On Tue, 10 Jan 2006 15:25:35 GMT, Cecil Moore wrote:

Amos Keag wrote:
A resonant rhombic without a termination resistor is
bidirectional.

It may be bidirectional, but it still has directivity in the direction
away from the feed.


Yes and, being open-ended, is a *standing-wave* antenna.
Contrary to what has been said here on r.r.a.a in the
past, Balanis says: "The current and voltage distributions
on open-ended antennas are similar to the standing wave
patterns on open-ended transmission lines. ... Standing
wave antennas, such as the dipole, can be analyzed as
traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by
traveling wave currents If and Ib in Figure 10.1(a)."

If the unterminated rhombic is long enough to be considered a -real-
rhombic, it is both a standing wave and a traveling wave antenna.

If you want to think of it in transmission line terms, it's a very
lossy line. The 100% reflection at the open end is 100% of what's
left after the effects of resistive and radiated loss are factored in.

The rhombic as the positive attribute of simplicity but that is about
the only positive. It takes a huge amount of space (in wavelengths)
and it's really a crappy antenna from the standpoint of minor (and
some not so minor) lobes. The Laport version is much better, but much
more complicated.