Split Frequency and Tone Operation on GMRS

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.

GMRS and FRS Defined

Operation

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 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 must be programmed separately with split frequency and 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