A New Ham’s Guide
How to Use Amateur (Ham Radio) Repeaters by N4UJW
Used with non-commercial rights from Ham Universe All Rights Reserved
Welcome to Part 2 on How to Use Amateur (Ham Radio) Repeaters
The Basic Repeater Components:
Most repeaters use only one antenna. The antenna is used on transmit and receive signals that are going into and out of the repeater. It usually is a high performance, high gain, heavy duty, and very efficient antenna located as high on a tower or structure as we can get it above the surrounding terrain.
Lots of repeater system antennas are located on a high hill or mountain.
Antenna systems for repeater use are usually very costly and have an omni-directional pattern.
The feed line used on most repeaters is not just a piece of standard 50 ohm coax cable. A type of specialized feed line called “hard line” is usually used. It is very similar to cable tv line that you see strung between power poles around town. Look just above the telephone lines and you will see much larger “aluminum” cable. The signal loss with hard line versus regular coax is much lower than in standard coax, so more power gets to the antenna and weaker signals can be received due to the very low loss of the “hard line”.
This device serves a major role in a repeater. The duplexer separates and isolates the incoming signal from the outgoing and vice versa. It prevents the receiver and transmitter from hearing one another by the isolation it provides. A duplexer has the shape of large tall cans and is designed to pass a very narrow range of frequencies and to reject others. It helps to reject very strong nearby frequencies from other repeaters or rf producers from getting into the repeater system.
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 is set to receive the input frequency. It’s also where CTCSS (Continuous Tone Coded Squelch System) or “PL” decoding takes place.
Most “machines”, as repeaters are sometimes called, have a transmitter composed of an exciter and a power amplifier. The exciter modulates the audio coming from the receiver which is tuned to the transmitting stations’s frequency at the proper transmit frequency, and the power amplifier simply boosts its level so the signal will travel further. Lots of repeaters use 100 watts or more. It simply takes the weaker received frequency from say a mobile or ht and re-transmits it (repeats) at a higher power level on a different frequency. On the 2 meter band, this separation between transmit and receive frequency is usually 600kHz either positive or negative in relation to the transmit frequency. On the 440 (70cm) band it is usually 5mHz positive or negative in relation to the transmit frequency.
The “transmit” frequency is the frequency you tune your radio to and is usually listed in various sources. For instance, if you see a repeater listing that says:
146.90mHz, minus or negative offset, Pl tone, 100….then you would program your radio to 146.90mHz , minus 600kHz with a PL tone of 100 into the memory for that specific repeater.
NOTE that most newer radios automatically set the offset (your actual transmit frequency) so you won’t have to.
This is the brain of the repeater. It handles repeater station ID using either CW or voice, activates the transmitter at the appropriate times, and sometimes performs many other functions depending on the sophistication of the repeater. Some also have a DVR (Digital Voice Recorder) for announcements and messages. The controller is a small computer that’s programmed to control a repeater.
What is Offset?
You may have seen the word “offset” mentioned in the Tramsmitter section above. So exactly what is a repeater offset you may ask?
In order to listen and transmit at the same time, repeaters use two different frequencies. One for it’s transmit frequency and another for it’s receive frequency. On the 2 meter ham band these frequencies are 600 kHz (or 600 kilohertz) apart. On other bands, the offsets are different. 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 automatically. Example: If the repeater output is 146.840 Mhz. The input, or the frequency the repeater receiver listens on is 146.240 Mhz ( 600 kilohertz below 146.840mHz).
If you have your radio tuned to 146.840 Mhz, (the repeater’s output frequency), when you push the mic button, your radio automatically transmits on 146.240 Mhz, 600kHz down from 146.840. When you release the mic button to listen, your radio switches back to 146.840 mHz to listen on the repeater’s output frequency. Note: There are exceptions to the rule so check your local repeater listings.
Standard Repeater Input/Output Offsets
6 meters 1 MHz
2 meters 600 kHz
1.25 meters 1.6 MHz
70 cm 5 MHz
33 cm 12 MHz
23 cm 20 MHz
(Note that input/output offsets are voluntary among local and regional “Frequency Coordination Groups”.
They are not fixed in stone by the FCC! They are “recommended” offsets for a particular area.
Your area may be different. Check with your local repeater operators.
Why do Repeaters use an Offset?
Without having an offset between the transmit signal and the receive signal frequency, the repeater would simply hear itself when it was transmitting on the same frequency it was listening on!
Therefore, to use a repeater a user must use a different transmit frequency than receive frequency. Your actual transmit frequency is the exact same one that the repeater receiver is listening on. 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. Cell phones use full duplex so each party can hear the other while the he/she is talking.
Even with the offset, the two frequencies are close enough that some isolation is required. Again, this isolation is done by the Duplexer. So you can see why some repeater components interact with each other and without the basic system components….nothing would work.
What’s all those tones about?
What is a PL or CTCSS 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. Tone Squelch is an electronic means of allowing a repeater to respond only to stations that encode or send the proper tone. In other words, if a repeater is set up to operate only when a PL tone of say, 136.5Hz is heard by it’s receiver, then it will allow the transmitting station access. If your station, (your mobile, base or hand held) does not transmit the tone that the repeater receiver has been programmed for, when you key up, then the receiver of the repeater does not hear you and will not be usable by your station until you set the proper tone in your radio to be transmitted when you key your mic. Any modern station may be set up to transmit this unique low frequency tone that allows the repeater to operate. If a repeater is “In PL mode” that means it requires a CTCSS tone (PL tone) to activate the repeater. Due to severe congestion of ham repeaters in some areas, most repeaters are “PL’ed”.
These repeaters were once called closed repeaters.
TABLE OF COMMON PL TONES (in Hz)
67.0 94.8 131.8 171.3 203.5
69.3 97.4 136.5 173.8 206.5
71.9 100.0 141.3 177.3 210.7
74.4 103.5 146.2 179.9 218.1
77.0 107.2 151.4 183.5 225.7
79.7 110.9 156.7 186.2 229.1
82.5 114.8 159.8 189.9 233.6
85.4 118.8 162.2 192.8 241.8
88.5 123.0 165.5 196.6 250.3
91.5 127.3 167.9 199.5 254.1