A Detailed Controduction of Repeater

This article introduces fundamental concepts of FM repeater operations and depicts a high level view of typical FM phone repeater anatomy and functioning. The goal is to demystify repeaters and help the newly licensed Technician overcome any initial bewilderment about repeaters. 

Repeater Basics:

As the name implies, an FM repeater repeats your radio signal. It is just an amateur radio station that has been designed for the special purpose of retransmitting your signal instantaneously as it is received. Typically, an FM repeater station will be located in a high position, perhaps on a hill or mountain, or on a sizable tower or building. A repeater station may also retransmit with higher power than an operator uses with a handheld transceiver or other station transmitting to the repeater. As a result, the FM repeater’s relay of your signal is transmitted over a much broader area than you can achieve with your station alone. The fundamental benefit is that operators who are geographically separated significant distances can use the repeater to achieve radio contact when simplex operations are not feasible or practical due to the separation or terrain.

Further, because an FM repeater uses published, unchanging frequencies, it is a convenient way for amateurs within its reach to convene on the air. Groups of amateur operators will use a repeater to operate nets, on-air meetings at prearranged times, usually for a specific purpose or topic of interest. Nets on repeaters may also be convened by amateur radio emergency response agencies to coordinate emergency aid efforts across a broad area within the repeater’s range.

While multiple types of repeaters are operated by amateurs around the world, the most common type is the FM voice repeater using VHF or UHF frequencies. But repeaters are operated using single sideband mode, digital modes, and HF frequencies, as well. In this article we will refer almost exclusively to those most common VHF/UHF FM repeaters for voice or phone mode.

FM Repeater Operation: What do you need to do to use an FM repeater with your transceiver? Let’s consider the practical aspects of repeater operation using an example. Then we can build on these basics for a more thorough understanding of repeater functions.

As an FM repeater receives your signal it must retransmit it on a different frequency. It cannot retransmit on the same frequency that you are using to reach the repeater – that would cause a feedback loop in which the repeater’s receiver would “hear itself” transmitting and then try to retransmit itself! Trouble lies there.

Frequency Pairs:

Instead, repeaters use frequency pairs: One frequency is used to receive signals from repeater users and another frequency is used to retransmit those received signals. Consider the UHF 70- centimeter band example in the graphic below. Each calling station transmits to the repeater using 442.725 MHz. Each station monitors the repeater using a frequency of 447.725 MHz. When the pedestrian HT radio operator transmits his signal (red arrows), the 442.725 MHz transmission is received by the repeater and it retransmits the signal on 447.725 MHz to be received by any other stations monitoring that repeater frequency. Any other station, such as the SUV mobile station (blue arrows) operates exactly the same way, transmitting on the lower frequency 442.725 MHz and listening on the higher 447.725 MHz frequency.
 

Squelch Tones: 

However, many repeaters utilize one additional wrinkle that you will need to include in your channel programming or the repeater will ignore your transmissions. A repeater often will use a special method of squelch in its receiver, and you must include the correct squelch information in your transmission to open the squelch of the repeater. One of the most common methods of repeater squelch employed in the US is a low frequency audio tone that is transmitted continuously along with your voice signal. If this continuous tone is not transmitted in your signal, the repeater’s squelch will not be opened and the repeater will not receive your transmission.

This squelch method is known as the Continuous Tone Coded Squelch System, or CTCSS. (Sometimes this is also referred to as PL tones by hams, a Motorola trademark referring to “private line,” but it is not at all private!) A repeater will use a single established tone from a set of 42 standard frequencies. When you program a channel into your radio for a specific repeater you must select the appropriate CTCSS tone used by the repeater and ensure your CTCSS transmit function is activated for the channel. When properly accomplished, your transmission will automatically include the selected continuous tone and the repeater will happily receive your signal. Repeaters often filter out the CTCSS tone from retransmissions so that it is not heard as audio by receiving stations.

Repeater Coordination: 

You can probably imagine that if two FM repeaters implementing the same frequency pair have transmission ranges that overlap, a significant interference issue arises. For example, a single operator in range of both repeaters could cause both repeaters to activate when only one activation is intended, possibly interfering with ongoing communications on the repeater of accidental activation. Since repeaters tend to have significant range it is important to ensure they are coordinated to avoid mutual interference. Consideration of repeater geographic separation, transmission power, as well as the use of different frequency pairs and careful selection of CTCSS or other squelch methods helps to avoid repeater conflicts.

Regional frequency coordinators are selected by operators and organizations whose stations are eligible to be repeaters. The coordinator recommends repeater pair frequencies and other station parameters to help avoid interference. The frequency coordinator may itself be a group or organization comprised of repeater operators or club representatives.

On-Air Protocols: 

Be aware that every FM repeater will have a community of regular operators who use it, and these communities tend to develop something of a personality for the repeater. Some repeaters may have policies or rules established for its use. For example, many wide area repeaters (repeaters that reach over a very wide range) may have a policy of priority-only traffic, or a “no rag chewing” policy. That is, the repeater operators wish to keep the repeater available for long-range communications of a more important nature than checking up on the health of your friend’s cat in the midst of an hour-long, idle chit-chat QSO. Still, other repeaters may be designated specifically for extended rag chewing, yucking it up, and generally having fun on the air within the regulations of FCC Part 97. It is a good idea to monitor a repeater for a while, contact the repeater operator, look for information about the repeater online or inquire with other hams before using a repeater extensively.

How Repeaters Work: 

Understanding a bit about how FM repeaters are designed and how they work will improve your understanding of their on-air operations. Repeaters are not very different from other transceivers, but a few specialized components have been added to affect the repeat function via frequency pairs and to automate the repeater control. Let’s take a surface glance under the hood of a typical FM repeater. Follow along in the simplified block diagram below of the most basic repeater architecture.

You may also like:

How to Input Repeater Frequencies in a Ham Radio

The Advantages of a Radio Repeater

How Do I Set Up a UHF Repeater?

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