Repeaters

Repeaters are the staple of life for a Ham operating on 6 meters or lower, but are legal for any amateur frequency higher than 28 MHz. Essentially, these bands have almost no propagation beyond line of site (Except for rare circumstances). In addition, many of these devices are mobile, including a fair number of hand held radios. These are limited on power, and are of a frequency that they could easily be obstructed by objects in the way.


Enter the repeater. A repeater is simply a device which will repeat a signal that is received back. They typically operate from a high location, with lots of power. They can be heard thus from a long ways away, sometimes even as much as 100 miles away! I personally have only talked with repeaters about 30 miles away, but this is still quite a bit further than I could otherwise! I was able to talk with people as far as 60 miles away, who lived the opposite direction of the repeater!

As repeaters can be seen from far away, it is possible that in some busy areas, repeaters could collide. Amateurs have come up with a system for preventing that. The most common is called a CTCSS, for Continuous Tone Coded Squelch System. Essentially what this is is a low frequency tone included in your signal so the repeater knows to forward your signal through the repeater.

If the same exact frequency was used to transmit and receive, there would be considerable difficulty in echos for those who could hear the uplink. The solution to this is to have an offset in the transmit and the receive frequency. The whole plan is quite complex, but essentially it boils down to what band is used, and tradition. Modern radios typically will change your frequency for you automatically, so you don't have to worry about this as much.

The offsets vary based off of the band, but I found this convinient chart from the online version of Ham Radio for Dummies.

Band	Output Frequencies of Each Group (In MHz)	Offset from Output to Input Frequency
6-meters	51.62 - 51.98	- 500 kHz
	52.5 - 52.98
	53.5 - 53.98
2-meters (there is a mix of 20 and 15 kHz channel spacing)	145.2 - 145.5	- 600 kHz
	146.61 - 147.00	- 600 kHz
	147.00 - 147.39	+ 600 kHz
220 MHz	223.85 - 224.98	- 1.6 MHz
440 MHz (local options determine whether inputs are above or below outputs)	442 - 445 (California repeaters start at 440 MHz)	+ 5 MHz
	447 - 450	- 5 MHz
1296 MHz	1282 -1288	- 12 MHz

In the United States, any amateur can put up a receiver, so long as it follows the law, but coordinated receivers will be given the priority. If a person decides to set up a repeater without coordination, and they interfere with a coordinated repeater, they are responsible for fixing it.

Now, for the important question. How can you find repeaters? There are basically two ways to do this. The first is to find a directory of some sort, and try dialing in. The second is to scan for one on the air and see if you can pick one up. I personally prefer to do a mixture of the two, to have a list in front of me of possible repeaters, and to scan for one. With the list in front of me, I can look up the appropriate tone and location.

In no particular order, here's a few sites I've used to find repeaters:


http://rptr.amateur-radio.net/
http://www.levinecentral.com/repeaters/
http://www.repeaterbook.com/repeaters/prox.php


In summary:

1. Repeaters rebroadcast your signal over a wide area
2. You might need to enter a CTCSS tone to use the repeater
3. 2m has an offset of +- 600 kHz, 70cm has a +- 5 MHz.
4. An uncoordinated repeater is responsible to fix interference with a coordinated repeater.