Base Frequency

[W3APX]

Im not sure if this is the right place for this however I'm looking at a Motorola System.

When looking at the system information it lists a base frequency. In this case it is 852.0625mhz and seems to be a common one around the country from my research. Its not a traffic channel or a control channel on this particular system anyway. So my question is what is this frequency? I can't find any thing indicating what it corresponds with and with offsets it doesn't line up with a transmit frequency.

 

[clbsquared]

What system are you looking at?
The base frequency is the lowest possible possible output of the system. The step and offset (all are combined to form the "band plan") will tell the radio which frequency to use.

I tried to simplify it, but there is a lot more to it than that.

 

[W3APX]

What system are you looking at?
The base frequency is the lowest possible possible output of the system. The step and offset (all are combined to form the "band plan") will tell the radio which frequency to use.

I tried to simplify it, but there is a lot more to it than that.

Im looking at the Carroll County system in Maryland. But that system has an 851.xxxx frequency listed so is that not a valid frequency on the system?

 

[merlin]

Base frequency, in your case, 800 Mhz band is a bit standardized at 852.0625mhz
In trunking, they use HEX channel numbers to set voice channels.
This frequency is channel 001h (lowest possible channel)
Channel 002 will be 6.25 Khz OR 12.5 Khz up depending the band plan.
This continues to a total of 4096 channels possible per system.
Most systems use from one to a dozen of these channels. This includes control channels.
I deal with a 700 Mhz system, a bit more complicated with several base frequencies.
A local site has 1 control channel, 3 alternate control channels and 5 voice channels.
As many as 56 groups use this so like 1000 radios.
Conservation of bandwidth is the idea behind all this.
Offset for 800Mhz band is -45 Mhz

 

[W3APX]

Base frequency, in your case, 800 Mhz band is a bit standardized at 852.0625mhz
In trunking, they use HEX channel numbers to set voice channels.
This frequency is channel 001h (lowest possible channel)
Channel 002 will be 6.25 Khz OR 12.5 Khz up depending the band plan.
This continues to a total of 4096 channels possible per system.
Most systems use from one to a dozen of these channels. This includes control channels.
I deal with a 700 Mhz system, a bit more complicated with several base frequencies.
A local site has 1 control channel, 3 alternate control channels and 5 voice channels.
As many as 56 groups use this so like 1000 radios.
Conservation of bandwidth is the idea behind all this.
Offset for 800Mhz band is -45 Mhz

Ok this changes everything that I thought I knew about these systems. So theres 4096 total frequencies that can be used in this system? I was thinking that 12.5khz was the band width which also didn't make sense as TDMA should be using a 6.25khz band width per time slot and 12.5khz total.

 

[clbsquared]

Ok this changes everything that I thought I knew about these systems. So theres 4096 total frequencies that can be used in this system? I was thinking that 12.5khz was the band width which also didn't make sense as TDMA should be using a 6.25khz band width per time slot and 12.5khz total.

No, there’s not that many frequencies that can be used on that system. The frequencies that can be used are only the ones listed on the license. Look up trunking system band plans in the Wiki on this site. That should help explain it to you in a better way.

 

[clbsquared]

Ok this changes everything that I thought I knew about these systems. So theres 4096 total frequencies that can be used in this system? I was thinking that 12.5khz was the band width which also didn't make sense as TDMA should be using a 6.25khz band width per time slot and 12.5khz total.

What Merlin is saying is that there is that many channels available to use ON a system.

 

[clbsquared]

Ok this changes everything that I thought I knew about these systems. So theres 4096 total frequencies that can be used in this system? I was thinking that 12.5khz was the band width which also didn't make sense as TDMA should be using a 6.25khz band width per time slot and 12.5khz total.

TDMA is Time Divided Multiple Access. It splits the frequency into two time slots.

 

[merlin]

With a TDMA system and 12.5 Khz step you only get 2048 possible frequencies. Each frequency is divided into two time slots so is like doubling the talk channels.
Each of these channel frequencies must be evenly divisible by the step size of 6.25 or 12.5 Khz depending on the system and site. Here is where knowing the system and site info comes into play.
This is not engraved in stone, but regardless the base frequency or step size, neither the site nor the radio can legally transmit outside the low or high splits of the band specs.
The radio TX frequency is always offset -45 Mhz for 800 Mhz band.
Perhaps I should note here that public safety band only licenses just some of these frequencies.
Using an SDR and app like DSD+, you can glean all the needed site and system info from the log files.

 

[nikronzo]

is it possible to calculate bandplans from CC data on RR database pages or do you need to run unitrunker or DSD to get that info. I remember reading a while back that their is a formula for all of that stuff

 

[merlin]

A complete bandplan, no. Just knowing base frequencies is simply a piece of the puzzle.
There is a mathematical relation between channel numbers, step size, and base frequency though.
Problem here is site administrators can change channel numbers at their call. so you can run into such system.
All I have encountered are the typical. decimal channel number minus one times the step size will give channel frequency. However, this is not true with all systems or sites.
If you are looking to program a trunking scanner, the RR DB will get you what you need.
Programming say a P25 radio, you need much more info. (hex channel numbers for a site and such)
Unitrunker is another source for the full scope.
The more you know about a system and site, the easier the programming is.
Especially true where your scan list is involved, root out your desired groups to scan.

 

[AM909]

@W3GXP: What do you mean by "base frequency"? Where are you seeing that?

I don't think 852.0625 would be a rational "base frequency" as the term is used in P25 (and other) bandplans (channel maps).

851.00625 or 851.0125, however, are common for this purpose as they are one channel below the start of the 800 MHz base-to-mobile band depending on the bandwidth/TDMA slots (i.e., making channel 1 come out to 851.0125 or 851.0250).

In the bandplan table, it's true that each prefix is limited to 4096 channels (usually 4095 because the 0 channel serves as the base and isn't used), but there can be (and usually are) many (16?) prefixes defined that identify different bands/channel widths. The network, site, and subscriber hardware is probably limited to a much smaller number of channels.

As others have said, Unitrunker and DSD+ can just listen to the system's control channel and will map out everything needed to monitor it from that. Unitrunker can even import the system info from RRDB (this site) without listening to the system (I think a premium account is required?).

In the case of the Carroll County system (BEE00.66A), the page shows just one simulcast cell with the primary control channel 852.9375. If you don't pick it up there, try one of the other alternate CC frequencies that are shown in blue.

 

[maus92]

The band plan for Carroll County is:



The system runs in both TDMA and FDMA modes, and subscribers are configured to operate on multiple systems. This band plan is typical for P25 systems in the NCR / Baltimore region.

 

[nikronzo]

The more you know about a system and site, the easier the programming is.

I know the feasibility would be a pain in the rear end, but it would be nice to eventually have people collect that data and put into the TRS pages, bandplans and such

 

[maus92]

I know the feasibility would be a pain in the rear end, but it would be nice to eventually have people collect that data and put into the TRS pages, bandplans and such

On P25 trunking systems, the band plan info is transmitted on the control channel, thus usually not necessary to know the specifics of the band plan when programming a consumer scanner. Some RRDB entries do contain band plan info, but usually only for systems that require band plan settings in a scanner. Motorola SmartNet systems would be an example of a system type that would require knowing the band plan to program a scanner or software packages.