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Communications for your Astronomy Event
"Sub Channels?"

by
John Huggins


For many years two-way radios had a feature called by many names including Motorola's Private Line, GE/Ericsson's "Channel Guard", E.F. Johnson's "Call Guard", RCA's "Quiet Channel." Others simple call it tone activated squelch. Some call it by the most correct name (probably) of Continuous Tone Coded Squelch System (CTCSS).

All this CTCSS stuff really is is a tone that is sent along with your voice to the receiving radio. If you were to listen to this combination on a good stereo, you would hear your voice along with a very annoying low tone. However, radio engineers long ago realized that communications via voice did not need superb audio fidelity. So the audio that is allowed through is between about 300Hz - 3,000Hz. This really does stink compared to what you expect when you play music, but for passing the human voice, this is fine.

So...

CTCSS takes advantage of the unused bandwidth below 300Hz by mixing one of the many tones listed below into the audio.

If the radio receiver chooses to, it can keep the radio squelch circuit silent unless a particular tone is found.

What this means for you is that each frequency can have multiple users... sort of. If your group chooses one tone, and another chooses a second tone, each group's radios will not open squelch when the other group transmits. However, if two transmitters key up at the same time, a receiver will only sense the closest transmitter.

The moral of the story is that CTCSS sub-channels are a good idea, but don't expect they provide you with a real channel; You are still sharing the same RF frequency with others just like a party line telephone system with all the same issues.

For completeness I have included an absurd table containing the available tone frequencies and a cross reference to what other names. Since most radios seem to have adopted a 1 - 38 sub-channel system we at last have a way to express what CTCSS tone we all wish to utilize with a simple number.

"Meet me on channel 16, sub-channel 5."

Tone
Frequency
(EIA in Bold)
Motorola
Old
Alpha
Code
FRS and FRS/GMRS Radios Ham Radios
Uniden
Code
Measured
Motorola
Talkabout
Code
Published
Cobra
Code
Published
Icom
Z1A
2M/440
Handheld
Witnessed
67.0 Hz XZ 1 1 1 67.0
69.3 Hz WZ - - - 69.3
71.9 Hz XA 2 2 2 71.9
74.4 Hz WA 3 3 3 74.4
77.0 Hz XB 4 4 4 77.0
79.7 Hz WB 5 5 5 79.7
82.5 Hz YZ 6 6 6 82.5
85.4 Hz YA 7 7 7 85.4
88.5 Hz YB 8 8 8 88.5
91.5 Hz ZZ 9 9 9 91.5
94.8 Hz ZA 10 10 10 94.8
97.4 Hz ZB 11 11 11 97.4
100.0 Hz 1Z 12 12 12 100.0
103.5 Hz 1A 13 13 13 103.5
107.2 Hz 1B 14 14 14 107.2
110.9 Hz 2Z 15 15 15 110.9
114.8 Hz 2A 16 16 16 114.8
118.8 Hz 2B 17 17 17 118.8
123.0 Hz 3Z 18 18 18 123.0
127.3 Hz 3A 19 19 19 127.3
131.8 Hz 3B 20 20 20 131.8
136.5 Hz 4Z 21 21 21 136.5
141.3 Hz 4A 22 22 22 141.3
146.2 Hz 4B 23 23 23 146.2
151.4 Hz 5Z 24 24 24 151.4
156.7 Hz 5A 25 25 25 156.7
159.8 Hz - - - - 159.8
162.2 Hz 5B 26 26 26 162.2
165.5 Hz - - - - -
167.9 Hz 6Z 27 27 27 167.9
171.3 Hz - - - - 171.3
173.8 Hz 6A 28 28 28 173.8
177.3 Hz - - - - 177.3
179.9 Hz 6B 29 29 29 179.9
183.5 Hz - - - - -
186.2 Hz 7Z 30 30 30 186.2
189.9 Hz - - - - -
192.8 Hz 7A 31 31 31 192.8
196.6 Hz - - - - -
199.5 Hz - - - - -
203.5 Hz M1 32 32 32 203.5
206.5 Hz 8Z - - - -
210.7 Hz M2 33 33 33 210.7
218.1 Hz M3 34 34 34 218.1
225.7 Hz M4 35 35 35 225.7
229.1 Hz 9Z - - - -
233.6 Hz M5 36 36 36 233.6
241.8 Hz M6 37 37 37 241.8
250.3 Hz M7 38 38 38 250.3
254.1 Hz 0Z - - - -
270.4 Hz - - - - -

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