Trunking Offset

[Renamon]

I do not understand what the trunking offset is used for, but here is what I have concluded based on my reading...

If a system looked like this:

Base: 136.0000
Spacing: 12.5
Offset: 380

Does that mean an output frequency would have in input 380 channels (4.75mhz) higher than the output?

Out-----------In
137.1925 -> 141.9425

137.1925 is just a random frequency I thought of for this example, btw.

It seems to me that 800meg systems have a standard split (45mhz) but uhf/vhf do not, so they use an offset to get the input.

[RFguy]

A quick Google of Base Spacing Offset came up with this page http://wiki.radioreference.com/index.ph ... la_Systems

It should give you the info that you're looking for.

[Renamon]

I have seen that page before, and the way it explains anything confuses me. It seems to tell me that the base freq is the offset # (380). If that is true, why would you need an offset in the first place?
To quote the page...

"Channel number 17C hex (380 decimal) is the base frequency and this is the lowest possible output frequency of the system. Channels are numbered sequentially upwards and the step between each channel in kHz is the offset."

I still am wanting to know what the offset (380 in this example) is used for, and I am slightly more confused.

[RFguy]

Maybe someone has some old Privacy Plus manuals that has this information in the system description section.

These scanners also work on "unstructured" bands like VHF. I suspect the offset may possibly vary in an unstructured band, so they provide the option of changing it. (Just a guess though).

[w2sjw]

The best way to describe it is to look at the 380 as the BASE, not as the offset.

Moto trunking system start at 380 (decimal) for the channel assignments, and I believe that they end at 759 or 760.

Trying to apply the terminology of a scanner to an actual radio is where many people get confused, as my APX doesn't even use data in that format. The OBT control channel settings on a TRS are just presented as starting & ending frequencies for a max of 3 pairs, along with the offset step (6.25k/12.5k/25.0k).

[d119]

I do not understand what the trunking offset is used for, but here is what I have concluded based on my reading...

If a system looked like this:

Base: 136.0000
Spacing: 12.5
Offset: 380

Does that mean an output frequency would have in input 380 channels (4.75mhz) higher than the output?

Out-----------In
137.1925 -> 141.9425

137.1925 is just a random frequency I thought of for this example, btw.

It seems to me that 800meg systems have a standard split (45mhz) but uhf/vhf do not, so they use an offset to get the input.

UHF does have a standard offset: Repeater RX is always +5 MHz above TX (451.000 TX = 456.000 RX). This applies to trunked systems as well.

UHF "T Band" (470-520MHz) is slightly different. Repeater RX is always +3MHz above TX (470.000 TX = 473.000MHz RX). Again, same for trunked or conventional.

800MHz is as you said, 45MHz offset. 900MHz is 39MHz offset. I don't recall what 700MHz is, but it's a few MHz as well.

VHF (both lowband and highband) is a whole different ballpark as there is no standard for offsets in that band.

I maintain several OBT systems, but for the life of me don't ask how the base offset stuff works. I just maintain the infrastructure and have never had to program a subscriber from scratch, Heaven help me if I ever do. On the infrastructure side, all of that information is maintained in the controller codeplug and to me that's just a file on a disk.

[BIODTL1997]

Sorry to bring a dead thread back to life, but I haven't actually seen anyone explain this clearly.

In the stated example above (now below as well), what would the end value be? the TX and RX offsets are easy, 380+step size has got me spinning. Is it NOT 4.75 MHz above the start value?

------------------
Base: 136.0000
Spacing: 12.5
Offset: 380
------------------

[motorola_otaku]

When you plug that into a scanner, you're telling it that channel 380 is 136.0125, and to count up from there in 12.5 kHz steps. Does that make sense?

edit: "Offset" in this case is a bit of a misnomer.. you're dealing with channel assignment offsets, not repeater input offsets. Scanners don't need that information, ergo there is no entry for it. And keep in mind this is only applicable to analog or mixed-mode Motorola Smartnet systems.. 9600 P25 systems are a whole 'nother can of worms.

[immelmen28]

When you plug that into a scanner, you're telling it that channel 380 is 136.0125, and to count up from there in 12.5 kHz steps. Does that make sense?

edit: "Offset" in this case is a bit of a misnomer.. you're dealing with channel assignment offsets, not repeater input offsets. Scanners don't need that information, ergo there is no entry for it. And keep in mind this is only applicable to analog or mixed-mode Motorola Smartnet systems.. 9600 P25 systems are a whole 'nother can of worms.

I would be interested in hearing about the 9600 can o worms...how is it different?

[RADIOMAN2002]

To be brief you only need the control channel, all other info is passed along on the CC.

[motorola_otaku]

To be brief you only need the control channel, all other info is passed along on the CC.

Sometimes. This is almost always the case on 700/800 systems, but not always on VHF/UHF systems. And even on 700/800 systems you still need to set up a control channel table with the base frequency and the repeater TX offset. Good explanation here: http://batboard.batlabs.com/viewtopic.p ... 62#p367195

[Wowbagger]

... but not always on VHF/UHF systems.

I don't know how a P25 system can operate without the control channel providing the offset information. However, the message sent for VHF/UHF systems is a different message than for 700/800MHz systems (IDEN_UP_UV and/or IDEN_UP explicit vs. IDEN_UP implicit), so if the scanner or decoder you are using only understands IDEN_UP then you might be fooled into thinking a VHF system doesn't have the channel plan announced on the control channel.

[immelmen28]

Yeah, I guess what had me confused/wondering about this, is my XTL5ks in VHF still have the Astro25 Channel ID tab in the trunking system pages, so if P25 systems are sending all the info needed for channel assignment over the CC then why the need for the channel ID page....I assumed it still worked like OBT did on 3600 systems and the correct parameters were required on the Astro25 Channel page for channel assignment. This was also based on seeing a VHF radio set up with internet( scanner) data for a TRS not land on the correct voice channels when a talkgroup was active.