Repeater Questions and Answers
What is a repeater?
A repeater is in concept a rather simple device. It listens on one frequency, then re-transmits what it hears on another. These machines are usually located in high-up places and are equipped with efficient antennas, extremely low loss feedlines, and a power amplifier for the transmitter. The end result? People using a repeater get much longer range from their radio equipment than would be possible talking simplex (point to point without a repeater). This is how an individual with a handheld transceiver can easily communicate with people many miles away and still have a low noise floor and great clarity.
How does it work?
A repeater might appear complicated at first, but if we take it one piece at a time it's really not so tough. A basic repeater consists of several individual 'blocks' that when connected, form a functional system. Here's a simple block diagram:
The antenna serves to both transmit and receive the RF (Radio Frequency) signal that is going in to and out of the repeater. It's generally a high performance, durable, and rather efficient antenna located as high on a tower as we can get it. Antenna systems of this type can easily cost $1000 or more.
The feedline on most repeaters isn't just a piece of standard coax cable, it's what's called 'hard line'. This stuff is more like a pipe with a center conductor than a cable. It's hard to work with and expensive. So why do we use it? Performance! The signal loss is much lower in hard line than in standard cable, so more power gets to the antenna and weaker signals can be received. Remember, the signal at a repeater site doesn't just travel a few feet to an antenna like in a mobile rig. It goes hundreds of feet up the tower to the antenna. Just for fun check out the specs on a roll of coax some time and see how many dB of loss you'll get from 200 feet of cable. Hard line also tends to be more durable than standard cable, which increases reliability and helps us minimize tower climbs to replace it.
This device serves a critical role in a repeater. To make a long story short, the duplexer separates the incoming signal from the outgoing and vice versa. Even though the repeater's input and output frequencies are different, the duplexer is still needed. Why? Have you ever been in a place where there's lots of RF activity, and noticed the receive performance of your radio degrades to some degree? This is called de-sense, and it's a Bad Thing. The receiver gets hosed up from the strong RF signals being radiated in its vicinity and confused about which signal it should receive. The result is poor receive quality, or in extreme cases, lack of receive capability. Keep in mind that in this example, the radios are picking up radiated power from one another and that's enough to cause trouble. Now imagine how much trouble there will be if you not only have the transmitter and receiver close together, but connect them to the same antenna! Transmitting only a few hundred kHz away in frequency would blow away the input to the receiver. That's where the duplexer comes in. It prevents the receiver and transmitter from 'seeing' one another on the antenna.
Receives the incoming signal. This receiver is generally a sensitive one which helps weaker stations to get into the repeater with a good signal. It's also where CTCSS (Continuous Tone Coded Squelch System) decoding takes place. CTCSS, or 'tone' or 'PL tone', is a sub-audible tone that's sent out by a repeater user's transceiver. This is used to help the repeater discern between a person who wants to use it, and spurious RF noise or intermod interference. The general thinking is "If the squelch gate is open, and there's a PL tone detected, then it's probably a Ham and not just noise". Many repeaters use this system with good results including the NARA machines. Most (if not all) NARA machines use a 100Hz tone.
This is the brain of the repeater. It handles station ID (through either CW or the "Romeo" voice), activates the transmitter at the appropriate times, controls the autopatch, and sometimes does many other things. The 147.180 and 146.940 machines also have a DVR (Digital Voice Recorder) for announcements and messages. The controller is a little computer that's programmed and optimized to control a repeater. The various models of controllers have different useful features like speed-dial for phone patches, a voice clock, facilities to control a remote base, etc. The controller gives the repeater its 'personality'. Whenever you're using a repeater, you're interacting with its controller.
The NARA machines have a transmitter composed of an 'exciter' and a power amplifier. The exciter modulates the audio at the proper transmit frequency, and the power amplifier simply boosts its level so the signal will travel further.
Operating using a repeater isn't difficult. A good source of info is the ARRL Repeater Directory. It's an inexpensive book with repeater listings all over the US. It contains frequency, offset and whether the repeater is + or - shift, and whether or not it requires a PL tone. The ARRL directory also has a great section in the front regarding proper, legal, and courteous operation that would be of great value for all hams, both new and old, to read once in a while. For the local Huntsville area, there's a list here as well.