Dan, N3OX wrote:
"Also should have said "a few turns over a few inches for a "TWT
OPERATING" at microwave frequencies".

That`s interesting. John D. Kraus invented the axial-mode helix antenna
after attending a lecture on traveling wave tubes given by a famous
scientist visiting Ohio tate University. Kraus asked the visitor if he
thought the helix could be used as an antenna. The visitor said no, so
Kraus went home, wound seven turns one wavelength in circumference and
discovered it made a sharp beam off the open end when he used a ground
plane across the driven end. The story appears on page 222 of Hraus` 3rd
edition of "Antennas".

Lenkurt described operation of the traveling wave tube in its August
1965 edition of the "Demodulator". Here is an excerpt:
"The signal to be amplified by the tube is coupled into the gun end of
the helix. This 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 of the signal is slower, of course, because of
the pitch and diameter of the helix. This forward movement of the wave
is analogous to the travel of a finely threaded screw where many turns
are required to drive it into position. The signal wave generates an
axial electric field which travels with it along the longitudinal axis
of the helix. This alternating electric field interacts or velocity
modulates the electrons in the beam."

Terman`s description in the 1955 edition of "Electronic and Radio
Engineering" starts on page 678 and is very similar to Lenkurt`s. i`d
bet that it is more than coincidental.

Kraus says of his new helical antenna on page 223 of his 3rd edition of
"Antennas":
"At a low frequency (helix circumference about lambda/2) there was
almost a pure standing wave (VSWR goes to infinity) all along the helix
(outgoing and reflected waves nearly equal) (Fig. 8-3a)--."

Surely an antenna loading coil resembles Kraus` low-frequency helix. It
has an open-circuit whip producing a reflection into one end. The
circumference is well below 1/2-wavelength, giving a current
distribution such as shown in Fig. 8-3a for a frequency below the axial
mode of operation.

Fig. 8-3c shows uniform outgoing and reflected currents over the middle
section of the helix. Kraus` figures were produced from actual
measurements.

Best regards, Richard Harrison, KB5WZI