Can I Determine Base/Offset/Spacing?

[Dewey]

Hello all,

Recently, the Metropolitan DC PD UHF system (603d) has started using new (additional) frequencies. One of the new frequencies is lower than the base frequency, yet the radios have not been reprogrammed since 2004. Therefore, I believe that they are now operating on what was a dormant 2nd base/offset/spacing. The old, used to be correct base/spacing/offset was 458.775/12.5/380, and the frequencies were 460.025, 460.100, 460.150, 460.200, 460.250, 460.275, 460.325, 460.350, 460.400, 460.425, 460.450, 460.475, & 460.500. The newly added frequencies are 453.450 (confirmed) and 460.375 (unconfirmed by me).

Now, I've said all that to ask this. Is there something I can look at, such as UniTrunker's data dump, or Treport, that will be able to pull out information to get the correct base/spacing/offset?

Thanks,
Dewey

 

[SCPD]

Now, I've said all that to ask this. Is there something I can look at, such as UniTrunker's data dump, or Treport, that will be able to pull out information to get the correct base/spacing/offset?

If you've been able to match each channel as it appears on UniTrunker. Unitrunker has a test or trial mode where you can plug in channel numbers for a given base / offset / spacing and it will compute the frequency. Use that to verify your work.

Let's make a good first guess on spacing. Take two known channels. By know I mean you known both the channel numbers and frequencies. Subtract frequencies - write down the difference. Subtract channel numbers - write down that difference. Divide the frequency difference by the channel number difference. That number will usually be a round number like 5000, 12500, or 25000.

In Unitrunker you can insert a new blank band plan entry. From the main menu, select "nsert -> and plan". Make a wild guess for base - for UHF let's say 406.0 mhz for starters. Use an offset of 380. This "just works" for Motorola systems because the offset is usually somewhere between 380 and 760 (which is the valid range for output channel numbers). Type in the spacing value you calculated above.

Near the bottom is a place to type in a channel number. Using the band map entry the program will calculate the frequency. Enter one of the known channel numbers. Does the computed frequency match the actual frequency? If not, how far off is it? If it is 1.5 Mhz too high, increase your base frequency by that same amount and try again. If it is too low (by 350 khz for example), reduce your base frequency by that amount.

Try other channel numbers to see if they also work for this base / offset / spacing set of band map values. If it matches all known channels then you're done. If it matches a group of them - but not others; you may need to create a second band plan entry and repeat the process for these other channels. If it only matches one or two channels (out of a dozen or so), then you may need to try a different spacing value.

Beware that because of the way the algebra works on these mappings, you can have two different looking base / offset values that actually map the same set of frequencies. At that point it's just a matter of preference. Some folks like an offset of 380, others like an offset of zero.

*shrug* maybe someday I'll get around to writing a "band wizard" that will figure all of this out.

-rick

 

[Dewey]

Take two known channels. By know I mean you known both the channel numbers and frequencies. Subtract frequencies - write down the difference. Subtract channel numbers - write down that difference. Divide the frequency difference by the channel number difference. That number will usually be a round number like 5000, 12500, or 25000.
-rick

Thanks Rick... there's one thing I would like to get a better grip on though. When you say "Channel Numbers", do you mean the LCN (offset channel number), or the channel frequencies in order; such as, 1, 2, 3, 4,..., etc?

If you mean the 1, 2, 3, 4,..., etc, this shouldn't be too hard, BUT, if you mean the LCN's, this isn't going to be easy. Up until the addition of the new frequencies last month, LCN 380 started at the old base of 458.775. Now that one of the new frequencies is 453.450, I have no idea where the LCN 380 will fall.

Either way, I going to start working on what I believe that you are saying. Since the new frequency is now at the top of the frequency list, I can see (in UniTrunker) where we are now missing A LOT of communications based on the hits to that frequency.

Dewey

EDIT: Ok, this is really bugging me now. I removed all of the Moto603d files, and ran UniTrunker again. When it populated the new frequency file, the new frequencies still had the proper LCNs to go with the 458.775/12.5/380 base/spacing/offset... all EXCEPT the newly added frequency of 453.450, which consisantly shows a LCN of 380. I've tried entering a second base/spacing/offset of 453.550/5,12.5,50/380, and all this does is stop all reception on this system. Oh well, back to the drawing board!

 

[Dewey]

Rick, THANKS for your help, I'm now getting somehwere! I guess the order of the tables count also. When I switched things around and made 453.450/12.5/380 the first table, and 458.775/12.5/380, the second table, things came to life! Now, I'm not too sure about that spacing of 12.5 in the first table, but since it only applies to a single frequency so far (453.450), I'm guessing that it's not important unless additional frequencies using that table come into play.

Again, THANKS,
Dewey

 

[SCPD]

I've tried entering a second base/spacing/offset of 453.550/5,12.5,50/380, and all this does is stop all reception on this system. Oh well, back to the drawing board!

Thanks Dewey. I'll log this as a bug. The worst that an incorrect bandplan should do is display an incorrect frequency - not cease decoding.

-rick

 

[Dewey]

Thanks Dewey. I'll log this as a bug. The worst that an incorrect bandplan should do is display an incorrect frequency - not cease decoding.

-rick

Rick,

I apologize for the misunderstanding. When I said that the reception stopped, I was referring to stand alone scanner reception. As I was trying different tables, I would plug them into the scanners to verify that they were working. I was going wrong because I would program the new 453 table into the scanner "behind" (table #2) the primary 460 table. Once I decided to see if order made a difference, and programmed the 453 table in as #1, and the 460 table in as #2, everything came to life.

Dewey

 

[SCPD]

When I said that the reception stopped, I was referring to stand alone scanner reception.

Good - one less bug to worry about.

-rick

 

[DaveH]

Rick, THANKS When I switched things around and made 453.450/12.5/380 the first table, and 458.775/12.5/380, the second table, things came to life! Again, THANKS,
Dewey

The base 458.775 was never really "correct" but it would make the system
trackable with older scanners that can only handle one base/offset/step, with
a base of 380. I strongly suggest adjusting 458.775 up to (say) 460.000 and
increasing the offset accordingly. Using two bases with the same offset is
going to cause confusion. The frequencies above 460MHz might actually be
in range 3, and range 2 might be in the 451-456 sub-band.

Also, LCN is not the right word here...it's an EDACS term for the channel
number which doesn't relate to Moto systems. If you can read hex channel
numbers from UniTrunker, you can figure out the real base frequency for
the lower range...just because 453.450 is the lowest frequency in use
doesn't mean its offset is 380, in fact, most likely isn't. You'll need to find it
exactly to make a scanner track.

I also advise against trying to use a offset of 0. The input frequencies for
range 1 start at 0, and if you start trying to apply this to the outputs, more
unnecessary confusion will set in... Trunker will show input and output
frequencies simultaneously for VHF/UHF systems, and channel numbers in
the range 0-379 should be ignored.

Dave

 

[DaveH]

just because 453.450 is the lowest frequency in use
doesn't mean its offset is 380, in fact, most likely isn't. Dave

Sorry, just reread your earlier post; if 453.450 shows up as channel 380
(decimal) then you can safely say that base frequency 1 is that frequency.
When I said hex channel, I mean the channel number whatever number
system is used.

Dave

 

[Dewey]

Sorry, just reread your earlier post

Thanks Dave. I've been scanning for over 30 years, and trunking since the 235 came out, but I'm just really getting into trying to interpret the trunking output from programs like Trunker and UniTrunker. I've been using the various Trunker releases for about the last 6 or 7 years, but have only started to really appreciate the data that it presents since the recent addition of MPDC's new freqeuncies in December. Eric Carlson recently explained the problems associated with trying to have more than one table that uses 380 as an offset, although it would probably work in some scanners (as it did with the BC250D and Pro-96). So I had already decided to "slide" the base/offset of the 458.775 table to 460.000/470 or 460.025/480 (the lowest freqeuncy currently in use by that table) when Eric posted the correction. Either way, I thank you for your help also.

Dewey