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Every repeater has a squelch system that determines what signals it will retransmit. The two main approaches - carrier squelch and tone squelch - work very differently, and understanding them is key to using repeaters successfully.
This trips up a lot of new operators. A GMRS repeater doesn't live on some special frequency of its own. It uses one of the 8 main GMRS channels (15-22, often labeled RPT-15 through RPT-22 on radios). For example, the Plumstead repeater "on 462.575" is just using GMRS channel 16 - the exact same 462.5750 MHz anyone can use for open, radio-to-radio simplex. The only thing that makes it a repeater is the machine itself: it listens 5 MHz up (467.5750, the input) and rebroadcasts what it hears back down on 462.5750 (the output).
So the repeater's output and ordinary simplex chatter share the same channel. What separates them is the tone - and that tone is a filter on what your speaker plays, not a change of frequency.
Worked example - Plumstead on channel 16 (462.5750, 229.1 Hz in/out):
Same frequency either way. The receive tone simply decides which traffic on that channel your radio bothers to play. Programming "229.1 in and out" buys you a clean, repeater-only experience - at the cost of not hearing un-toned traffic on the same channel.
Note: this only works because Plumstead sends a tone on its output. Many repeaters use a tone on the input but carrier (no tone) on the output - see Mixed-mode repeaters below, where a receive tone would mute the repeater itself.
Carrier squelch is the simplest approach: the repeater retransmits any signal it receives, as long as the signal is strong enough. No special tone or code is needed. If you key up on the correct frequency, the repeater opens.
Tone squelch adds a gate: the repeater only retransmits signals that include a specific sub-audible CTCSS tone. Your radio transmits this tone continuously whenever you key up (you can't hear it - it's below normal audio frequencies). The repeater checks for this tone before opening its squelch.
DCS (Digital-Coded Squelch) serves the same gating purpose as CTCSS but uses a digital data stream instead of an analog tone. Your radio sends a continuous three-digit code (like D023 or D156) below the voice audio. DCS offers more unique codes than CTCSS - 104 standard codes vs. 50 tones - which helps in areas with many overlapping repeaters. Programming is identical: enter the DCS code in the transmit tone field just as you would a CTCSS frequency. If a repeater listing shows a DCS code, use DCS; otherwise use CTCSS.
Monitor the repeater output before you transmit. Listen for how it behaves when it opens:
If a repeater listing shows a CTCSS frequency or DCS code, it uses tone squelch and that code is required. If no tone is listed, assume carrier squelch.
Some repeaters use tone squelch on the input (what they receive from users) but carrier squelch on the output (what they transmit to listeners). This means you must send the correct tone to key up the repeater, but any radio can receive the retransmitted signal without programming a receive tone. This is the most common real-world configuration - program a transmit tone and leave your receive tone set to none (open squelch).
Most GMRS repeaters use tone squelch. If a repeater listing shows a CTCSS tone (like 141.3 Hz) or DCS code, that tone is required to activate the repeater. Program it as your transmit tone. If the listing shows no tone, the repeater likely uses carrier squelch.
The squelch type affects what you hear when conditions are bad. On a carrier-squelch repeater, nearby interference - a malfunctioning device, a distant signal on the same frequency, or intermod from two strong transmitters mixing - causes the repeater to open and retransmit garbage. You'll hear bursts of noise or distorted audio with no one talking. On a tone-squelch repeater, that same interference doesn't include the correct sub-audible tone, so the repeater ignores it and stays silent. Tone squelch effectively filters out most unintentional interference at the cost of requiring everyone to know the access tone.
When programming a repeater channel, you have several tone-related options:
Use this to choose your settings for any repeater:
You can hear a repeater just fine, but when you key up, nothing happens - the repeater doesn't retransmit your signal. The most likely cause: the repeater requires a CTCSS/DCS tone that you haven't programmed. Your signal reaches the repeater, but without the correct tone, the repeater's squelch stays closed and it ignores you.
Check the repeater listing in our GMRS repeater directory, on myGMRS.com, or RadioReference. The listing will show the required CTCSS tone frequency (e.g., 141.3 Hz) or DCS code (e.g., D023). Program this as your transmit tone on the repeater channel. For more on CTCSS and DCS, see CTCSS and DCS Tones Explained.