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For UHF and 900 MHz
By Robert W. Meister WA1MIK
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To expand the UHF frequency limits or allow 900 MHz amateur frequencies, you have to modify the Model Definition File (that's the file with the MDF extension) using a hex-editor. I recommend and use Hex Workshop by www.bpsoft.com. Before modifying anything, save a copy of the original MDF file, incase you make a mistake. Also generate the checksum of the file and make sure it is the same after you finish modifying the file. With Hex Workshop, choose "Tools" from the top-line menu, then "Generate Checksum"; select "Checksum (16-bit)" and "On Entire File" then click "Generate". Standard disclaimers apply; you're on your own. The data shown in this article came from the latest release of the MaxTrac conventional program (R07.02.00a) but the same value changes will also work on the MaxTrac trunking program. Locations may vary with other programs or versions, and this same procedure may work with other programming packages. All file addresses are shown as hexadecimal values.

Some versions of RSS allow you to enter out-of-band frequencies using the SHIFT-NUM method, where you hold down the SHIFT key while entering the various digits for the TX or RX frequency. For example, if you want 927.41250, you hold down SHIFT and type 927, then release SHIFT and type the period, then hold down SHIFT and type 41250. The field must be completely filled in. It will now display (@&.$!@%) but will show up correctly if you ever go back to look at it.

There are three steps involved with expanding the band limits for amateur use; we'll go through each one:

  1. Find the frequency range table
  2. Find the entry for the band you desire to change
  3. Change the transmit and receive band limits

Step 1:

The frequency range table is one of several distinct areas in the MDF. Each of these areas can move around in the MDF; it depends on the amount of data in each area that precedes it. There's a table of contents (index) at the front of the file that will get you quickly to each area. Each entry is 8 bytes long (addresses 0000, 0008, 0010, 0018, etc.) and the 3rd entry (at address 0010) points to the frequency range table. Here you will find a 32-bit value. Like all hex values in this file, the low and high 8-bit bytes are reversed, as well as the low and high 16-bit words. This means that a value displayed as AABB CCDD really represents a hex address of DDCCBBAA. Here's the entire index:

AddressData Bytes Shown As 16-bit WordsASCII Text
000000002800 0000 0E00 1500 4E01 0000 0E00 1A00(.......N.......
00000010BA02 0000 1200 0F00 C803 0000 1000 1200 ................
00000020E804 0000 1500 8C01 5374 616E 6461 7264........Standard

Index entries consist of three fields: a 32-bit address field, a 16-bit field that indicates the length of each entry in the area, and a 16-bit value that tells the number of entries in the area.

The first entry has an address value of 2800 0000. If you reverse the order of the bytes, this represents a hex address of 00000028, and this is the address of the first area in the file. If you look at address 0028, you'll see the value 5374. This is the hex value for the two ASCII characters that begin the word "Standard", which can be seen at the right side of the table above.

The 3rd entry (at address 0010) is the one that points to the frequency range table. It has been highlighted with red text in the table above. The value BA02 0000 represents the hex address 000002BA; this is the beginning of the frequency range table. The 4th index entry at address 0018 (C803 0000) tells us the next area starts at address 000003C8, so we know the frequency range table ends immediately before that.

The remaining fields for this index entry tell us that each frequency range table entry is 0012 bytes long (18 bytes or 9 16-bit words) and that there are 000F (15 decimal) entries. So now we know where to find the frequency range table and how long it is (15 entries of 18 bytes each).

Step 2:

The frequency range table, shown below, runs from 02BA to 03C7 inclusive. All of the frequency range data has been highlighted in red. Entries start at 02BA, 02CC, 02DE, 02F0, etc. There are two entries that are all zeroes; obviously these would not be used by any radios.

AddressData Bytes Shown As 16-bit WordsASCII Text
000002B07261 6320 3235 2020 2020 E405 5406 E405 rac 25    ..T...
000002C05406 E405 5A06 5005 7206 0200 9411 5C12 T...Z.P.r.....\.
000002D09411 5C12 9411 5C12 2611 6612 0500 5005 ..\...\.&.f...P.
000002E05406 5005 5406 5005 5A06 3C05 7206 0200 T.P.T.P.Z.<.r...
000002F0B405 CC06 B405 CC06 B405 CC06 9605 E006 ................
000003000300 BE0F CC10 BE0F CC10 BE0F CC10 AA0F ................
000003103011 0400 8A11 5C12 8A11 5C12 8A11 5C12 0.....\...\...\.
000003202611 6612 0500 5C12 8813 5C12 8813 5C12 &.f...\...\...\.
000003308813 5C12 8813 0600 4C13 5014 4C13 5014 ..\.....L.P.L.P.
000003404C13 5014 4C13 5014 0700 0000 0000 0000 L.P.L.P.........
000003500000 0000 0000 0000 0000 0800 7C1F FC21 ............|..!
000003607C1F 3A20 3E21 FC21 721F 0622 0900 0023 |.: >!.!r.."...#
00000370C224 0023 3C23 8624 C224 F622 CC24 0A00 .$.#<#.$.$.".$..
000003800000 0000 0000 0000 0000 0000 0000 0000 ................
000003900B00 2801 6801 2801 6801 2901 6801 2201 ..(.h.(.h.).h.".
000003A07201 0C00 6801 A401 6801 A401 6801 A401 r...h...h...h...
000003B05E01 AE01 0D00 A401 F401 A401 F401 A401 ^...............
000003C0F401 9A01 1C02 0E00 0000 0000 0000 0000 ................

Finding the actual data for the band you're interested in takes a bit of searching. Data at any location can be viewed with Hex Workshop in many formats. The ones that make sense here are 16-bit Unsigned Short or 16-bit Signed Short. Either one will display the hex data values as decimal numbers. Start at the beginning of the area above and you'll see recognizable decimal values. The first value in the area above (E405) will be displayed as 1508 in decimal. The values are shown in increments of 0.1 MHz, so 1508 represents a frequency of 150.8 MHz. As you move down the table, you will encounter other values. When you get to values in the range of 8960 to 9410 (896.0 MHz to 941.0 MHz), you've found the 900 MHz entries shown below. Similarly, when you get to values in the range of 4500 to 4700 (450.0 MHz to 470.0 MHz), you've found the UHF entries shown below. We've now completed step 2.

Step 3 (900 MHz):

Here is the data for the 900 MHz frequencies extracted from the table above:

Use or
Meaning		Hex
Addr.		Hex
Value		Flipped
Hex Value		 Decimal
Value
FCC Low0000036E0023 23008960 (896.0)
FCC High00000370C224 24C29410 (941.0)
TX Low000003720023 23008960 (896.0)
TX High00000374 3C23233C 9020 (902.0)
RX Low00000376 86242486 9350 (935.0)
RX High00000378C224 24C29410 (941.0)
RSS Low0000037AF622 22F68950 (895.0)
RSS High0000037CCC24 24CC9420 (942.0)
Entry Index0000037E0A00 000A10

Basically there are four values we care about: TX Low, TX High, RX Low, and RX High. The values are shown in increments of 0.1 MHz, so 8960 represents a frequency of 896.0 MHz. You don't enter the decimal point, but you do have to enter the data in the reversed order as shown in the Hex Value column.

We only need to change TX High from 9020 to 9410, and we only need to change RX Low from 9350 to 8960. This boils down to changing the data at location 0374 from 3C23 (9020) to C224 (9410), and changing the data at location 0376 from 8624 (9350) to 0023 (8960). The two original values are shown with red text in the table above. We're expanding the range by making the TX and RX values the same as the FCC values. They're sitting right next to the locations you'll be hex-editing, so you don't even have to calculate the new values. These are the exact same value changes that work on the GTX MDF (see the GTX Information article on www.repeater-builder.com for more details). Fortunately the checksum also remains the same after making these two changes, so these really are the only locations that need to be modified. This completes step 3 for the 900 MHz band.

Save the modifications back to the file and go program your MaxTracs in the amateur part of the band.

Step 3 (UHF Band):

Here is the data for the 450-470 MHz frequencies extracted from the table above:

Use or
Meaning		Hex
Addr.		Hex
Value		Flipped
Hex Value		 Decimal
Value
FCC Low000002CC 94111194 4500 (450.0)
FCC High000002CE 5C12125C 4700 (470.0)
TX Low000002D0 94111194 4500 (450.0)
TX High000002D2 5C12125C 4700 (470.0)
RX Low000002D4 94111194 4500 (450.0)
RX High000002D6 5C12125C 4700 (470.0)
RSS Low000002D82611 11264390 (439.0)
RSS High000002DA6612 12664710 (471.0)
Entry Index000002DC0500 00055

Basically there are six values we care about: FCC Low, FCC High, TX Low, TX High, RX Low, and RX High. The values are shown in increments of 0.1 MHz, so 4500 represents a frequency of 450.0 MHz. You don't enter the decimal point, but you do have to enter the data in the reversed order as shown in the Hex Value column.

We will need to change FCC Low, TX Low, and RX Low from 4500 to 4400, and similarly increase FCC High, TX High, and RX High from 4700 to 4800. This boils down to changing the data at location 02CC, 02D0, and 02D4 from 9411 (4500) to 3011 (4400), and changing the data at location 02CE, 02D2, and 02D6 from 5C12 (4700) to C012 (4800). The six original values are shown with red text in the table above. We're expanding the range by decreasing the Low values and increasing the High values. Because we made the changes in equal amounts, the checksum doesn't change, so these really are the only locations that need to be modified. This completes step 3 for the 450 MHz band.

Save the modifications back to the file and go program your MaxTracs in the amateur part of the band.

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.


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This article first posted 18-Nov-2007 and covered 900 MHz only.

Hand-coded HTML © Copyright 2007 by Robert W. Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.