Art wrote:
"For my interest, what is the unit that must be used for front to back
ratio of a directive antenna?"

I must be an idiot for venturing an answer, but ratios can be just
numbers, but numbers have origins. If radiated power in one direction is
twice that in another (reference), we can say it has a directive gain of
two or we can say it has a 3 dB gain. Front to back ratios have the same
origins and units. For legitimacy, Terman says on page 871 of his 1955
opus:
"The directive gain can be expressed either as a power ratio, or in
terms of the equivalent number of decibels.

Best regards, Richard Harrison, KB5WZI