Choosing frequencies and RF filtering equipment.
In order to properly connect multiple repeaters to an antenna system a multicoupler; combiner and (pass-reject) duplexer must be used. The combiner can consist of cavity filters or hybrid combiners depending on the transmitter frequency separation.
Generally, cavity combiners are better suited for applications where insertion losses must be kept to a minimum but this requires a transmitter frequency separation of at least several hundred kilohertz. For narrower transmit frequency separations, a hybrid combiner must be used.
In either case, the transmitter frequency separation (TX-TX separation) must not be less than about 100kHz – anything less will result in a degradation of performance – in any case, the multicoupler combiner manufacturer may refuse to produce any equipment if the frequencies are not correctly chosen.
Using a TX-TX separation of less than 50 kHz may cause interference. This interference will be significantly more if a (amateurish) system of multiple duplexers and multiple antennas is used. The multiple antenna, multiple duplexer arrangement does not provide sufficient transmitter isolation resulting in the repeaters transmitter signal triggering the FCC Type 1 threshold of the repeaters.
If the FCC Type 1 threshold is triggered, the repeater will not transmit until the channel is clear. This will in turn hinder the correct operation of the system –particularly with Capacity Plus and Linked Capacity Plus purely because these systems have multiple repeaters on a site.
There are three simple rules in this regard:
· Choose intermodulation product free frequencies which have a transmitter separation of a few hundred kilohertz or even one or two megahertz.
· Always use a multicoupler; combiner and (pass-reject) duplexer when there are multiple repeaters on the same site.
· Unless there is a legal requirement (i.e. the equipment is to be installed in the USA where there is a requirement for FCC Type 1 monitoring), turn the FCC Type 1 Threshold up as high as you want. Increasing this threshold will not have an effect on sensitivity.
Observing these simple rules will ensure trouble free operation – no matter which manufacturer and no matter if analogue or digital is used.