2.2 Simplex vs. Repeater Operations

2.2 Simplex vs. Repeater Operations

For the newly licensed Technician, the VHF (Very High Frequency) and UHF (Ultra High Frequency) bands are where you will likely spend most of your time. Operating on these bands primarily involves two different methods of communication: simplex and repeater operations. Understanding how they work and the etiquette associated with them is essential for successful local and regional communications.

Simplex Operations

Simplex operation is the most basic form of radio communication. It involves transmitting and receiving on the exact same frequency. When you talk directly to another station without any intermediary equipment, you are operating simplex. This is effectively how walkie-talkies work.

Because VHF and UHF signals travel primarily by line-of-sight, the range of simplex communications is limited by terrain, buildings, and the curvature of the Earth. A handheld radio communicating simplex might only reach a few miles, while a mobile radio with an external antenna could reach 10 to 20 miles depending on the elevation.

To facilitate finding other operators on simplex, the amateur community has designated specific calling frequencies. The most important one to remember is the national 2-meter FM simplex calling frequency: 146.520 MHz. The idea is to monitor this frequency to find people looking for a contact. Once a contact is established, the polite practice is to quickly move (QSY) to a different, agreed-upon simplex frequency to leave the calling frequency clear for others.

Repeater Fundamentals

When you need to communicate beyond your simplex range, you use a repeater. A repeater is an automated station, typically located on a high point like a mountain, tall building, or tower. It receives your weak signal and simultaneously re-transmits it at a much higher power level from its advantageous location, vastly extending your communication range.

Because a repeater must transmit and receive at the exact same time, it cannot do this on a single frequency without its powerful transmitter overwhelming its own sensitive receiver. Therefore, repeaters use a pair of frequencies: an input frequency (what the repeater listens to, and you transmit on) and an output frequency (what the repeater transmits on, and you listen to).

The difference between these two frequencies is called the offset.

• On the 2-meter band, the standard offset is 600 kHz (0.6 MHz).
• On the 70-centimeter band, the standard offset is 5.0 MHz.

Offsets can be positive (input is higher than output) or negative (input is lower than output), depending on where the repeater's output frequency sits within the band plan. Most modern radios have an automatic repeater shift (ARS) feature that automatically applies the correct standard offset when you tune to a known repeater output frequency.

CTCSS and DCS: Keeping the Noise Out

Because repeaters are situated on high elevations with sensitive receivers, they are prone to picking up distant signals, intermodulation, and random RF noise. To prevent the repeater from constantly keying up (transmitting) every time it hears a burst of static, most repeaters use an access tone system.

The most common system is CTCSS (Continuous Tone-Coded Squelch System), often referred to by Motorola's trademarked name, PL tone (Private Line). CTCSS is a sub-audible low-frequency tone that your radio transmits continuously along with your voice. The repeater is programmed to only open its receiver (unsquelch) if it detects that specific tone. If you do not have the correct CTCSS tone programmed into your radio for that specific repeater, you can transmit all day, and the repeater will ignore you.

Another system is DCS (Digital-Coded Squelch), which serves the same purpose but uses a continuous stream of digital data instead of an analog tone. When programming a repeater into your radio, you must know its output frequency, its offset direction, and the required CTCSS or DCS tone.

Time-Out Timers

If a user were to sit on their microphone button (a "stuck PTT") or talk endlessly, they would monopolize the repeater and eventually cause its transmitter to overheat and fail. To prevent this, repeaters are equipped with a time-out timer.

This timer automatically cuts off the repeater's transmitter if it receives a continuous signal for a predetermined amount of time—typically 3 minutes. When communicating through a repeater, you should pause briefly between transmissions. This allows the timer to reset and gives other stations a chance to break in if they have emergency traffic or want to join the conversation.

Repeater Etiquette and Systems

When using a repeater, the etiquette is slightly different from simplex. Because you are using a shared community resource, conversations should be polite, inclusive, and relatively concise. You don't need to call "CQ" on a repeater. Instead, you simply announce your presence by stating your call sign, such as "KN4XYZ listening."

Most repeaters are open, meaning they are available for any licensed amateur to use. Some, however, are closed repeaters, restricted to members of a specific club or group that funds the equipment. You should respect the owner's wishes and not use a closed repeater unless invited or in an emergency.

Modern repeaters are often part of linked systems. Technologies like EchoLink and IRLP (Internet Radio Linking Project) connect local RF repeaters to the internet via Voice over IP (VoIP). This allows a user with a handheld radio in California to talk through their local repeater directly to a repeater in London, England. This hybridization of RF and internet technology has massively expanded the capabilities of the Technician class license.