Repeaters

Picture: The view here is that at the base of the KGW towers located in Portland Oregon. The red and white tower is 1100' tall. It's so tall it has an elevator. Without the elevator, crews trying to access the top would take around 2 hours to get there.

Overview

The goal of this section is to provide some understanding to what repeaters are and what they do. In amateur radio, analog only repeaters have been the mainstay of the hobby. Since there isn't any legal objection, repeaters have been deployed to support lots of different modes. Some are open standard commercial standards based and others are specific to amateur radio. I won't be going into any detail into any other types other than DMR repeaters.

Repeaters – what are they and how to use them 

*Repeaters – what are they and how to use them (arrl.org) 

More hams use frequency-modulated (FM) voice than any other communications mode. Most hams have an FM rig of some type. They use it to keep in touch with their local friends. Hams often pass the time during their morning and evening commute talking on the air. In most communities, amateurs interested in a specialized topic (such as chasing DX) have an FM frequency where they meet regularly to exchange information. At flea markets and conventions, hand-held FM units are in abundance as hams compare notes on the latest bargain.

Generally, it's a good idea to use VHF or UHF for all local communications. The HF bands should be reserved for longer-distance contacts to reduce interference on the HF bands.

VHF and UHF FM voice operation takes two forms: simplex and repeater. Simplex operation means the stations are talking to each other directly, on the same frequency. This is similar to making a contact on the HF bands.

FM voice operation is well-suited to local VHF/UHF radio communication because the audio signal from an FM receiver is not affected by static-type electrical noise. Car engines and ignition systems produce quite a bit of static electrical noise, and many hams like to operate their FM radios while they are driving or riding in a car. (This is called mobile operation.) An AM or SSB receiver is affected much more by static-type electrical noise.

The communications range for VHF and UHF FM simplex is usually limited to your local area (5-15 miles). If you live high on a mountain and use a high-gain directional antenna, you may be able to extend your range considerably. Unfortunately, most of us do not have the luxury of ideal VHF/UHF operating conditions. Often, we want to make contacts even though we live in a valley, are driving in a car or are using a low-power, hand-held transceiver.

Enter repeaters. A repeater receives a signal and re-transmits it, usually with higher power and from a better location, to provide a greater communications range. Often located atop a tall building or high mountain, VHF and UHF repeaters greatly extend the operating range of amateurs using mobile and hand-held transceivers. If a repeater serves an area, it's not necessary for everyone to live on a hilltop. You only have to be able to hear the repeater's transmitter and reach the repeater's receiver with your transmitted signal.

A repeater receives a signal on one frequency and simultaneously retransmits (repeats) it on another frequency. The frequency it receives on is called the input frequency, and the frequency it transmits on is called the output frequency. To use a repeater, you must have a transceiver that can transmit on the repeater's input frequency and receive on the repeater's output frequency. The input and output frequencies are separated by a predetermined amount that is different for each band. This separation is called the offset. For example, the offset on 1.25 meters is 1.6 MHz. A repeater on 1.25 meters might have its input frequency on 222.32 MHz and its output on 223.92 MHz. Repeater frequencies are often specified in terms of the output frequency (the frequency you set your receiver to listen on) and the offset. Your transmitter operates on a frequency that is different from the receive frequency by the offset amount.

Most transceivers designed for FM repeater operation are set up for the correct offset. They usually have a switch to change between simplex operation (transmit and receive on the same frequency) and duplex operation (transmit and receive on different frequencies). So, if you wanted to use the repeater in the preceding example, you would switch your transceiver to the duplex mode and dial up 223.92 to listen to the repeater. When you transmit, your rig will automatically switch to 222.32 MHz (1.6 MHz lower in frequency), the repeater input frequency.

When you have the correct frequency dialed in, just key your microphone button to transmit through ("access") the repeater. Most repeaters are open -- that is, available for use by anyone in range. Some repeaters, however, have limited access. Their use is restricted to exclusive groups, such as members of a club. Such closed repeaters require the transmission of a continuous subaudible tone or a short "burst" of tones for access. These are called CTCSS (continuous tone-coded squelch system) or PL (Private Line PL is a Motorola trademark) tones. There are also some repeaters available for use by everyone that require the use of special codes or subaudible tones to gain access. The reason for requiring access tones for "open" repeaters is to prevent interference from extraneous transmissions that might accidentally key the repeater. If you wish to join a group that sponsors a closed repeater, contact the repeater control operator. 

Adding On

Something that is important to remember, is that when you use a traditional analog repeater, only one person can talk at a time and everyone else must listen. To plant the seed, start to think about the limitations surrounding the repeater only allowing one person to talk at a time. Here are a couple of examples where that can become problematic:

• The topic being discussed isn't of interest

• Multiple users need to communicate to each other to accomplish their communication goals.

  • Think about an emergency situation where multiple groups are working to resolve an emergency, but they need to have their own uninterrupted communications. Police are trying to control traffic while the Fire Department is working to put out the fire. If they were forced to share, it could threaten lives of either group. 

• Also think about what happens if you move outside the coverage area of the analog repeater.

If you think I am asking setup questions, you'd be right.

Additional Notes

• Many examples of repeater operations use a geographic (mountains or terrain) or other obstacles like structures to show that simplex operations isn't possible and that a repeater must be used. This is sort of misleading. When you think about radio propagation or range, there are several factors that impact it.  Those factors include: Terrain, Environment, Weather, Power, Antenna, and Free Space Loss (FSL) along with other physical  losses. If you simply think about radio range as being related to Line of Sight (LoS) then it will start to make more sense. One of the best similarities would be a flash light. The brighter your flash light and the more focused the beam is, the further it will travel. Radio waves are very similar. 

• Another common issue with repeaters is an imbalance to the output of a repeater and the output of a subscriber (portable, mobile, or base station radio) in the field. A great example of where this typically impacts users is during portable use. Typical portable radios are limited to 5 watts of output power. Repeater output power is only limited by license. For this example, let's use 50 watts. Meaning, the repeater is 10 times more powerful than the portable. (Or is it?) What this means is that the repeater will likely be heard further than the portable can "talk back" to the repeater. Systems must be designed with the lowest powered subscriber in mind. There are some tricks that can be used like "preamps" or Pre-Amplifiers that increase the level of the receive signal, but they have limits and they cannot amplify a signal that isn't there.

Summary

As you can tell, repeaters have become key to our communications throughout our communities. Repeaters are great for allowing us to communicate over great distances. Understanding a repeaters functional design to support radios in the field will let you understand what to expect for coverage,