Jopl wrote:
"What is a traveling-wave antenna?"

A traveling-wave antenna has only weak if any reflections. So, it is
defined.

A traveling-wave antenna is the opposite of a standing-wave antenna. In
a standing-wave antenna, the wave reaches the tip end to find an
impedance significantly different from the characteristic impedance of
the wire or wires carrying the wave.

In the standing-wave antenna, the wave often encounters a short-circuit
or an open-circuit at the antenna tip which reflects 100% of the energy
remaining in the wave after its trip from the feedpoint. This produces a
standing wave pattern on the anternna wire just as a short or open would
cause on a lossy trannsmission line. In the standing-wave antenna case,
most of the energy escaping from a good antenna is radiated as r-f in
place of being converted to heat at the wire.

A traveling-wave antenna is usually equipped with a resistor or
resistors to absorb energy which is not radiated on its trip to the end
or ends of the wire or wires (if it is a balanced antenna). To absorb
100% of the remaining energy and to produce no reflection requires the
chosen load resistance to match the charcateristic impedance of the
wire, or wires in a balanced configuration, of the antenna.

The ideal horizontal rhombic antenna, for example, is a balanced
traveling-wave antenna which is terminated at its far end with a
resistor equal to the characteristic resistance of the antenna at its
far end so that no energy will be reflected at the far end to travel
back toward te feedpoint and produce radiation toward the feedpoint end
of the antenna. Well constructed and with the proper termination
resistor, the rhombic is unidirectional. Without the resistor, the
rhombic is bidirectional and its front to back ratio depends on how many
wavelengths are in a side of the antenna.

The ideal dipole antenna, for example, is a balanced standing-wave
antenna which is terminated at its far end in an open-circuit in the
usual case, or it is terminated in a short-circuit if the dipole is the
"folded-dipole" type. Both types radiate identically. Only the
drivepoint resistance is different between the two types of dipoles.
Both dipoles have energy left over from the first trip between feedpoint
and antenna tip and this is nearly 100% reflected at the tip. If the
dipole has a 1/4-wave length between drivepoint and tip, the open or
short is inverted and at the feedpoint the open-circuit tip appears a
low impedance. On the contrary, the short-circuit at the tips of the
folded dipole appear as a high impedance back at the feedpoint when the
elements are 1/4-wavelength.

The usual horizontal balanced rhombic looks like a transmission line of
600-900-ohm Zo. It is pulled apart for extremely wide-spacing which
allows radiation. This widening as distance from the drivepoint
increases also raises the Zo to a high value but this is symmetrically
reduced back to the drivepoint impedance by the reduction in spacing
between the rhombic wires toward the far end. Ideally the whole assembly
just looks like a transmission line terminated in its Zo, which happens
to have a wide spot within which allows radiation in and out. At high
frequencies where the rhombic has many wavelengths per side, much of
energy fed a rhombic will be radiated before reaching the termination,
often a dissipation line made from stainless steel resistance wire.

Best regards, Richard Harrison, KB5WZI