[TrunkCom] Base, spacing freq.'s and offsets ???

[Brian J Cathcart]

On Tue, 23 Apr 2002 22:10:10 -0400 Brian J Cathcart
<scannerdude@juno.com> writes:
> On Tue, 23 Apr 2002 16:00:26 EDT Campd715@aol.com writes:
> > I need some help with programming my 780XLT. I want to track a 
> > Motorola UHF trunked system that uses frequencies from 461 to 464 
> > MHz.I need more explanation of BASE, SPACING FREQUENCIES
> > and OFFSET CHANNEL. Pages 55 & 56 in the 780XLT operating
> > guide aren't enough info for me.

SORRY about that, I hit the "send" key too quick.

Here is a section from one of my previous books that explains it:

----------

For Motorola VHF and UHF systems, you must enter a base frequency,
spacing, and the offset channel.  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 as
3-digit hexadecimal numbers, which the radios interpret as frequencies to
use.  On 800-MHz and 900-MHz systems, these hexadecimal numbers always
equate 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 Trunk Tracker but are displayed on programs such as TRUNKER
and TREPORT]

The system designers have to program the trunking controller and radios
with the "base" frequency and the "spacing".  Now perhaps you're used to
"base" being a base station, but in Motorola VHF/UHF trunking, this term
means something completely different.  The "base" is the starting
frequency of the system, while the "spacing" 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 spacing, 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
spacing 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 780xlt.  It has to know what 17C is, and what the step
size (spacing) is in order to know what frequencies the rest of the
hexadecimal numbers represent.  By setting the proper starting frequency
and step size (spacing), it will know what frequency to tune when it is
told, for example, to tune hex 193.  (NOTE: the "Offset Channel" setting
is there mainly for those systems that use more than one set of these
base and spacings, and is explained below)

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 you find the Base and Spacing?  Using a program like Trunker or
Treport.  They give 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  spacings until you find the correct
combination.

NOTE FOR SOME SYSTEMS....
Most UHF systems use just one set of base and spacing frequencies, and
you'll find them set up as described above.  But some systems will use
more than one base frequency and spacing.  In fact, all Motorola VHF/UHF
systems can be configured with up to THREE base and spacings defined. 
Thus, it could look like this:  

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

Now what is the "Offset Channel"?  It is simply the decimal value of the
starting frequency for each set.  For multi-set and single-set systems,
Offset channel 380 will almost always be used for the first set and
should not have to be changed. For the remaining sets, you will need to
know what the starting frequency is, and its hex value.  In the example
above, you would set the scanner to:

        Set 1: Base = 451.550, 25-KHz spacing, Offset channel 380
        Set 2: Base = 462.600, 12.5-KHz spacing, Offset channel 426
        Set 3: Base = 464.300, 25-KHz spacing, Offset channel 530

As you can see, Set 1's starting hex was 17C, which converts to decimal
380; Set 2's starting hex was 1AA, which converts to decimal 426; and Set
3's starting hex was 212, which converts to decimal 530.

Multi-Set systems are not as common in the U.S. as single-set systems,
but they are numerous in Canada and elsewhere.

--

The Scanner Dude
Brian J. Cathcart - KE4PMJ
South Florida Frequency & Trunking Guide - 6th Edition (On CD-ROM too!)