Novel designs of wideband patch antennas for mobile communications
Student thesis: Doctoral Thesis
Related Research Unit(s)
|Award date||4 Oct 2004|
The aim of this thesis is to propose novel techniques to improve the performance of the dual polarized, circularly polarized and vertically polarized patch antennas that consist of the L-shaped feeding probe(s). They included the isolation enhancement techniques for the dual polarized patch antenna, the axial ratio bandwidth enhancement techniques for the circularly polarized patch antenna and the impedance bandwidth enhancement techniques for the vertically polarized (monopolar) patch antenna. In fact, L-shaped probe is a kind of wideband feeding method that was first proposed in the literature in 1998. In the design of dual polarized patch antenna, the basic structure is developed through using two L-probes to couple the adjacent corners of a patch. This antenna has poor input port isolation due to the strong coupling between the vertical metallic parts of the feeding probes. In this thesis, three techniques are proposed to reduce such coupling. The first technique employs two L-probe-fed patches that are arranged in a “T” shape for dual polarization operation. This antenna has an impedance bandwidth (S11 ≤ -14dB) of 17% for both ports, which can cover the PCS operating bandwidth from 1.71 to 1.88 GHz. The input port isolation is 30dB across this bandwidth. To construct an antenna array, these antennas are connected by the feeding networks. However, its input port isolation is poorer than the single element case due to the strong coupling between the array elements. To tackle this problem, a technique called “probe cancellation” is utilized. A 2x1 antenna array, which has an impedance bandwidth (S11 ≤ -14dB) of 20% and an isolation of 34dB over the PCS operating bandwidth, is developed. In fact, the first technique has the advantage of very simple feeding network is required, however, it has the drawback of a large ground plane is needed. To avoid this weakness, the other two techniques are suggested to improve the input isolation level of an antenna that consists of a patch operating for both polarizations. One of them uses a coupled line directional coupler mounted at the back of the ground plane to feed the L-probes. The antenna based on this technique has an impedance bandwidth (S11 ≤ -14dB) of 18% and an isolation of 34dB over the PCS operating bandwidth. For the other one, two L-probes that have 1800 phase difference are used to feed the patch for each polarization. This technique can enhance the isolation of the antenna to 35dB across the CDMA800 and GSM900 operating bandwidths. Such antenna can also achieve an impedance bandwidth (S11 ≤ -14dB) of 21% for both ports. To develop an antenna array, the above “dual feed” antennas are connected by the feeding networks. However, its isolation level is poor due to the coupling between the array elements. A technique, which involves mounting two vertical sidewalls on the two edges of the ground plane and using two special arranged feeding networks to connect the array elements, is proposed to solve this problem. Based on this technique, a 2x1 antenna array is produced. It has an impedance bandwidth (S11 ≤ -14dB) of 18.4% and an isolation of 37dB over the CDMA800 and GSM900 operating bandwidths. In fact, this technique can also be employed to design arrays with even number of elements. A 4x1 antenna array with an impedance bandwidth (S11 ≤ -14dB) of 18.2% and an isolation of 33.5dB over the same operating bandwidth (CDMA800 and GSM900) is developed. In the design of circularly polarized patch antenna, two techniques are proposed to enhance the axial ratio bandwidth. The first technique is called hybrid feeding. It employs two different feeding methods, included L-probe and aperture, to excite the patch in orthogonal directions. Based on this technique, two CP antennas operating at different cavity resonance modes are developed. The first antenna is operated at TM01 mode, it consists of a rectangular patch that is orthogonally fed by an L-probe and an aperture. By utilizing a Wilkinson power divider with equal amplitude but quadrature phase differences at the outputs to connect the feeds, axial ratio bandwidth (AR ≤ 3dB) of 28.9% and impedance bandwidth (S11 ≤ -14dB) of 36.9% can be obtained for this antenna. The second antenna is operated at TM21 mode, it consists of a slotted circular patch that is orthogonally fed by a pair of L-probes and a pair of apertures with relative phase of 00, 00, 900 and 900, respectively. Such phase arrangement is provided by a matching network that is mounted below the ground plane. For this antenna, axial ratio (AR≤3dB) and impedance (S11 ≤ -14dB) bandwidths of 24% and 21% can be achieved. The second technique is called sequential rotation. It employs four L-probes that have relative phases of 00, 900, 1800 and 2700 to excite the patch in sequential directions. Factually, this technique is equivalent to feed a dual polarized patch antenna, which has employed the proposed isolation enhancement technique called “dual-feed”, by signals with 900 phase difference. Based on this technique, two CP antennas with single and dual band operations are constructed. The first antenna consists of two circular patches in stacked configuration which are excited by four L-probes that are located sequentially under the circumference of the lower patch. The L-probes have relative phase of 00, 900, 1800 and 2700 provided by the matching network. This antenna possesses an impedance bandwidth (SWR≤2) of 54.9% and an axial ratio bandwidth (AR≤ 3dB) of 47.7%. The second antenna, which operates at dual frequency bands, is developed by adding a larger circular patch below the patches of the first antenna. The additional patch operates at the lower frequency band while the others operate at the upper frequency band. The patch for each band is sequentially coupled by four L-probes. This antenna has an impedance bandwidth (SWR≤ 2) of 43.9% and an axial ratio bandwidth (AR ≤ 3dB) of 33% for the lower band, while an impedance bandwidth of 55.2% (SWR ≤ 2) and an axial ratio bandwidth (AR ≤ 3dB) of 44.7% can be obtained for the upper band. In the design of vertically polarized (monopolar) patch antenna, three techniques are proposed to enhance the impedance bandwidth. The first technique uses a rectangular-loop to feed the center of a square patch for monopole-like radiation patterns. The loop is constructed by shorting the open-end of an L-probe. Based on this technique, an antenna with impedance bandwidth (S11 ≤ -10dB) of 16% is developed. For dual band operation, two rectangular patches of different sizes are excited by a feeding structure that is composed of two rectangular-loops. This antenna has impedance bandwidth (S11 ≤ -10dB) of 16.7% for the lower band and 10% for the upper band. The second technique employs an L-probe to excite the center of a circular patch. To reduce the lateral size, the patch is shorted to the ground plane by four shorting wires. Based on this technique, an antenna with impedance bandwidth (S11 ≤ -14dB) of 22.4% is developed. For dual band operation, two shorted circular patches of different radii are used. The center of each patch is excited by an L-probe. This antenna has impedance bandwidths (S11 ≤ -14dB) of 26.7% and 25.9% for the lower and upper bands, respectively. For the last technique, the L-probe of the above single band antenna is replaced by a planar monopole, which is formed by connecting four trapezoidal plates orthogonal to each other. This monopole is directly connected rather than proximity-coupled to the patch. This antenna possesses an impedance bandwidth (S11 ≤ -14dB) of 107.2%, corresponding to the frequency range from 0.77 to 2.55 GHz.
- Equipment and supplies, Mobile communication systems, Microstrip antennas, Antennas (Electronics), Design and construction