Richard Harrison wrote:
. . .
Terman says about twice the bandwidth is required for FM as compared
with AM on pages 589 and page 590 of his 1955 opus.
. . .

You really should try to understand the context of the various
quotations from Terman.

In his _Radio Engineering_, Third Edition (1947), he points out that
"When the modulation index is less than 0.5, i.e., when the frequency
deviation is less than half the modulating frequency, the second and
higher order side-band components are relatively small, and the
frequency band required to accommodate the essential part part is the
same as in amplitude modulation." This is, of course, what is considered
to be narrow band FM.

Unlike an AM signal with its one pair of sidebands containing replicas
of the modulating signal, any FM signal contains an infinite number of
pairs of sidebands. However, as Terman and any other communications text
points out, the relative strengths of some of those sidebands can be
made to be very small by the choice of modulation index. In the case of
NBFM, all but the first pair are small. That first pair are spaced the
same distance from the carrier as AM sidebands, so the bandwidth is
essentially the same as for AM.

You can, of course, increase the modulation index which increases the
bandwidth by increasing the amplitudes of higher order sideband pairs,
making wideband FM. The advantage of doing this is that you can improve
the signal/noise ratio of the received signal as a trade for the
increased bandwidth.

So FM can be as narrow in bandwidth as AM, or any greater bandwidth, all
depending on the modulation index. Saying that "twice the bandwidth is
required for FM as compared with AM" is simply not a correct statement
and, if said by Terman, was taken out of context which surely qualified it.

Roy Lewallen, W7EL