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

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)."
--
73, Cecil http://www.qsl.net/w5dxp