The site that a repeater is physically located at can have a dramatic effect on its overall performance. The first thing that we think of is, of course, terrestrial height. We all know that at VHF frequencies, height is our friend. Thus, the higher the elevation on which our repeater is located is, the better the repeater’s reach.
INTRODUCING THE NOISE FLOOR
But there is something else that is very important which is little discussed. That is the electromagnetic noise floor at the site. A site that exists in the middle of nowhere will have relatively little wide-spectrum electromagnetic noise. But another site in the heart of an industrial complex…well, you figure it out.
WHAT IS WIDE-SPECTRUM ELECTROMAGNETIC NOISE?
Virtually all electronic devices emit electromagnetic noise at some level. Some devices are particularly susceptible for the emission of electromagnetics and are therefore regulated by the FCC if they are to be sold across state lines. An example of this is the charging device that an electric car uses to charge its battery from 120 or 240 VAC household power.
AN EXAMPLE FROM THE ANCIENT PAST
The author remembers, as a child in the 1950s (when T Rex still roamed the earth), watching Saturday morning cartoons on the TV. He also vividly remembers, while watching those cartoons, his mother vacuuming around the house. Her vacuum cleaner had a powerful electric motor with a very large armature. The vacuum motor was not designed to limit or block the electromagnetics that the motor amateur would naturally develop as its current was constantly commutated. The result was a TV cartoon that had both its audio and video 100% obliterated by the QRM. The author also clearly members opening his child’s mouth to holler out, “Mom, stop that vacuuming. I’m missing my cartoons.” But then logic wisely set in realizing I would be very sorry.
MUZZELING IN ON THE REPEATER’S INPUT
Guess what… A repeater is much like me as a child watching my TV with my mom vacuuming somewhere else in the house. If the repeater is located in a highly urbanized environment, it is likely that there will be industrial electromagnetic noise that pervades the environment. The operative question of the day is, how bad is that noise? This defines a “noise floor.” This noise is always present. Any amateur radio signals that the repeater is to hear, must be heard above this noise. If the incoming signal is lower in magnitude than that noise, the repeater receiver cannot “hear” it and it goes unrepeated. The obvious effect is that a noise floor limits the range of the repeater. It will be heard much beyond that distance from which it can receive for a typical power transmitter. Someone trying to reach the repeater with a more powerful transmitter will likely help but the repeater’s height becomes somewhat irrelevant. This is a case where power may have an advantage.
CIRCUMVENTING A NOISE FLOOR
Sorry. There is no help for canceling a noise floor. This is a hard limit to repeater operation. Someone might suggest a receive preamp for the repeater’s receiver input. But think about it…What are you going to amplify? The signal of interest is obscured by the localized QRM (the noise floor).
A POSSIBLE REMEDY
It will take some legwork and a lot of patience but there may be some device(s) local to the repeater area that are operating out of the assigned FCC limitations. If you can find one of these offenders and nullify or silence it, you will have effectively increased the range of your repeater by lowering its noise floor.
WHAT IS TOO LOW OF A NOISE FLOOR?
The next obvious question is, how low is low? When is low TOO low? The answer is that it can never be too low, but nevertheless, there is a low beyond which more low is irrelevant. That is because it competes with your receiver sensitivity and selectivity. It all boils down to who is more powerful. Is the noise floor too powerful or is the receiver sensitivity more powerful? But in any case, a receiver’s sensitivity and selectivity is non-applicable for overcoming an objectionable noise floor.