Richard Harrison wrote:
Hohn Popelish wrote:
"I guess this depends on the official definition of "slow wave."

My dictionary of electronics defines "slow-wave circuit" as: "-A
microwave circuit in which the phase velocity of the waves is
considerably below the speed of light. Such waves are used in
traveling-wave tubes."

William I. Orr wrote on page 6.11 of the 22nd edition of the "Radio
Handbook":
"Spaced closely around the (TWT) beam is a circuit, in this case a helix
of tightly wound wire, capable of propagating a slow wave. The r-f
energy travels along the wire at the velocity of light but, because of
the helical path, the energy propagates along the length of the tube at
a considerably lower velocity than is determined primarily by the pitch
of the helix." (I think Mr. Orr probably wrote: that is determined
primarily by the pitch of the helix.)

I`ve seen several mentions of "slow wave" in the literature and it
always meant slower than the speed of light. There is no need to limit
the definition to microwaves as the coil slows the velocity across
because the wave is guided by the wire wrapped around its form. The wire
is longer than the coil form.

It seems that these references are not particularly concerned with the
dimensions of the coil with respect to wavelength, or with the mode of
propagation within the coil. I suspect that the term has different
meanings to different specialties.