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Owen Duffy · May 11th 07, 01:54 AM posted to rec.radio.amateur.antenna

(Richard Harrison) wrote in news:18308-
:

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.

This seems to introduce a new concept of "directive gain", which isn't
Directivity as it is commonly used / understood, and it isn't Gain as it
is commonly used / understood. It also seems to be based on a flexible
reference (being "another" direction).

What is wrong with the concept that Directivity (either the maximum, or
in a specific direction) is the ratio of the power density (more
correctly) in that case to the power density averaged over all
directions, and that Gain=Directivity/Loss.

Richard, you seem to have used a textbook to define something that it
didn't.

Owen