When proposing to coordinate a repeater, one of the primary considerations is how far it will be from other repeaters on the same frequency. These are called co-channel repeaters. It may surprise you that the main consideration is not the transmitter location; instead, it is the user area. This is the main misunderstanding many Amateurs have about repeater spacing.
Repeaters have users who may be transmitting at any time from anywhere the repeater receiver covers. If the user areas overlap, a repeater may hear a signal from a user talking to another repeater. It will often override a signal transmitting into this repeater.
Sub-audible tone (PL) may be used to block unwanted signals from being repeated, but unwanted signals can still override the intended signal and prevent it from being heard by the repeater.
If we reduce receiver sensitivity in an attempt to reduce user area, users will have to use higher antennas and higher power to achieve readable quality. A poor receiver can actually increase interference to co-channel repeaters.
Often, with the exception of high profile systems, repeaters can operate at 100 mile spacing, provided they stay at their coordinated location and parameters. This permits a user area of up to 50 miles from the repeater. When we place repeaters much closer, problems start to develop. A 25-watt mobile on a quarter wave antenna or a 10-watt mobile on a 3-db gain antenna is good for 35 to 45 miles to most repeaters. It is not unusual for a Ringo Ranger on the roof of a house and a 10-watt radio to access a repeater from 50 miles.
If satellite receivers are used, the user area will be greater and the location of the users will be even less clearly defined.
Radio Amateurs usually do not own the site from which they operate their repeaters. From time to time, situations occur that require repeaters to move. Good operating locations are not always plentiful. Often Amateurs must settle for what they can get. If the new site is closer to a co-channel repeater, the spacing will be closer.
If in a few years the other repeater needs to move, the spacing could be closer yet. It seems prudent to provide for some flexibility to permit reasonable future change. We do not want to create a situation where a popular, long existing repeater cannot utilize the only decant site that is available.
The distance between repeaters on adjacent channels must also be considered. If a user of a repeater 15 kHz from another repeater is located close the other repeater site, both are going to have problems hearing.
It would be easy to become popular with Amateurs who want new repeaters by coordinating short spaced repeaters without regard to the interference that would be caused to existing systems. This would compromise the quality of our Amateur repeaters. Those who are promoting the misconception that, through some hocus-pocus they can magically grant coordinations that we cannot, are setting the stage for some serious interference problems now and in the future.
Whenever an Amateur applies for repeater coordination, The Indiana Repeater Council makes every effort to find a frequency that can be used in accordance with good amateur practice. Unlike the old days, we now have the technology to search the entire band with great accuracy for frequencies you can use.
Unfortunately, throughout most of Indiana, there are no frequencies on two meters where a repeater can be added without resulting in harmful interference.