Study on Horizontally Polarized Omnidirectional Microstrip Antenna

A horizontally polarized omnidirectional microstrip antenna is proposed in this paper. The structure of designed antenna is with two back-to-back horizontally polarized microstrip antenna elements. Gain variation on main radiation plane of this new antenna is analyzed and radiation theory is deduced; formula of directivity onmain radiation plane is given. Better omnidirectional characteristic of this antenna can be obtained by decreasing patch physical length. Both simulated and measured results verify the omnidirectional radiation patterns and input impedance characteristics. Good omnidirectional radiation patterns (gain variation in E-plane less than ±0.4 dBi) and input impedance characteristics are obtained; moreover, cross polarization less than −20 dBi is achieved.


Introduction
Omnidirectional antennas were applied in many communication systems.It's well known that an omnidirectional antenna can be easily realized by a vertical dipole like in [1][2][3][4].Microstrip antennas were also easily designed with omnidirectional radiation pattern, microstrip antenna array mounted on a circular cylinder was presented in [5], it needed more than eleven patches to achieve good omnidirectional pattern.The drawbacks of those cylindrical conformal microstrip antennas were difficult to fabricate and they need more than three patches to obtain omnidirectional radiation pattern.All back-to-back omnidirectional microstrip antennas in [6][7][8][9] were with omnidirectional radiation pattern, however they are either vertically polarized or circularly polarized, none of those works were horizontally polarized.A horizontally polarized omnidirectional microstrip antenna was described in [10], the antenna consists of three main components: a probe-fed main patch and two parasitic patches, placed conformally on a cylindrical structure, drawback of this antenna is a performance with only 1.46 % narrow bandwidth.
Horizontally polarized (HP) omnidirectional radiation pattern can be achieved by printed antennas.In paper [11], there were four notches cut out from the bottom conductor layer, and four microstrip lines fed them respectively on the upper layer to obtain HP omnidirectional radiation pattern.In paper [12], a horizontally polarized omnidirectional planar printed antenna for WLAN applications was presented.The printed Alford-loop-structure antenna consisted of two Z-shaped strips printed on the top and bottom plane of the FR-4 printed-circuit-board substrate.A horizontally polarized omnidirectional planar antenna in [13] is developed for mobile communications.The proposed antenna consists of four printed arc dipoles that form a circular loop for HP omnidirectional radiation.But it needs a complex feeding network which includes four baluns and an impedance matching circuit to excite the four printed arc dipoles.
Slot antenna or array may also obtain omnidirectional radiation like in [14][15][16][17].Paper [14] presented a three-element CPW-fed leaky wave folded slot antenna array with omnidirectional radiation pattern.By using the columnar structure, a gain variation of less than 1.1 dB was achieved in the azimuthal plane.A planar slot array antenna with omnidirectional radiation pattern in the horizontal plane was proposed in [15], the antenna with eight back-to-back slots was designed by employing the genetic algorithm implemented on a cluster system to achieve omnidirectional radiation characteristics.In paper [16], a dual-polarized diversity antenna with azimuthally omnidirectional patterns was designed inside a slender and low-profile columnar structure.The proposed antenna was composed of a cavity-backed notch for horizontal polarization and a folded slot for vertical polarization.Slot antennas or arrays may obtain horizontally polarized omnidirectional radiation pattern, but the structure were large in size, weight in heavy, and difficult to be fabricated.Some other special structures obtained horizontally polarized omnidirectional radiation pattern like in paper [18].Circular dipoles in the shape of a "C" produced a horizontally polarized omnidirectional radiation pattern, but the antenna was very large in size, hard to fabricate and not suitable for application.A traveling-wave antenna based on a tapered half mode substrate integrated waveguide was introduced in paper [19], the antenna used a direct transition from a coaxial connector and radiated from the open side of the waveguide, but this antenna was with low 80% efficiency for the whole bandwidth.
A new horizontally polarized omnidirectional microstrip antenna was proposed in this paper.The new designed microstrip antenna, which was a pair of patches, was with back-to-back structure.Two patches were fed by a Wilkinson power divider.The patches were symmetrically placed along vertical plane, adding that patches were horizontally polarized, so fields from the two patches of the designed antenna interfered constructively (add) in the horizontal direction and interfered destructively (cancel each other) in vertical direction.
The radiation theory of the back-to-back structure was studied.Software Ansoft HFSS was applied to design and simulate the antenna for the purpose to optimize the omnidirectional characteristic, drew the best technical parameters.The new proposed antenna was fabricated and measured.Measured results were shown to make the comparison with the simulated ones.Advantages of the designed antenna were compact in size, easy to be fabricated, with good omnidirectional characteristic, and good horizontal polarization performance.

Antenna design
The designed antenna consists of four layers as shown in Fig. 1(a), two external layers are single-fed rectangular-patch microstrip antennas with back-to-back structure.And two internal layers formed to be a classic stripline-type Wilkinson power divider, input port 1 is connected with a SMA connector, port2 and port 3 are two output ports.Two patches are connected with output ports of the power divider differently by probes, port 2 is used to feed the right microstrip patch shown in Fig. 1(a) and port 3 feeds the left patch.Outline size of the antenna in Fig. 1(b) is: length b=32mm, width a=10mm, height h=65mm.Fig. 1(c) is the front view of the designed antenna, length of the patch is l, width of the patch is w, length between the center point of the patch and the up edge of the substrate is c, and e is distance of the feeding point away from edge of the patch.Fig. 1(d) is the size of the equal-split Wilkinson power divider, which is for 50Ω system impedance in working band.Width of the 50Ω transmission lines is w1=2.5mm.And width of the quarter-wave transmission lines, in the divider should have a characteristic impedance of 70.7Ω, is w2=1.2mm.The dielectric constant of the external layer substrate is 16 and thickness is 4mm, and dielectric constant of the internal layer substrate is 2.65 and thickness is 1mm.

Analysis of the antenna structure
For main TM 01 mode, the E-field radiation is given by [20] where 0 0 sin cos sin cos , w is the effective width, l is the effective length.U 01 is the voltage of mode TM 01 at patch corner point.
The designed antenna under investigation can be treated as an array of two horizontal polarized microstrip antenna elements (antenna element 1 and antenna element 2) positioned along z-axis, as shown in Fig. 2. Antenna element 1 is placed on positive z-axis while antenna element 2 is on negative z-axis, so main radiation field of antenna element 1 covers upper hemisphere and that of antenna element 2 covers lower hemisphere.Two antenna elements are fed by signals with same amplitude and phase because of power divider.The total field radiated by the two elements, assuming no coupling and no difference in excitation between the elements, is equal to the sum of the two antenna elements.And radiation field radiated by element 1, placed on positive z-axis, in the y-z plane is given by Since effective length l is a little bit longer than physical length l ' , and effective length l is in proportion to physical length l ' , it's tenable that using physical length l ' to replace effective length l in formula (5).Thus for the two-element array of constant amplitude, total approximate formula for directivity in y-z plane of designed antenna is given by: Table I shows the formula calculated roundness and gain variation against physical length l ' .Taking l ' equal λ/4 for example, V  dBi.Roundness and gain variation with other physical length l ' can be calculated by using the same procedure as shown in the table.Roundness and gain variation against physical length l ' are plotted Fig. 3, as physical length l ' decreases, the roundness increases and gain variation decreases, respectively.From table I and Fig. 3, the following results are clear: (a) The back-to-back microstrip antenna radiates omnidirectional pattern; (b) By decreasing the physical length l ' , roundness increases and gain variation decreases, respectively.In general, physical length l ' of patch is less than λ/2, and it can be decreased by increasing dielectric constant of substrate.In this way, gain variation of the omnidirectional pattern will be decreased and better omnidirectional radiation pattern be achieved by using higher permittivity material as shown in Fig. 4. however, it's well known that such patch size reduction (antenna is up to 1/20th of free space wavelength in this paper) often brings decreased bandwidth, increased losses (lower efficiency) and matching problems, so selecting a proper substrate dielectric constant and physical length l ' is quit important to achieve better antenna performance.In order to verify the accuracy of formula ( 6) calculated directivity in y-z plane of the proposed antenna, gain variation comparisons between formula calculated and HFSS simulated are made.For simplify the simulation process, power divider is removed from designed antenna, two patches are fed by two ports with same amplitude and phase respectively, as shown in Fig. 5.By choosing different substrate dielectric constant, different physical length l ' is got and different gain variation on main radiation plane is obtained.
There are total four different patch sizes, physical length l ' 1 of patch No. 1 is 61.5mm with substrate dielectric constant  r is 2; l ' 2 of patch No. 2 is 44.1mm with substrate  r is 4; l ' 3 of patch No. 3 is 31.4mmwith substrate  r is 8; l ' 4 of patch No. 4 is 25.2mm with substrate  r is 12.Four patch antennas from No.1 to No.4 are working in the same center frequency 2.35GHz.Normalized simulated radiation pattern (by Ansoft HFSS) in y-z plane with different patch size is indicated in figure 6.It's clearly seen that as physical length decreases, better HFSS simulated omnidirectional radiation pattern is obtained, which is with the same variation tendency as formula (6) calculated results.Table II summarizes the specified numeric comparison of gain variation calculated by formula (6) and by HFSS simulator by given different patch physical length.Physical length l ' of patch No.1 is 61.5mm, which is nearly 0.323 , and approximate formula for directivity in y-z plane is .By using the same procedure, gain variation of other patch physical length can be calculated as shown in table II.
Gain variation difference between formula calculated and HFSS calculated results is shown in Fig. 6.Gain variation difference of patch No. 1 is 1.6dBi, that of patch No. 2, patch No. 3, patch No. 4 is 0.6dBi, 0.2dBi, 0.03dBi, respectively.As physical length getting smaller, less difference between formula-calculated result and HFSS calculated result is achieved.Both results show that better gain variation performance is got as patch physical length gets shorter.
Fig. 8 gives the object of the designed horizontally polarized omnidirectional microstrip antenna connects with a SMA connector.Simulated and measured S11 of the designed antenna is shown in Fig. 9, band-width of the antenna is about 6.4% with center frequency 1.575GHz, and measured result agrees well with the simulated one.
Simulated three-dimensional radiation pattern of the designed antenna is shown in Figure 10, peak gain of the designed antenna is more than 2dBi.It is obvious that the radiation pattern is omnidirectional in y-z plane, gain variation in y-z plane less than ±0.5dBi is achieved.
Fig. 11 shows the simulated and measured main polarization pattern and cross-polarization pattern in E-plane and H-plane.Peak gain of the designed antenna is about 2.2dBi, antenna efficiency is higher than 88% in the operating band.Gain variation of the HFSS simulated result in E-plane pattern is about 0.58dBi.Physical length of the designed antenna l ' =22.1mm=0.116λ,formula calculated gain variation in E-plane pattern is 0.5dBi, which is coincides with the HFSS simulated result.Measured gain variation is about 1dBi and cross polarization level is -20dBi lower than that of main polarization.

Conclusion
A new horizontally polarized omnidirectional microstrip antenna was presented in this paper.And formula of directivity in main lobe radiation plane of designed antenna was deduced.The measured results agreed well with the simulated ones.Band-width of the designed antenna was about 6.4%, peak gain was about 2.2dBi, antenna efficiency was higher than 88% in the operating band, and cross polarization level was -20dBi lower than that of main polarization.The designed antenna was with good omnidirectional radiation patterns, gain variation in E-plane less than ±0.5dBi was achieved in both simulated and measured results.
Fig.1.model of the designed antenna

Fig. 2 .
Fig.2.Geometry of a two-element array positioned along the z-axis.
antenna element 2 rotates 180deg around X-axis and faces to negative z-axis compared to antenna element 1, far field radiation pattern of antenna element 2 in y-z plane is 1 and element 2 share one ground, in this case d=0, r1=r2=r.The total field radiated by the pair elements is given by

TABLE I ROUNDNESS
AND GAIN VARIATION AGAINST PHYSICAL LENGTH L