Roy, W7EL wrote:
"I maintain there`s no such group as "other coils", but that coils act
quite differently depending on their physical sizes and the amount of
coupling between turns."

I wrote:
"That`s how the experts say the coil in a TWT works, and is no different
from other coils."

All coils aren`t inside TWTs, but all coils do create inductance.

Bill Orr wrote this concerning the coil in a Traveling Wave Tube:
"Figure 25 is a simplified sketch of a basic helix-type TWT tube. Spaced
closely around the 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, because of the helical path,
the energy progresses along the length of the tube at a considerable
slower veloity than is determined primarily by the pitch of the helix."

Terman wrote this concerning the coil in a TWT:
"The beam is shot through a long, loosely wound helix, and is collected
by an electrode at anode potential as shown."

Lenkurt wrote:
"The RF signal travels as a surface wave around the turns of the helix,
toward the collector, at about the velocity of light. The forward or
axial velocity is slower, of course, because of the pitch and diameter
of the helix."

Orr`s example was a helix of tightly wound wire.

Terman`s example was a long, loosely wound helix, and Lenkurt did not
specify how tight or loosely the coil was wound. In all cases the coil
retarded the signal well below the velocity of light along the axis that
the electron beam traveled so that the beam could keep up with the
signal along the path. The beam needs to be speeded as well as slowed
for velocity modulation.

Point is that group velocity does not exceed the velocity of light even
in W8JI`s coil no matter how he makes it. There is no way to coerce
actual energy to exceed the velocity of light. It would turn into a
pumpkin or something.

Also, electric current follows the course of maximum potential
difference and that`s along the conductor supplying the electrons. The
wave impels electrons to move in the conductor.

Kraus wrote:
"The helical antenna, which is discussed in this chapter, may be
regarded as the connecting link between the linear antenna and the loop
antenna, discussed in preceeding chapters. The helical antenna is the
general form of antenna of which the linear and the loop are special
cases. Thus, a helix of fixed diameter collapses to a loop as spacing
approaches zero. On the other hand, a helix of fixed spacing between
turns straightens out into a linear conductor as the diameter approaches
zero.

This thread has been about a coil loaded whip. This is a standing wave
antenna. When the signal gets to the antenna tip it has no where else to
go but return over the path which brought it. The coil has an incident
wave impinging from the transmitter and an out-of-phase signal reflected
from its tip. These two waves have the same origin so they are locked in
step to make standing waves in both voltage and current. These determine
the ratios of voltage to current at each point along the signal route.
In this respect the coil behaves as a conductor in the antenna. It has
more opposition to the signals traversing it than a straight wire but
the volts and amps at each of its ends can obviously be very different.
Thus, current in one end of the coil can be very different from the
current at the other end of the same coil.

Best Regards, Richard Harrison, KB5WZI