What is the minimum and maximum possible frequency separation on a VHF Capacity Max system?
Question
What is the minimum and maximum possible frequency separation on a VHF Capacity Max system?
Answer
The Fixed Channel Plan (defined in ETSI TS 102 361-4 [v1.10.1] §4.7.1) defines a Base Frequency and Duplex Split from which a range (1 to 4094) of Channel Numbers are derived. This only works if all the system frequencies have the same TX-RX separation.
In terms of TX-RX separation, if Fixed Channel Plan is used, the (technical) minimum is 25kHz. To be honest I've never ever come across a system that has such a small TX-RX spacing. The smallest practical/real-world TX-RX separation on VHF is 600kHz and requires RF filtering hardware the size of a small car.
The maximum is 50MHz, up or down. By this I mean the TX frequency can either be 50MHz above or below the RX frequency. Having such a huge TX-RX separation on VHF is also problematic, as either frequency may deviate significantly from the resonant frequency of the antenna, which will have a negative impact on performance. Also, on VHF, having a huge TX-RX separation requires a bespoke duplexer or combiner - which will increase costs I guess.
In my experience, the TX-RX spacing on VHF is usually somewhere between 4 and 7 MHz. On UHF anywhere between 5 and 10MHz. This is for LMR systems of course.
If the Flexible Channel Plan is used, there is no real limit on the TX-RX separation however both frequencies have to be within the range supported by the radio or repeater. So in the case of a VHF system, this would be anything between 12,5kHz and 38MHz. Again, you have to consider the limitations of any external hardware.
If the Flexible Channel Plan is used, there is no real limit on the TX-RX separation however both frequencies have to be within the range supported by the radio or repeater. So in the case of a VHF system, this would be anything between 12,5kHz and 38MHz. Again, you have to consider the limitations of any external hardware.
The Flexible Channel Plan allows you to define discrete frequencies and associate these with a logical channel number. This feature is useful if the TX-RX spacing on any system channel is not the same. This could occur when a customer is migrating from an existing system that used odd frequency pairs.
The separation between the lowest and highest channel number is normally 51,175MHz if Fixed Channel Plan and 12,5kHz channel spacing is used.
The separation between the lowest and highest channel number is normally 51,175MHz if Fixed Channel Plan and 12,5kHz channel spacing is used.
It is also possible to use a combination of the Fixed and Flexible Channel Plans.
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