Repeater Basics

Started by Z.O.R.G., April 15, 2024, 03:40:41 PM

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Z.O.R.G.

Repeaters can greatly extend you range for Very High Frequency (VHF – 30-300 MHz) and UHF (300-3,000 MHz) radio communications by overcoming terrain obstacles.  This is a quick guide on how the most common ones work and what you need to program into your radio to use them. 

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The two handhelds show in the picture can't communicate directly due to the mountain between them.  However, both can "see" the repeater at the top of the mountain.  When one of the handhelds keys up the repeater receives its transmission on it's receive frequency and immediately rebroadcasts it on it's transmit frequency.  The second radio is listening on the repeater's transmit frequency and receives the first radio's transmission. 

Z.O.R.G.

A repeater is actually two radios, one for receive and one for transmit.  They generally have very good antenna(s) that will amplify both the signals it receives and transmits and are located as high in elevation as is practical.  The antenna(s) are typically on a tower or other structure and is usually not located right next to the radios that make up the repeater.

A repeater uses two different frequencies, a transmit frequency and a receive frequency.  Radios accessing the repeater use the same two frequencies, but their role is reversed.  Handhelds, mobiles (vehicles) and base stations transmit on the frequency that the repeater listens (receives) on and receive on the repeater's transmit frequency. The difference between the transmit and receive frequencies is commonly refereed to as the offset.  Typically for each band there's a standard offset.  This doesn't have to be case for every repeater – but it is strongly suggested. 

The offset is expressed in MHz above or below the transmit frequency of the repeater and can be positive or negative.  For the 144-148 MHz Ham the standard offset is usually 0.6 MHz.  If the transmit frequency of the repeater is 147.000 MHz, the receive frequency is likely 147.600 or 146.400 MHz.  This is typically written as 147.000 +/- 0.6.

As there are specific frequencies set up for Ham and GMRS repeaters, it's possible that a repeater on the same frequency could be keyed up accidentally.  For example, you're in a vehicle with a 50-watt radio, a good antenna, on top of a very high hill and call on your local repeater.  There may be another repeater several counties away on a mountain on the same frequency and you end up keying both repeaters. 

To prevent this "tones" or "codes" are typically used.  A "tone" signal below human hearing that is transmitted as soon a you key your radio.  Tones typically run from 67.0 to 254.1 Hz.  A code is a digital signal that's also sent as soon a you key your radio.  Digital codes typically run from 023 to 754.  To key the repeater no now have to have the correct repeater receive frequency and the correct tone or code.  Now you'll only key the repeater you want to, not the other repeater with the same input frequency. 

You can also have tone or code control on the receive frequency of your handheld if the repeater supports it.  This is useful if you're located where two you can hear two repeaters on the same or similar frequencies.  This is a convenience to you and not required to communicate with the repeater. 

So to use a repeater, you need the following:
1. A valid license to use the frequencies
2. The transmit frequency of the repeater (what you will listen on)
3. The offset that the repeated uses for its receive frequency
4. The tone or code of the repeater.

Hopefully this is useful.

NT2C

Let me see if I have pictures of the 40W UHF repeater I built in my trailer...

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The repeater uses a pair of Motorola radios, a homebrew controller, a UHF duplexer tuned to the operating frequencies (the trailer is coordinated as an "itinerant repeater"), fed through a switch to either an exterior N connection to work with mast-mounted antennas (I carry a 9db homebrew colinear) or to the roof-mounted UHF 5dbi antenna.

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It all sits on a shelf I built across the V-nose of the trailer, above the bank of AGM batteries connected to my solar panels. (the trailer also has shore power capability)

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Solar controller and 2kw pure sine wave inverter.
Nonsolis Radios Sediouis Fulmina Mitto. - USN Gunner's Mate motto

Current Weather in My AO
Current Tracking Info for My Jeep

Z.O.R.G.


NT2C

Quote from: Z.O.R.G. on April 15, 2024, 04:39:56 PMVery cool!
Oh, and I set it up so that either radio can be the receiver or the transmitter.  This provides transmit redundancy to a point.
Nonsolis Radios Sediouis Fulmina Mitto. - USN Gunner's Mate motto

Current Weather in My AO
Current Tracking Info for My Jeep

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