A standard slow wave meander line having sections of alternating impedance relative
to a conductor plate is provided with a top shield connected to the conductor plane
for the purpose of lowering the resonant frequency of narrow band antennas and
lowering the low frequency cutoff limit of wide band antennas due to a higher delay
per unit length occasioned by the use of the top shield. The shielded meander line
may be utilized as a coupling device to truncated antennas such as a whip antenna
or grounded loop antenna for the purposes of loading the antenna so as to provide
lower frequency performance. Since the propagation constant of the meander line
structure depends upon the number of high impedance/low impedance transitions per
unit length, the utilization of the top shield results in more phase shifts per
unit length and thus more delay per unit length, with the symmetric double sided
version having double the number of transitions per unit length. When configured
to provide a miniature antenna, the utilization of the top shield both lowers the
cutoff frequency and eliminates down firing typical of wireless phone antennas
due to the ground plane effect. Moreover, the top shield provides a uniform low
VSWR over wide bandwidths and by virtue of lowering the operating frequency solves
a skip-induced blackout problem due to the lower frequencies that can now be used.
Further, for frequency switched meander lines, voltage stress is reduced when using
the top shield. Finally, reducing the volume requirement by over 30% permits mobile
use where real estate is at a premium.
