uhf hi channel in custom table

[dorslv]

Hi All, need some help please, programming aUHF Motorola type II APCO 25 on a GRE psr 500. I got the base as 406.0000, 410.0000, step 12.5, offset 380 & 680. I get the part about lo channel is the same as the offset, what i don't remember is how i arrived at getting the hi channel for the custom tables. I would appreciate if someone could point me in the right direction or link please. Thx, Dorian

 

[mikey60]

For the hi channel, use one less than the first channel of the next block. Use 759 as the high channel for the last block.

So if I understand your post, your tables would be:

1: Base: 406.0000 Offset: 380 Ch-Lo: 380 Ch-Hi: 679 Spacing/Step: 12.5
2: Base: 410.0000 Offset: 680 Ch-Lo: 680 Ch-Hi: 759 Spacing/Step: 12.5

Mike

 

[dorslv]

Thx Mike, I'm just trying to understand how you got the 759 for the last table? I had used 979 in last table cause the first table has 299 units between the lo and hi channels so i quessed at that number and added 299 to 680 to get the hi channel for the last table if that makes sense...

 

[Psudds87]

perfect it worked thanks

 

[Spitfire8520]

Thx Mike, I'm just trying to understand how you got the 759 for the last table?

I believe it deals with how Motorola trunked systems are setup. They only have a total of 380 channel pairs to work with, channels 0-379 for transmitting and 380-759 for receiving. It was basically defined by Motorola whenever they designed their trunking technology way back when. Setting a Hi Channel of 979 won't do anything bad, it just won't reference anything past 759 from what I understand.

If you're programming Denver Area Federal / Military, which coincidentally Psudds was looking for these exact same tables, then you can reverse some of the math required that can be found in Determining Base/Step/Offset for VHF/UHF Motorola Systems to calculate the highest channel actually used. I walk through it in the next paragraph if you were interested for a little fun fact, but it doesn't affect anything you have programmed.

The highest channel used can be calculated using ((Highest Frequency Used - Base Frequency)/Step Size) + Channel Low = Highest Channel Used. The highest frequency ends up being 410.6750 MHz for the above system. If we look at the first table with a 406.0000 MHz and a max of 299 steps, we can multiple the step of 0.0125 MHz with 299 steps to get 3.7375 MHz positive offset. This means the first table, covers frequencies from 406.0000 to 409.7375 MHz, so the highest frequency/channel used does not reside within the first table. Using the second table, we take 410.6750 MHz - 410.0000 MHz (the base frequency), we end up with a 0.6750 MHz shift. Taking 0.6750 MHz / 0.0125 MHz (the step), we end up with a 54 channel offset. Since the second table starts with 680 we add the offset to get a highest channel used being 734. This means that you could theoretically use 734 as the Hi Channel, but we use 759 in order to make sure that all possible Motorola trunking channels are defined.

 

[dorslv]

Thx Spitfire8520, i appreciate the help everyone has provided, I ordered a ws1080 to upgrade my scanning abilities...