How long do I cut the limbs for PMR 446?

[Lightspeed]

Yes, halving the coax length will half the losses from it, but not the insertion losses of the connectors.

Getting the antenna higher and in the open is always a good idea, and for sure will improve things a good deal.

This antenna has good reviews. It appears to be tuned specifically to PMR446 band, so may be worthwhile considering. I have not played with one myself, so only have web reviews to go on.

Re burners: don't forget that your signal is being attenuated both ways. If you install an amplifier, then you'll need to install a receive pre-amplifier as well. Make sure that both are tuned to 446 Mhz

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Personally I'd forget about amplifiers and pre-amps and concentrate on getting a well tuned antenna high and in the clear.

I'd feed this with as short a run as possible using the best coax I could afford.

Connectors should ideally be N-Type.