what is the best UHF antenna for remote/ fringe areas..4.5db or 6db?

Answer 1

I cant give any detailed information because you didn't say if it was for TV or radio.
Anyway...

dB is the difference (or ratio) between two signal levels.
It is used as a term of gain , or more simply pulling power.

An antenna that has 4.5dB gain should have better signal pulling power over a dipole.
Expanding on that a 6dB antenna will be even better again.


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Answer 2

1. bigger is better.
2. so is higher.

if the numbers you refer to are signal gain, see rule # 1.