VHF and UHF Trunked System Frequency Assignments
These systems use a number of variables to arrive at an actual frequency with regard to the hex value sent out. People have in the past demonstrated ways to easily determine these frequencies with a few simple formulas. Below is one method suggested by Jim Conrad():
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Data Channel:408.750Hex:1acDec:428
408.750 - (428 * .0125) = 403.400(This is ref channel if 12.5 Khz increments)
408.750 - (428 * .0250) = 398.050(This is ref channel if 25.0 Khz increments)
408.750 - (428 * .0500) = 387.350(This is ref channel if 50.0 Khz increments)
"Now once you have the ref channel(s) you take the voice channel hex numbers
that will show up in the display of trunker and computer the voice
frequencies as shown below. Most systems that I have found use 25 Khz."
Example Voice Channel - Hex: 1ecDec: 492
Formula;
Ref Channel Freq + (Dec Channel # x Channel Increment) = Voice Channel Freq
403.400 + (492 * .0125) = 409.550
398.050 + (492 * .0250) = 410.350
397.350 + (492 * .0500) = 421.950
"You can eliminate the 421.950 because that goes out of bound so it's not
possible. You need to take all the hex channel numbers and run though
looking for the impossibility's like the one above. Monitoring the system
with another scanner to verify the channels as they switch will also help
to verify you have selected the proper perimeters and have the correct
resultants.
Many of the 400 Mhz systems also display the input frequency channel number
so these must be disregard initially. It is difficult to figure out the
input frequencies without knowing what the frequency is on at least one.
The input reference frequency will not normally be the same as the output.
Output hex channels usually start at 380 decimal (17c hex) so anything
lower in value in a 400 Mhz system will be an input channel.
This task could be automated in the Trunker program quite easily. It would
just have to prompt for the required parameters (i.e. channel increments).
Since all systems that I have encountered broadcast the data channel hex
number it would be quite a simple task for Trunker to compute the channel
frequency's once it was told what the increments were. I have monitored
over a dozen 400 Mhz systems and they all are compliant with my resolution
for computing the frequencies."
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Though the above is correct to a point it does not work for systems which use multiple base frequencies. Here are the values that are taken into account by a radio and used to compute values from hex values sent out over the control channel:
1 Channel Spacing
2 Number of Channel Ranges (Up to 3)
3 Base or Starting Frequency
The following are allowable channel spacings:
Channel Starting Freq Channel Starting Freq
Spacing divisible by: Spacing divisible by:
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5.00 kHz 5.00 kHz 25.00 kHz 5.00 kHz
6.25 6.25 31.25 6.25
10 5.00 30 5.00
12.5 6.25 37.5 6.25
15 5.00 35 5.00
18.75 6.25 43.75 6.25
20 5.00 40 5.00
25 6.25 50 6.25
In order for the channel spacing to be valid the starting frequency must be evenly divisible by either 5.00kHz or 6.25kHz depending on the value in question. For example:
You want your Starting Frequency to be 406.1375MHz...
406.1375 / 0.00625 = 64982 <no remainder so value is GOOD>
406.1375 / 0.005 = 81227.5 <a remainder of 5 which makes 5.0kHz invalid>
The above frequency is available to use any channel spacing which is a multiple of 6.25Khz such as 6.25kHz, 12.5kHz and 18.75kHz. 25kHz is evenly divisible by either 6.25 or 5.0 so it is listed twice. 25kHz seems to be the default channel spacing for analog VHF or UHF system. 12.5kHz for ASTRO.
There are a maximum of six channel ranges allowed per system. Up to three ranges for Transmit and up to three ranges for Receive. All channel ranges must equal 380 channels. If one channel range is chosen then the number of channels available would be 380. If you chose two channel ranges, both ranges would have to equal 380. If Range 1 was to be 100 then Range 2 would have to be 280.
Each channel range has its own starting frequency. The starting frequency cannot have a maximum frequency which is out of range of the radio. Take for example a Government UHF system. A radio used within this band generally goes from 403 to 441 but Government frequencies cannot exceed 420. Therefore your starting frequency could never be a value such as 415MHz because the ending frequency, if your channel spacing is 25kHz and one Channel Range, would be 424.475Mhz. Choosing a channel spacing of 12.5kHz or lowering your Base Frequency would be better.
Channel values sent out have two groups, receive and transmit.
Hex:
000 - 17B Receive
17C - 2F7 Transmit
Decimal:
000 - 379 Receive
380 - 759 Transmit
Channel ranges are really groups of bandwidth with there own unique starting frequency and channel spacing. One range cannot begin within another. If I had a starting frequency of 406MHz and a 25kHz spacing with 137 channels my highest frequency would be 409.425MHz. The second channel range could be any frequency AFTER 409.425MHz. Whatever it is it cannot start at a value, with 243 channels in mind, which will exceed the limit of 420. And the second frequency range MUST have 243 channels so that it equals a total of 380. You could divide up the 243 one more time keeping the rules into account. Here's an example of what I just mentioned...
Receive Number of
Frequency Range Channels Spacing
406.0000 to 409.4250 137 25kHz
410.0000 to 415.0000 100 50kHz
415.0125 to 416.8000 143 12.5kHz
This is only one part. You must also define for Transmit. Transmit frequencies can exist within the above ranges but will generally be a few Megahertz apart. Most systems will only use one Channel Range though.
Can you see how the original formula is not always correct? Here is an actual system which uses more than one Channel Range as an example:
Hex values sent out cannot be determined which channel range they are from if there is more than one channel range. If you are using the control channel as a reference to figure out the frequencies by formula you can only figure for which frequencies the range the control channel is in. Here's an example of a system that uses two or possibly three channel ranges.
Travis Air Force Base (Solano County, CA)
UHF Astro Simulcast
1 406.5750,182,386
2 407.1750,1b2,434
3 407.3250,1be,446
4 408.0000,1f4,500
5 408.1750,202,514
6 408.2500,208,520
7 409.2250,21e,542
8 409.6500,240,576
9 409.9250,256,598
10 410.0000,25c,604 Control Channel at the time
Consider all frequencies except the Control Channel to be unknown.
I'll make it simple and include the channel spacing which is 12.5kHz. Generally all ASTRO systems use 12.5kHz spacing.
Ch10 (604 x 0.0125) - 410 = 402.45
Ch 8 (576 x 0.0125) + 402.45 = 409.65 (OK)
Ch 7 (542 x 0.0125) + 402.45 = 409.225 (OK)
Ch 6 (520 x 0.0125) + 402.45 = 408.95 (Incorrect)
409.225 is where the Channel Range changes. If you used the formula to find 1 through 6 you would have incorrect frequencies. In actuality 1 and 2 are also control channels so you would still have a reference value but how would you know which to apply it to? That is why you cannot trust the formulas sometimes.