What is a Repeater?       
What is a Repeater?
A duplex repeater, in concept, is not really a complicated device. It's a 'duplexed' two-way radio set that listens on one frequency, then re-transmits what it hears on another; at exactly the same time. These systems are usually located in places of high elevation (on mountains and tall buildings, in Florida its high towers from 200ft to 1500ft) and are equipped with large - efficient antennas, extremely low loss Feedline, and a transmitter and receiver that is very durable and rated for continuous duty. The end result? People using a repeater get much greater range from their radio equipment that would not be possible talking simplex. This is how an individual with a portable walkie-talkie (handheld) transceiver can communicate with people many miles away with good clarity. A ham (Amateur) repeater is often times referred to as a "machine". Repeaters are used in Commercial (Business) Communications, Emergency Communications (either by 'hams' or by Federal or Local Government agencies), and even Pleasure Communications. These machines might have multiple sources of power, including batteries for when commercial power is lost. Repeaters can be built that are extremely power efficient, and may run exclusively from batteries; recharged by solar, wind or water power.

What is Simplex?
Simplex is point to point communications without the use of a repeater. Simplex operation utilizes the same frequency for receive and transmit. I.E. Portable to Portable or Mobile to Mobile.

What is a Simplex Repeater?
These machines listen on the frequency for activity, when it recognizes something it will begin to record that activity for a pre-determined time; usually 1/2 to 1 minute. After the activity ceases or the time has expired, the unit will repeat what it has recorded. This method of communications is somewhat cumbersome over a conventional repeater; because you are forced to listen to what you said earlier in time; however it should not be discounted as these types of systems can be very beneficial.

What is Duplex?
Full duplex operation is, where both people can talk at the same time. In contrast, 2 mobiles or a pair of handhelds can operate in half-duplex mode because only one person can talk at a time. Since the 'repeater' listens and talks at the same time in relaying your message, it operates in full duplex mode.

How does a Repeater work?
A repeater might appear complicated, but if we take it apart piece by piece,  it's really not so difficult to understand. A basic repeater consists of several individual pieces, that when connected, form a duplex repeater system.
Here's a simple block diagram of a repeater:
Antenna -
Most repeaters use one antenna, some use 2 beams to target a certain perimeter. The antenna serves both the transmit and receive RF (Radio Frequency) signals that are 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 or structure as we can get it. Antenna systems of this type can easily cost $800 or more.

Feedline -
The feedline on most repeaters isn't just a piece of standard coax cable, it's what's called Hardline. 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 Hardline 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 may go 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. Hardline also tends to be more durable than standard cable, which increases reliability and helps us minimize tower climbs to replace it.

Duplexer - (cans)
This device serves a critical role in a repeater. To make a long story short, the duplexer separates and isolates the incoming signal from the outgoing and vice versa. Even though the repeaters 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 on a repeater. The receiver goes deaf or gets desensitized 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 if the equipment was simply connected together with a Tee. That's where the duplexer comes in; it prevents the receiver and transmitter from 'hearing' one another by the isolation it provides. A duplexer is a device that is referred to by several different names like cavities or cans. A duplexer has the shape of tall canisters and is designed to pass a very narrow range of frequencies and to reject others. There is some loss to the system because of the duplexer, however, the advantage of being able to use a single antenna usually outweigh the drawbacks.
Telewave Duplexers
Receiver -
Receives the incoming signal. This receiver is generally a very sensitive and selective one which helps weaker stations to be heard better by the repeater. It's also where CTCSS (Continuous Tone Coded Squelch System) or "PL" decoding takes place.

Transmitter -
Most 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.

The "Station" -
The term "Station" is used to describe a stationary two way radio set; which includes the transmitter, receiver and sometimes the control circuitry. A 'Repeater Station' is a station designed to be used as a duplex repeater.

Controller -
This is the brains of the repeater. It handles station identification (through either CW or voice), activates the transmitter at the appropriate times, controls the autopatch, and sometimes does many other things. Some 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 or linking, etc. The controller gives the repeater its 'personality'. Whenever you're using a repeater, you're interacting with its controller. NBARC has a CAT 1000DXL for its controller.
What is a Phone Patch?
Many repeaters have a feature that allows you to place a telephone call from your radio. Phone calls are generally restricted to the local calling area of the repeater to avoid long distance charges to the repeater's sponsors. If in doubt, ask if the repeater has an open patch and how to access it. When using the patch it is common courtesy to announce your intentions, e.g. " This is WN3DHI on the patch". This may help to prevent anyone from keying up while you are trying to use the function.

A DVR is a Digital Voice Recorder, NBARC's CAT 1000DXL is equipped with a DVR for Skywarn.

Repeater Operation -
Operating using a repeater isn't difficult. A good source of info is NBARC
Repeater Courtesy.

What is Offset?
In order to listen and transmit at the same time, repeaters use two different frequencies. On the 2 meter ham band these frequencies are 600 kc's (or 600 kilohertz) apart. As a general rule, if the output frequency (transmit) of the repeater is below 147 Mhz then the input frequency (listening) is 600 kilohertz lower. This is referred to as a negative offset. If the output is 147 Mhz or above then the input is 600 kilohertz above. This is referred to as a positive offset. Virtually all ham radios sold today set the offset once you have chosen the operating frequency. As an example one repeater output is 145.250 Mhz. The input, or the frequency it listens on is 144.650 Mhz ( 600 kilohertz below). If you have your radio tuned to 145.250 Mhz, when you push the PTT switch (Push-To-Talk) your radio automatically transmits on 144.650 Mhz. When you release the PTT to listen, the radio reverts back to 145.250 Mhz to listen on the repeater's output frequency. Note: There are exceptions to the rule so check local repeater listings.

Why do Repeaters use an Offset?
To use a repeater a user station must use a different transmit frequency than receive frequency. This is a form of duplex, or two frequency operation. It is known as half-duplex as you do not receive and transmit at the same time but normally use the push-to-talk button on your microphone to switch between the two. Most repeater installations use the same antenna for transmit and receive. Without having an offset of 600 kHz the repeater would simply hear itself when it was transmitting on the same frequency it was listening on. Even with the offset, the two frequencies are close enough that some isolation is required. Again, this isolation is afforded by the Duplexer.

What is CTCSS or a PL Tone?
PL, an acronym for Private Line, is Motorola's proprietary name for a communications industry signaling scheme called the Continuous Tone Coded Squelch System, or CTCSS. It is used to prevent a repeater from responding to unwanted signals or interference. Any station may be set up to transmit this unique low frequency tone that allows the repeater to operate. If a repeater is "In PL" that means it requires a CTCSS tone to activate the repeater. In days of old, repeaters that used PL were considered to be closed or private. This is no longer the case as PL operation has become more the rule instead of the exception. CTCSS is often referred to as PL, Channel Guard, Call Guard, Quiet Channel, and others.    Tone Squelch is an electronic means of allowing a repeater to respond only to stations that encode the tone to open their radio's squelch.