A few tips for choosing system frequencies

I recently had this question at work. The customer wanted to know what the rules are for selecting frequencies - in this case, on a Capacity Max system. Although the question was about Capacity Max, the below would apply to any system topology.

Very broadly (and as a rule of thumb), UHF is best suited for indoors and built-up areas whereas VHF works better in suburban and rural areas.

For multi-channel systems (i.e. more than one repeater on the same site), the general rule is to use a multicoupler and combiner to distribute the RF between the repeaters and (single) antenna, when two or more repeaters are on site and in the same band.

When only one repeater is installed on site, a duplexer can be used. When there are two repeaters, it becomes a little bit of a grey area: either a multicoupler-combiner can be used, or one can use two antennas; two duplexers and two feedlines (to save cost). If you choose to go with the latter, you need to be extra careful about frequency selection - unless there is a good reason for using two duplexers and antennas, I would advise against it.

In order to use a cavity combiner, the transmitter frequencies need to be separated by more than 300kHz (also known as the TX-TX spacing). If this is not possible, a hybrid combiner can be used, but this introduces more complexity and insertion loss.

Insertion loss is the amount of signal lost between the antenna inlet and repeaters. A four-port cavity combiner will have an insertion loss of around 4dB, whereas a four-port hybrid combiner will have an insertion loss of around 9dB.

If you're concerned about the high insertion loss, be aware that not using adequate RF filtering on the RF site can have disastrous consequences in terms of RF interference. In any case, if you're going to be using a multicoupler-combiner, you probably also would want to use a high gain antenna which would recoup any insertion loss.

There are no hard rules for the TX-RX spacing (the separation between the receive and transmit frequencies) but for a cavity combiner, this should be more than 3MHz.

The transmit frequencies should be within 20-30MHz of each other but this mostly depends on the combining equipment manufacturer. The same would apply to the receive frequencies but this is not as critical.