Is there a good rule of thumb when to expect to need a repeater (when broadcasting in the VHF and UHF ranges)? Is it dependent on line of sight? Wattage of the signal? What else? And, since the answer may depend on what obstructions may block the signal, let’s keep this simple. Let’s assume distance is the only limiting factor.
Well it is highly variable between situations.
The best way to answer is translate the question to " why would a repeater work to get the signal to the receiver ? Why can’t the receiver just pick up a weaker signal ? ".
I translate this way, as it I feel the answer to my question puts the OP back on track.
The receiver can’t pick up a signal that is either
- below its sensitivity … its minimum signal strength requirement
- lost in noise… signal to noise margin is too low
The repeater, with the same or similar sensitivity, is places at a location where the
- signal is level is strong enough to be received
and - the signal to noise margin is still good enough,
and retransmit it with a higher signal level. Noise introduced along the way is down at the lower level and noise attenuates too, so the result is that repeater can get the signal to the receiver at a higher level, and with a large enough signal to noise margin.
All the other things just cause lower signals and/ or more noise…
The antenna and coax are usually the most important part of the station. What type of radios and antennas are you expecting to be used on both ends?
Repeaters are generally mounted in locations with good antennas at high elevations. If a mobile radio has a car to car range of say 20 miles, he can’t talk to a car 25 miles away. Put a repeater with a good antenna at 30 to 35 miles from the car and the repeater can probably hear the car and retransmit the signal with more power. So with a repeater two cars could probably talk with maybe 60 miles between them. Many repeaters are fitted with preamplifiers to increase the signal into the receiver.
The preamp on a repeater is usually needed to offset the insertion loss of the duplexer…the set of filters that allow a repeater to transmit and receive simultaneously on frequencies within a given band .
I’ll also mention that repeaters are often connected to their antenna with hard line rather than the flexible coax used in mobile and residential installations. Hard line is a less flexible coaxial transmission line with less signal loss than common coax.