Split Frequency and Tone Operation on GMRS

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GMRS and FRS Defined

General mobile radio service (GMRS) and family radio service (FRS) are two radio services in the ultra-high frequency (UHF) radio band. GMRS radio service requires a license in the United States, and FRS is license-free. GMRS and FRS share the same frequencies except for the GMRS repeater inputs.

Operation

GMRS and FRS services allow line-of-site operation. GMRS allows higher power operation, and the service allows the use of repeaters. Repeaters expand coverage. For example, a GMRS repeater on a mountaintop allows two users not in the line of sight area to communicate with each other.

Challenge

Because GMRS and FRS share frequencies and FRS does not require a license, sometimes it is difficult for GMRS users to operate in a crowded environment. GMRS and FRS users can use a continuous tone-coded squelch system (CTCSS) or digital-coded squelch (DCS) to filter out conversations that are not part of their group. Occasionally, a radio user outside the group may scan the CTCSS or DCS codes on a channel and communicate with the group. Sometimes the outside radio user might engage in unwanted or undesirable (harassment) conversations with the group.

The Solution

GMRS users of the Wouxun KG-805G GMRS Radio (sponsored link) and others can program advanced radios with split frequency and split tone settings. Such settings can mitigate the effects of undesirable interference by obscuring the conversation. Splitting the frequency operation makes it harder for an outside listener to pick up on the conversation. The user would only hear one side of the conversation. With the split-tone operation, the transmitting and receiving tones are different. If an outside user tries to interfere with the conversation, split tones make it harder for the outsider to be heard within the group. To interfere with the group conversation, the outsider must scan the transmitting and receiving tones on two separate channels. Suppose the conversation between the two users in the group is short enough. In that case, the outside interfering user may not have enough time to scan channels and tones to engage in undesired conversation.

Split Frequency Example

In the example below, radio one receives on channel three and transmits on channel six. Radio two receives on channel six and transmits on channel three. The casual outside radio listener would hear one side of the conversation, and the outside radio user can communicate with the group on one of the channels.
Radio One Radio Two
Channel 3 Receive Channel 3 Transmit
Channel 6 Transmit Channel 6 Receive

Split Frequency and Tone Example

In the case above, the outside users hear one side of the conversation, and the outsider can engage in undesired communication with the group. By adding split tones to the channels, the outsider has more difficulty engaging in undesired communication. In the example below, radio one receives on channel three with a DCS code of DN174 and transmits on channel six with a DCS code of DN174. Radio two receives on channel six DCS code of DN205 and transmits on channel three DCS code of DN205.
Radio One Radio Two
Channel 3 (DN174) Receive Channel 3 (DN174) Transmit
Channel 6 (DN205) Transmit Channel 6 (DN205) Receive

Programming

The two radios need to be programmed separately with split frequency and split tone operation or both. For example, if both radios are programmed with one of the above configurations in the radio’s slot, they must be programmed separately. In the example below, both radios are set up for split frequencies on channels three and six and split DSC tones DN174 and DN205 on radio slot 39.

Radio One Program Setup

Radio Slot Radio Receive Radio Decode Radio Transmit Radio Encode
39 Channel 3 DN174 Channel 6 DN205

Radio Two Program Setup

Radio Slot Radio Receive Radio Decode Radio Transmit Radio Encode
39 Channel 6 DN205 Channel 3 DN174

Appendix

FRS and GMRS Channel Table

Channel Frequency FRS Power FRS Bandwidth GMRS Power GMRS Bandwidth Notes
01 462.5625 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
02 462.5875 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
03 462.6125 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
04 462.6375 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
05 462.6625 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
06 462.6875 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
07 462.7125 MHz 2 W 12.5 kHz 5 W 20 kHz Shared FRS and GMRS simplex.
08 467.5625 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
09 467.5875 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
10 467.6125 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
11 467.6375 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
12 467.6625 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
13 467.6875 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
14 467.7125 MHz 0.5 W 12.5 kHz 0.5 W 12.5 kHz Shared FRS and GMRS simplex.
15 462.5500 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
16 462.5750 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
17 462.6000 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
18 462.6250 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
19 462.6500 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
20 462.6750 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
21 462.7000 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
22 462.7250 MHz 2 W 12.5 kHz 50 W 20 kHz Shared FRS and GMRS simplex. GMRS repeater output.
467.5500 MHz 50 W 20 kHz GMRS repeater input only.
467.5750 MHz 50 W 20 kHz GMRS repeater input only.
467.6000 MHz 50 W 20 kHz GMRS repeater input only.
467.6250 MHz 50 W 20 kHz GMRS repeater input only.
467.6500 MHz 50 W 20 kHz GMRS repeater input only.
467.6750 MHz 50 W 20 kHz GMRS repeater input only.
467.7000 MHz 50 W 20 kHz GMRS repeater input only.
467.7250 MHz 50 W 20 kHz GMRS repeater input only.

Midland CTCSS and DCS Codes

CTCSS Privacy Codes Frequency Chart

Code Freq. Code Freq. Code Freq. Code Freq. Code Freq.
1 67.09 9 91.5 17 118.8 25 156.7 33 210.7
2 71.9 10 94.8 18 123.0 26 162.2 34 218.1
3 74.4 11 97.4 19 127.3 27 167.9 35 225.7
4 77.0 12 100.0 20 131.8 28 173.8 36 233.6
5 79.7 13 103.5 21 136.5 29 179.9 37 241.8
6 82.5 14 107.2 22 141.3 30 186.2 38 250.3
7 85.4 15 110.9 23 146.2 31 192.8
8 88.5 16 114.8 24 151.4 32 203.5

DCS Privacy Codes Chart

No. Code No. Code No. Code No. Code
1 23 27 165 53 413 79 731
2 25 28 172 54 423 80 732
3 26 29 174 55 431 81 734
4 31 30 205 56 432 82 743
5 32 31 223 57 445 83 754
6 43 32 226 58 464 84 36
7 47 33 243 59 465 85 53
8 51 34 244 60 466 86 122
9 54 35 245 61 503 87 145
10 65 36 251 62 506 88 212
11 71 37 261 63 516 89 225
12 72 38 263 64 532 90 246
13 73 39 265 65 546 91 252
14 74 40 271 66 565 92 255
15 114 41 306 67 606 93 266
16 115 42 311 68 612 94 274
17 116 43 315 69 624 95 325
18 125 44 331 70 627 96 332
19 131 45 343 71 631 97 356
20 132 46 346 72 632 98 446
21 134 47 351 73 654 99 452
22 143 48 364 74 662 100 454
23 152 49 365 75 664 101 455
24 155 50 371 76 703 102 462
25 156 51 411 77 712 103 523
26 162 52 412 78 723 104 526
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