Thread: Transmitter range

Does anyone know the general rule that defines transmitter range? i know it varies massively dpending on location, height, power, aerial type etc. But whats the general rules for good performance? is there a power/distance equation? Im experimenting with some circuits in the CB band, but im clueless when it comes to aerials, besides impediance matching.

It is quite a complex subject really. There are path loss calculators on-line but these assume a line-of-sight path. 27 MHz CB does not propagate well even over short distances. It does open up from time to time either very long distance. (e.g .transatlantic) via F layer ionosphere or shorter Sporadic E layer (Europe). But for normal commnmications beyond 30 miles it tends to be poor. Low VHF is better and higher VHF is better still for comms up to 100 - 300 km under normal conditions. Troposcatter and tropo bending are always present to some extent at VHF (70 MHz and higher) and UHF and ducting at times of high atmos pressure can extend the range to a 1000 km or more.

Receivers can work down to incredibly low levels of signal but there is a limit when receiver noise and external noise or interference masks the signal.

Think of a lamp sending out light in all directions. At some distance r away from the lamp the light will cover the inner surface of an imaginary sphere of radius r. The surface area of the sphere is 4 pi r2 (r sqrd). So the power per sq metre is inversely proportional to the square of the distance. At 1 km you will get X picowatts/sq m at 2 km it will drop to X/4 at 10 km it will drop to X/100.