NEC-LIST: Difference between F/B and F/R ratio?

Hello

I know that front-to-back ratio expresses the difference between the
mainlobe on boresight and the rear lobe response at 180 degrees to that, but
what is front-to-rear ratio?

I've seen F/R ratio mentioned a couple of times on a few websites, and
indeed 4NEC2 displays F/R as well as gain and F/B results from simulations.
However, I can't find anything in the literature. Is it something that is
peculair to HF antennas, by chance? Can anyone explain what it is please?

I apologise if this is a little off-topic seeing as it isn't expressly
NEC-related, but it does get encompassed c/o Arie Voor's excellent 4NEC2
(actually, 4NEC2 displays F/R as a ratio of the mainlobe and the orthogonal
lobe which may be an error).

Thanks for any comments.

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