Duplexers and Towers Comments from Falcon Direct

Duplexers and Towers
Comments from Falcon Direct

A duplexer is what makes a repeater different from a base station!

There are different types of duplexers (band pass, notch, etc.) but in essence they all do the same thing. A duplexer allows a single antenna to allow simultaneous transmission and reception of two different operating frequencies. The type, cost, and size is dependant upon two primary factors. The first is the separation between the two frequencies (3.5 MHz for VHF and 5.0 MHz for UHF) are considered optimal. The second is keeping dB loss to a minimum.

A loss of 1 dB is considered acceptable. Higher dB losses will adversely affect the talk back range of mobiles and in particular, handheld radios. A variation of a duplexer, known as a combiner allows multiple transmitters to use one antenna and multiple receivers to use another. Combiners are commonly used with trunking systems and/or on commercial rental towers. The losses can be significant (3 dB or more is not uncommon). High power (over 50 watts) typically results in a higher priced duplexer, often with higher loss. You can learn more about duplexers by clicking here. For now, we will simply say that obtaining an FCC frequency assignment for optimally separated frequencies, keeping power to 50 watts or less, and selection of the proper duplexer is a major consideration in system planning.

Tower height is another important consideration. Your greatest benefit generally comes with the first 50 feet (typically a 3 dB improvement over ground level). 100 feet will provide approximately a 5 dB improvement. 200 feet results in approximately a 6 dB improvement, 300 feet around 8 dB and 500 feet, a little more than 9 dB. This is an expensive way to improve system performance. The better alternative, at least in our opinion would be the use of multiple repeater stations, less powerful transmitters, and smaller towers designed to provide coverage into specific overlapping areas. For additional information on other elements of repeater station planning, click here.