like this:

 17C 451.525
 17D 451.5625
 17E 451.575
 ... ...
 ... ...
 all the way to...
 2F7 456.2625

Now, think of the 245xlt.  It has to know what 17C is, and what the step
size (offset) is in order to know what frequencies the rest of the
hexadecimal numbers represent.  By setting the proper starting frequency
and step size (offset), it will know what frequency to tune when it is
told, for example, to tune hex 193.

Even though the designers set the starting frequency, they do not have to
use it in the system.  In other words, instead of using 451.525 as the
first frequency, they might use 451.600.  451.575 is still the base
frequency for the system, it is just not the lowest-numbered frequency
used.

SO HOW DO I FIND THE BASE AND OFFSET?
Using Trunker!  It gives you the hex values of each frequency used in the
system.  Once you know what Hex 17C is (the Base frequency), you can try
different combinations of offsets until you find the correct combination.

THERE'S ALWAYS A CATCH....
Most UHF systems in the U.S. use just one set of base and offset
frequencies, and you'll find them set up as described above.  But some
systems will use more than one base frequency and offset.  In fact, all
Motorola VHF/UHF systems can be configured with up to 3 base and offsets
defined.  Thus, it could look like this:  

 17C - 1A9 = base is 451.550 with 25-KHz offset
 1AA - 211 = base 462.600 with 12.5 KHz offset
 212 - 2F7 = base 464.300 with 25-KHz offset

ENTER THE 780xlt
The 780xlt introduced the ability to enter 3 sets of base & offset, and
something new called the "Offset Channel".  This Offset Channel is simply
the channel number of the Base frequency for each set.  The channels, 380
to 759, are simply the decimal equivalent of Hex 17C to 2F7.  Thus, what
the 780xlt is looking for is the hexadecimal value of the base frequency,
but in decimal form.  Thus, using one of the examples above where the
starting frequency is 406.950 with a 25-KHz offset:

 17C 406.950
 17D 406.975
 17E 407.000
 ... ...
 ... ...
 all the way to....
 2F7 416.425

The "Offset Channel" in this case is Hex 17C, which translates to decimal
380, the setting the 780xlt is looking for.  Since the systems can have
up to 3 sets of these, the second and third "Offset Channel" will be the
next Base frequency value.  Againm, an example from above, a system with
3 sets of Base & Offset:

 17C - 1A9 = base is 451.550 with 25-KHz offset
 1AA - 211 = base 462.600 with 12.5 KHz offset
 212 - 2F7 = base 464.300 with 25-KHz offset

Offset Channel 1 would be Hex 17C (decimal 380), Offset Channel 2 would
be Hex 1AA (decimal 426), and Offset Channel 3 would be Hex 212 (decimal
530).

[ does that make sense?  it's midnight now and I have to go to bed  : O ]



> If I have a 'default' system of say.. Base: 406.0MHz,
> Offset: 25MHz.  What does the 245 default to for
> it's 'Offset Channel'?

Internally the 245xlt is using decimal 380 (hex 17C) as the "Offset
Channel".

--

The Scanner Dude
Brian J. Cathcart - KE4PMJ
South Florida Trunking Guide - 5th Edition (On CD-ROM too!)
Palm Beach County Frequency Directory - 4th Edition (On CD-ROM too!)