how to calculate F/B ratio in a antenna field pattern

the pattern's F/B=2.93-(-12.3)=15.23?
anybody can tell me it is correct?how to calculate F/B?
F/B=10log(Forward_G/backward_G)?

You are correct. The front to back ratio is calculated for opposite directions at the same elevation.

but in my pattern the F/B' unit is dBi,not the dB?WHY?
DO you think i need use the expression 10lg(F/B)?

105s said:

but in my pattern the F/B' unit is dBi,not the dB?WHY?
DO you think i need use the expression 10lg(F/B)?

Click to expand...

this give more information an standard F/B ratio!

you see how much more/lesser radiating compare to
'perfect' isotropic antenna (always 0 dB gain, dBi = stands for
isotropic reference) with 2.93 dB gain in frontside and -12.3 dB
suppression on backside

dipol for example can give around 2 dB gain on all direction
in horisontal compare to isotropic antenna (and very low power
in vertical mode) ie. take power from up/down to make more
power in horisontal mode by construction.

your antenna have approx same gain at dipol in front side
but have screen backside to make high F/B ratio.

screeing itself helping up power radiating on frontside
approx. 1 dB compare to standard dipol antenna.

another 'reference' to dBi is a dBd, ie how much
better/lower radiation than 'standard' dipol antenna.

Yor antenna have 0.93 dBd gain on front side and -14.3 dBd
gain in backside - but always same F/B ratio

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bad designed antenna (like cheep mobilantenna) can have
15 dB F/B also, but best side only give -10 dB radiation
compare to isotropic antenna (ie. negative gain compare
to isotropic antenna)