Codan Radio Transportable Radio Systems User Guide - Manual (Page 19)

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4.4.2 How do I Provide Isolation to the Repeater?
The simplest method of creating isolation between a transmitter and a receiver is to use two separate antennas.
Based on the isolation requirements of the repeater at the frequencies it is set for, the antennas must then be
physically separated in either horizontal or vertical distance. The following graphs show the relative isolation that is
achieved in the different frequency bands based on the distance between the transmit and receive antennas.
A transportable VHF repeater is deployed with a transmitter frequency of 155.3125 MHz and a receiver frequency of
155.8125 MHz. The difference between these two frequencies is 500 kHz. It is known that this particular repeater re-
quires isolation between the transmitter and the receiver equal to 85 dB to avoid any desensitization of the receiver.
How much vertical and horizontal distance is required between the two antennas to achieve this isolation?
If the two antennas can be mounted 100 feet apart from each other horizontally, the graph tells us that this will
give approximately 42 dB of isolation. The remaining 43 dB of isolation will need to be made up by separating the
two antennas vertically as well. To achieve the remaining 43 dB of isolation, the vertical separation graph shows that
approximately 16 feet of vertical separation will give 43 dB of isolation. Ultimately this will mean that the repeater
will have one antenna located on the ground beside the repeater, and a second antenna located 100 feet away and
suspended 16 feet above the ground on some kind of mount. While this achieves the desired results, it often is a
challenge to deploy a Transportable Radio with these kinds of set up constraints.
4.4.3 Duplexer Characteristics
The simpler alternative to using two antennas is to use a duplexer and a single antenna. This makes the deployment
requirements for the repeater much simpler, but it also comes with some trade-offs.
First and most importantly, the biggest difference between using a duplexer and two antennas is channel band-
width. For a duplexer to achieve the high level of isolation, it sacrifi ces the ability to have a wide channel range. Most
small, portable duplexers in fact are tuned only for a single frequency pair which means that the repeater can only be
used on that one channel and no other. The two antenna system allows for much wider channel capability.
Duplexers