WHAT IS A REPEATER? 
A normal radio conversation takes place from one unit to the next. This is usually accomplished with both stations being on the same channel or frequency. This is known as simplex, or single frequency operation. 

A repeater is a special radio station set up with the sole purpose of relaying the radio signals of other stations. It is normally set up in a high location. A repeater works by recieving a signal on one frequency and retransmitting it instantaneously on another frequency. 

To use a repeater a user station must use a different transmit frequency than recieve frequency. This is a form of duplex, or two frequency operation. It is known as half-duplex as you do not recieve and transmit at the same time but normally use the push-to-talk button on your microphone to switch between the two. 

WHY USE A REPEATER? 
Repeaters in the amatuer radio service are used on all bands above 28 megahertz. By far the most repeaters are used in the 144 to 148 megahertz band, known as two meters, with use on the 440 to 450 megahertz band (70 centimeters or cm) coming in second. 

On the 2 meters band, a typical modern mobile station will have a range of about 10 miles to other mobile stations. This varies with terrain, vegatation, buildings, and other conditions. This range increases for mobile to base operations, and even more for base to base operations. Conversely, it dwindles rapidly for hand held walkie talkies (handies or HTs). 

Since a repeater is located at a high location, it is the same situation as a mobile to base operation. Or better, since repeater antennas tend to be placed even higher than home station antennas. This range is now closer to 30-50 miles from the repeater. This applies to all mobiles using the repeater, no matter in which direction. So by using the repeater as a relay two mobile stations can be up to 60-100 miles apart, and still communicate!! And of course repeaters extend the usable distance for base stations and HTs too. The only limits are: 

• Ability to hear and be heard by the repeater, based on range, power, etc.
• Ability of the other station to do the same.

Now let's fill in some other cities. Again to make it easy let's put our cities 30 miles apart. City B is 30 miles west of City A, City C is 30 miles east of City A, and City D is 30 miles east of city C. 

So we have a repeater in city A, wiht a range of 30 miles. The range circle just touches cities B and C. Any mobile station within the circle can talk to any other mobile within the circle. So a mobile in B can just talk to a mobile in C. But none of the can talk to anyone in D. 

So Fred puts up a repeater in D with a 30 mile range. It cannot be on the same frequency pair (one receive, one transmit) as the repeater in A due to interference problems. 

Now the mobile in C can talk to the mobile in B, or change channel and talk to Fred in D. But not both at the same time. And Fred in D still cannot talk to B. So what are we going to do. 

Well, what a some groups, including the SouthEast Iowa Technical Society, have done is to link repeaters in various locations together. To do this without interfernce problems, SEITS made use of the 70cm band to link 2 meter repeaters. 

AND THE RESULT 
Since each linked repeater shares information with all the others, the range is greatly improved. As a mobile station leaves the coverage area of one repeater in the system, the operator simply changes his frequency to the next repeater and continues his conversation. It resembles most closely a manual switched cellular system, where each individual repeater is a cell. In the SEITS LINK system, the ranges of most of the repeaters overlap that of their neighbor, thereby providing almost continuous coverage over the entire system area. 

Comments or submissions for publication can be sent to muldoon@seits.org.