02-07-2013, 04:50 PM
Dual-Band Reconfigurable Antenna With a Very Wide Tunability Range
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Abstract
A new technique for designing dual-band reconfigurable
slot antennas is presented. Dual-frequency operation is
achieved by loading a slot antenna with two lumped variable
capacitors (varactors) placed in proper locations along the slot.
Loading the slot antenna with lumped capacitors shifts down the
resonant frequencies of the first and second resonances of the
antenna. However, these frequency shifts depend not only on the
values of the capacitors, but also on their locations along the slot
antenna. Here, it is shown that by choosing the locations of the varactors
appropriately, it is possible to obtain a dual-band antenna
whose first and second resonant frequencies can be controlled
individually. In other words, the frequency of either the first or
the second band can be fixed, while the other one is electronically
tuned. Using such a design, an electronically tunable dual-band
antenna is designed and fabricated using two identical varactors
having a capacitance range of 0.5–2.25 pF.
INTRODUCTION
AVAILABILITY of different unlicensed frequency bands
in the congested spectrum of electromagnetic (EM)
waves, compatibility of wireless devices with different standards
(i.e, GSM and CDMA), and interference reduction in a
two-way radio system are typical reasons why many existing
wireless systems make use of two different simultaneous frequency
bands. Whatever the reason may be, this undoubtedly
will add to the complexity of these systems. In particular,
the RF circuitry and antennas must be designed to be able to
operate at multiple frequency bands. Recent advancements in
antenna technology, the availability of efficient computer-aided
design (CAD) tools, and the availability of fast and powerful
computers have resulted in a variety of different techniques
for designing low-profile, cost effective, and highly efficient
multiple-frequency antennas [1]–[9].
Field Distribution Along the Loaded Slot Antenna
In addition to having a large tuning range for , having consistent
radiation patterns at both bands and over the entire tuning
range is also of significant importance. The radiation patterns of
a slot antenna are mainly determined by the electric field (i.e.,
voltage) distribution over the aperture. For an unloaded slot antenna,
the electric field (magnetic current) distribution at the first
and second resonance modes resemble that of a half sine wave
and a full sine wave, respectively. The symmetric field distribution
at the first band results in a radiation pattern with maximum
directivity at the antenna’s broadside whereas the antisymmetric
field distribution of the second band, generates a pattern with a
null at broadside. To mitigate this undesired characteristic, it is
necessary to study the electric field distribution of the loaded
slot antenna.
CONCLUSION
A new technique for designing dual-band reconfigurable slot
antennas is proposed. The technique is successfully applied to
design a dual-band slot antenna with consistent radiation patterns
at both bands and over the entire frequency range of the
antenna. Measurement results of the antenna indicate that its
frequency ratio can assume any value in the range of
by changing the dc bias voltages of its two varactors
in the range of , . Furthermore, good
simultaneous matching is observed at both bands for the entire
range of bias voltages, low levels of cross-polarized radiation is
observed, and the radiation patterns of each band remain practically
unchanged as the dc bias voltage is changed.