  A conceptual method to realize an impedance-matched electrically small
antenna is discussed, using a transmission line modulated only in time. The
time-periodic space-uniform modulation of a distributed shunt array of
varcapacitors excites a mode with very small frequency but still a small
(microwave) wavelength: the matching line then retains a practical size. The
modulated transmission line brings the problem of impedance matching to a
frequency $f_0$ much higher than the signal, where the large antenna reactance
can be easily compensated with a much higher bandwidth for the same $Q$ factor:
actually the bandwidth $\Delta f= f_0 / Q$ becomes large when $f_0$ is much
larger than the signal frequency. A simple way to obtain the space uniform
modulation of the transmission line is illustrated, making use of a
Wheatstone-bridge arrangement of the modulated varicaps. The signal and
modulating lines, running in parallel on a section of the same length, see two
very different equivalent shunt capacitances, so that the modulating voltage is
almost uniform while the signal voltage may change significantly across the
transmission line.
