406 trunking frustraton

[E5911]

i am having frustration decoding a mil 406 system. It has to do with base and offset. I can decode the talkgroup when I monitor the control channel only , but during scan the display just flashes between scanning and the talkgroup but will not pick up any audio.

I am picking up the control channel fine, and i know I have the right frequencies.

[3mary2]

What scanner are you using? If you are using software for your scanner, what trunking tables if any are you using. Here in Las Vegas monitoring Nellis AFB I use the trunking table of:
380 710 380 406.1000 12.5

710 759 710 410.5000 25.0
Don't know if this will help you or not, just a thought

[N7YUO]

You speak of a 406 Mil system, but where is it located? Maybe someone has already figured out the Base-Offset-Step for it. Is it Motorola or EDACS?

For Motorola Systems, the lowest channel number (in DEC) is 380. The highest channel number is 759. So if you use those for Lo & Hi you can't go wrong. The new military systems have steps of 12.5 kHz, so use that step. Since the lowest channel is 380, use 380 as the offset. Now all you need to determine is the Base. Try 406.1, when the scanner tries to access a voice freq, does it show on the display? how far is it off? How well do you understan algebra? The radios use an algebraic formula to dtermine the Voice Freq from the Base, Offset, Step, and Channel Number.

[N7YUO]

E5911:
Tell you what, give me any two of the voice freqs and I will try to correctly determine the BOS for you.

3Mary2: Give me two of the voice freqs for Nellis. I want to see what my calculations would be.

Examples: Lets begin with Base 406.15, subtract the offset 380, & step (.0125 in MHz):
406.15 - (380 x .0125)
406.15 - 4.75 = 401.4 ( I call this freq the Offset Freq)
406.9625 - 401.4 = 5.5625. > 5.5625/.0125 = 445 = (channel 445)
407.3625 - 401.4 = 5.9625. > 5.9625/.0125 = 477 = (channel 477)

What does all this mean?
From the Base, deduct the offset, (380 steps of 12.5)
Now count (445 steps of 12.5) to reach freq 406.9625.
That is how the radio does it's work. But if you have the wrong numbers, it won't work

[E5911]

ok here we go
409.1125
409.6875

im interested in what you get

[N7YUO]

I recall when I was trying to decipher one of these systems, I did this:
One scanner was programmed with all the voice freqs. The other scanner was set up to trunk. I was trying various Base, Offset, & Steps.

When a voice freq came active, I would look at the trunking scanner to see what freq it was trying to find. What ever amount it was off frequency, that is what I had to adjust my base frequency to get it right.

Suppose I used 406.0, 380 offset, and 12.5 step.
The voice freq 406.9625 comes active.
The trunking scanner shows 406.8125.
406.9625-406.8125=0.1500
I have to increase the Base freq by 0.1500,
so the correct base freq is 406.1500

From the freqs you provided, my guess is they are for Yuma Proving Ground. I used to scan that when I was stationed at MCAS Yuma in 1974. Very little activity.

Here is a partial list of NTIA narrowband federal frequency assignments. Notice the lowest freq is above 406.100. (I would think that the base freq should be higher than 406.1)
CH RECEIVE. TRANSMIT
01 406.1125 415.1125
02 406.1250 415.1250
03 406.1375 415.1375
04 406.1500 415.1500
05 406.1625 415.1625
06 406.1750 415.1750
07 406.1875 415.1875

Hill AFB 406.15, 380, 12.5
Nellis AFB 406.1, 380, 12.5
Holloman AFB 406.1125, 380, 12.5
Luke AFB 406.0, 380, 12.5

Try each of these base freqs and see what you get with 380 offset and 12.5 step.
00 406.0000
00 406.1000
01 406.1125
02 406.1250
03 406.1375
04 406.1500
05 406.1625
06 406.1750
07 406.1875

[N7YUO]

I have been looking up US Military bases in the various states. In the old days, it was simple.
The Motorola trunked systems were mostly 406/380/25. Now it seems that no two bases are set up
the same. I believe this is intentional. It makes it much more difficult to scan if you don't
know the setup of the system. The quickest way to determine what is going on is with Trunker.
But if you don't have that, Set the offset at 380, the step at 12.5 and have a radio scanning
the voice freqs. Begin trying base freqs 406.0, 406.0125, 406.025, 406.0375, 406.050, etc.
Continue going up one increment at a time until you find the right number.

These bases have Motorola systems. Notice that no two of them are alike.
Alabama Anniston Army Depot Calhoun Co Motorola Type II SmartZone 406.1625 380 12.5
Alabama Marshall Space Flt Ctr Madison Co Motorola Type II Smartnet 406.3500 380 25.0
Alabama Redstone Arsenal Madison Co Motorola Type II SmartZone 406.2375 380 12.5
Colorado Air Force Academy El Paso Co Motorola Type II SmartZone 406.3375 380 12.5
Colorado Air Force Academy El Paso Co Motorola Type II SmartZone 407.1000 435 12.5
Georgia Fort Stewart----------- Liberty Co Motorola Type II SmartZone 406.0125 380 12.5
Hawaii Hickam Air Force Base Honolulu-- Motorola Type II Smartnet 413.0000 380 25.0
Utah Hill air Force Base Davis Co-- Motorola Type II SmartZone 406.1500 380 12.5

Lets look at a simple system. Tooele Army Depot, 406.0/380/25, follow the arrows.

Offset Freq <-- -(Offset x Step) <--- Base Freq
396.5 <----- -(380 x .025MHz) <---- 406.00

396.5 -----> +(394 x .025MHz) ----> 406.35
396.5 -----> +(426 x .025MHz) ----> 407.15
396.5 -----> +(458 x .025MHz) ----> 407.95
396.5 -----> +(490 x .025MHz) ----> 408.75
396.5 -----> +(522 x .025MHz) ----> 409.55
This system uses Motorola Channels 394, 426, 458, 490 & 522.
This diagram shows how the Base, Offset, Step, & Channel numbers formulate the voice freqs.

[dste]

You really need trunker... but.... I just did the following.

I have 2 scanners. Scanner A is a pro95 set to trunk with a base 406, step 12.5 and offset 380. I put in the control channel and set it scanning.

Scanner B is a Pro96 with the known voice channels programmed in FM mode - no trunking.

Scanner A is going to run and LIE to me. It will stop on a frequency without any sound. I noted this frequency. I also noted the real voice frequency on Scanner B at the same time.

Freq B - (Freq A - 406) = Base

Note: If the base drops below 406 - this solution will not work. You will have to use more math to calc the correct offset at a 406 base. This just should get a lot of systems

[N7YUO]

Motorola UHF Trunked systems number the channels
from 380 to 759, so 380 is usually used as the offset.

Offset Freq <-- -(Offset x Step) <--- Base Freq
401.4 <----- -(380 x .0125MHz) <---- 406.15
If this system used a channel numbered 380, it would work like this:
401.4 -----> +(380 x .0125MHz) ----> 406.15

System 6227 in Salt Lake City has these freqs:
401.4 -----> +(429 x .0125MHz) ----> 406.7625
401.4 -----> +(461 x .0125MHz) ----> 407.1625
401.4 -----> +(493 x .0125MHz) ----> 407.5625
401.4 -----> +(525 x .0125MHz) ----> 407.9625
401.4 -----> +(557 x .0125MHz) ----> 408.3625

[N7YUO]

System 5F10 in Colorado Springs has two base & offset:
Offset Freq <-- -(Offset x Step) <--------- Base Freq
401.5875 <----- -(380 x .0125MHz) <---- 406.3375
401.6625 <----- -(435 x .0125MHz) <---- 407.1000

System 5F10 in Colorado Springs has these freqs:
401.5875 -----> +(382 x .0125MHz) ----> 406.3625
401.5875 -----> +(430 x .0125MHz) ----> 406.9625

401.6625 -----> +(440 x .0125MHz) ----> 407.1625
401.6625 -----> +(456 x .0125MHz) ----> 407.3625
401.6625 -----> +(568 x .0125MHz) ----> 408.7625
401.6625 -----> +(600 x .0125MHz) ----> 409.1625
401.6625 -----> +(632 x .0125MHz) ----> 409.5625

The system could use only the 406.3375 Base & 380 Offset
and still get all the same frequencies:
401.5875 -----> +(446 x .0125MHz) ----> 407.1625
401.5875 -----> +(462 x .0125MHz) ----> 407.3625
401.5875 -----> +(574 x .0125MHz) ----> 408.7625
401.5875 -----> +(606 x .0125MHz) ----> 409.1625
401.5875 -----> +(638 x .0125MHz) ----> 409.5625
Conclusion: The only reason to use Two Bases & Offsets
is to make the system difficult to scan.

[tracker1]

>Conclusion: The only reason to use Two Bases & Offsets
>is to make the system difficult to scan.

You've present some great information, but I'm not sure Motorola's reason for implementing multiple base frequencies was this. If you do a little more math you see that with .0125MHz channel spacing and a single base frequency, you can only channelize a total of 4.7375 MHz.

759-380 = 379 available channels
379 * .0125Mhz = 4.7375MHz

If by chance your one and only base frequency was 406.0125, your last channel could be no higher than 410.750 MHz. If you appropriately choose a second base frequency you can go higher. I routinely monitor a trunk that has its last channel at 410.900 MHz, and yes it uses two base frequencies.

[DaveH]

Originally Posted by tracker1

>Conclusion: The only reason to use Two Bases & Offsets
>is to make the system difficult to scan.

but I'm not sure Motorola's reason for implementing multiple base frequencies was this. If you do a little more math you see that with .0125MHz channel spacing and a single base frequency, you can only channelize a total of 4.7375 MHz.

Scanner types sometimes fall into thinking trunked system complexity
is all about keeping them from listening...for the most part not so.

Mutliple ranges (therefore base/offset/step) are required to work
with available (or future) frequencies, without running out of
channel numbers. Sometimes these can be accommodated within
a single large block, but frequently not.

For example, in commercial systems in the 451-455 and 460-465
MHz output ranges, you can't cover the whole 9MHz with one
range using stepsize 25kHz. If your channel assignments force
a smaller step and/or the frequency spread is too wide, more than
one range is needed.

There are cases where channel assignments need to jump around a
block of frequencies that can't be assigned e.g. VHF marine band
and 2m ham band.

Finding one base/offset/step can be challenging, but multiple
ranges is more fun...done that. A latent problem is that all three
ranges are assigned and programmed into radios, but the system
may only use frequencies in one range. New ones are added
in another range, which scanner users (without "inside" info)
could not have determined decoding over the air. The new
channels aren't properly decoded, parts of conversations get
missed, until listeners realize this and figure it out.

There is a similar problem with older Type I systems, where
not all blocks are initially used; similarly can't be determined from
listening because the necessary info is never sent over the air
(the fleetmap is known to both system and radios, but not
transmitted explicity).

Regards, Dave

[N7YUO]

Originally Posted by tracker1

>Conclusion: The only reason to use Two Bases & Offsets
>is to make the system difficult to scan.

You've present some great information, but I'm not sure Motorola's reason for implementing multiple base frequencies was this. If you do a little more math you see that with .0125MHz channel spacing and a single base frequency, you can only channelize a total of 4.7375 MHz.

759-380 = 379 available channels
379 * .0125Mhz = 4.7375MHz

If by chance your one and only base frequency was 406.0125, your last channel could be no higher than 410.750 MHz. If you appropriately choose a second base frequency you can go higher. I routinely monitor a trunk that has its last channel at 410.900 MHz, and yes it uses two base frequencies.

You can find the NTIA "Redbook" at this web address:
National Telecommunications & Information Administration
Look for chapter 4
open the .pdf file: 4.2 Frequency Allotments,
page 4-181: 4.3.9 Channeling Plan for Assignments in the Band 406.1-420 MHz
page 4-183: Table 1, Paired Channels

My Note: These are the paired freqs used in trunking. There are a total of 391 paired freqs.
beginning with 406.1125/415.1125 and ending with 410.9175/419.9875
If a 406 MHz trunked system can get all of their freqs by using a single base, then what is the purpose of using two?
System 6227 in Utah uses twenty channels from 413 to 717 with five sites in three counties and uses one base freq.
Also, the US Govt Motorola systems I have checked on so far have all been set up differently. That would indicate to me that there is probably a directive.
You can't set up the base, offset and step for a military base and go to another base and scan. The base, offset & step can be looked at like a password.
Each system has a different one, so they are more difficult to scan.
Those of us who have knowledge and understanding of these systems accept this as another challenge.
For the lesser skilled, or newbie, this can be frustrating or overwhelming.
This forum is about facts, ideas, and opinions. I try to make sure my ideas or opinions are expressed as that.
I welcome challenges to any of my posts, and I don't expect to present my ideas as superior to anyone else's.
So I hope everyone keeps this thread on a friendly basis and we can bring some light on a topic that has
frustrated many in our hobby. Mike Smith

[DaveH]

Originally Posted by N7YUO

If a 406 MHz trunked system can get all of their freqs by using a single base, then what is the purpose of using two?
System 6227 in Utah uses twenty channels from 413 to 717 with five sites in three counties and uses one base freq.

If all system channels can fit into one range, that's fine, but I think you'll find that radios are
programmed for three ranges, even if one or two are never used. Actually it's six ranges, three
each for TX and RX.

Dave

[N7YUO]

Originally Posted by DaveH

If all system channels can fit into one range, that's fine, but I think you'll find that radios are
programmed for three ranges, even if one or two are never used. Actually it's six ranges, three
each for TX and RX.

Dave

Thanks for the good information Dave. I think this thread is beginning to stray from the original topic.
I am sure that posting my personal opinions had something to do with that. Now, back to the original topic:
E5911 is having difficulty with the Base/Offset/Step for a 406 Motorola TRS.
I have read a lot of threads on similar topics and they all had something in common:
A lack of understanding how Base/Offset/Step work together, and what each one does.
Look at these posts from the McChord AFB thread:

Originally Posted by Mainsail

http://www.radioreference.com/modules.php?name=RR&sid=2200
In the database for McChord, there is this little table:
Quote:
Custom Frequency Table
1
Base: 407.0000
Spacing: 25.0
Offset: 380
What do these numbers mean/do?

Originally Posted by LEH

The 407 is the base frequency (I recall reading that is the lowest whole non decimal frequency in the system). You need this in your table.
The 380 is an offset, I am still not sure what it does, but you will need that value as well.
The last value is the step spacing between frequencies. What you are showing seems to indicate a step value of 12.5 rather than 25 kHz.

"What do these numbers mean/do?" "The 380 is an offset, I am still not sure what it does"
Those two statements are examples of the lack of understanding of these Motorola systems.
I have tried several ways to provide a simple explanation of how they work.
Using a diagram is the simplest method I have found.
I used to think that most systems used a base around 406-406.2, but now I see that I was wrong.
There are at least two systems that use base freqs above 406.3, and so far, I have found no two systems alike.