Abstract:
Circularly polarized (CP) antennas are currently being used in many wireless
communication systems like mobile communications, satellite communications,
radio frequency identification (RFID), wireless local area network (WLAN), etc.
In case of LP antennas, strict orientation between transmitter and receiver is
needed to avoid polarization mismatch losses; however this is not required for
CP antennas. Furthermore, „Faraday rotation‟ in earth‟s ionosphere region
results in significant signal loss i.e. about 3-dB or more if LP antennas are used,
while CP antennas are immune to this effect and that is why they are employed
for satellite communication. Similarly, LP antennas are severely handicapped in
multipath environment, while CP antennas are very effective to counter
multipath fading. With the evolution in wireless communication, the demand for
small mobile devices with efficient radiating elements is increasing day by day.
Many techniques have been used to produce wide band CP antennas but size
constraint has been observed in most of the cases. So, designing a CP antenna
with large 3-dB axial ratio bandwidth (ARBW) over a wide range of operating
frequencies with compact size of the antenna is a challenging task.
This thesis presents two potential designs of CP antenna for wireless
communications systems. First design is a simple monopole antenna with broad
impedance and 3-dB axial ratio (AR) bandwidth. The antenna is fed by a 50 Ω
coplanar waveguide (CPW) and it operates in WLAN/WiMAX bands with CP
characteristics. The 3 dB-AR band is achieved by using inverted L-shaped strips
on both sides of the monopole antenna. The CP BW is improved by simply
iv
introducing asymmetry and a stub on the ground plane. The overall size of the
antenna is 45 × 45 mm2
. Second design is a dual-band CP microstrip antenna
with compact size. The 3-dB AR-bands are achieved by introducing the
asymmetrical square slots in the corners of the patch antenna. This is a probe fed
antenna which is fabricated on FR4 substrate with height of 1.6 mm and area of
25 × 25 mm2
. It is suitable for WLAN applications with operating frequencies
of 3.5 and 5.8 GHz. Both designs have achieved broad impedance matching and
3-dB ARBW at the WLAN/WiMAX operating frequencies.
