How a Repeater Works
One of the first contacts a new amateur radio operator makes is generally through a VHF or UHF repeater. The new ham gets his or her new little walkie talkie, dials up a local repeater, gets up their nerve, and then pushes the Push To Talk (PTT) button and freezes. After two or three of these failed attempts, the operator finally remembers to push the button, and quickly blurts out their callsign, in that order. If the repeater is working properly, the HT has been programmed correctly, and the operator is within receiver range of the repeater, that callsign is blasted out a high power radio located high on a tower to over 7,000 square miles of space, and hundreds of amateur radio receivers.
While most of us do this every day, we rarely think through how this process works. This page will walk through what happens to make that 4 second transmission happen. While this process is typical of all repeaters, the photos and exact procedures are based on the W5NOR VHF repeater. This system includes a single transmitter, a main receiver, remote receiver linked by UHF radios, a UHF control link, and a controlling analog phone line. In addition, this system is monitored by a remote receiver that listens to the repeater, digitizes the audio, and streams the output to the servers at Broadcastify.com, where those four seconds of audio are recorded and stored on their servers for 180 days.
Yes, this is a complex environment, but it gives us a chance to run through how all of this works, every time you press the button.
This entire process starts with the press of a PTT button. To send a signal toward the W5NOR VHF repeater, the operator sets the receiver frequency to 147.06 MHz, the transmit offset to +600 KHz, which results in a transmit frequency of 147.66 MHz. The operator then needs to set the Continuous Tone-Coded Squelch System (CTCSS) code to 141.3 Hz for the main receiver, or 100 Hz for the remote receiver. You may also hear this referred to as a PL code. PL is Motorola's abbreviation of their term for this feature, called "Private Line".
- First, the handie talkie turns off the receiver, and the receiver audio in that radio. This keeps the operator from hearing their own voice from their own radio, and stopping any possible audio feedback loop.
- Most radios will also turn on a local light, or LED, that will signify that the radio is in transmit mode.
- The radio turns on the transmitter, by powering up a stable signal generator at 147.66 MHz. This is the carrier wave that the repeater receiver(s) will be listening for.
- Next, the CTCSS tone is mixed with the carrier wave, using a frequency modulation technique. This varies the carrier wave frequency in relation to the input signal of the CTCSS tone. This image shows you a graphic representation of the input CTCSS wave, and the resulting output (Amplitude Modulation) AM & FM waves:
The repeater receiver(s) will sense this tone, and use it to properly switch the received signal.
- Using the same FM technique, the audio from the microphone is mixed with the carrier wave, and the CTCSS tone to produce the output signal.
- The transmitter section of the radio will amplify this signal, and use the attached antenna to send this signal out to the local area. Increasing the output power and the antenna efficiency will result in a stronger signal, that will reach a farther distance.
- Some of the newer, digital handie talkies contain a transmitter timeout (TO) timer. These timers help the operators control the unintended output of their radio. Should the operator hold the PTT button down longer than that time, the radio will automatically unkey.
- Once the radio has been unkeyed, the radio will turn off the transmitter section of the radio, turn off the transmit indicator light, or LED, and turn the receiver section back on.
The first part of this process is now complete, and the signal is flying from the radio, toward the receivers, at the speed of light.