Антенны, СВЧ / Баланис / Balanis.Modern_Antenna_Handbook_Ch4_MS Antennas (pp157-200)
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4.4 FEED/EXCITATION METHODS |
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Tear-drop probe
Cylindrical probe
FIGURE 4.15 Tear-drop and cylindrical-shaped feed probes for relatively thick substrates.
50 ohms (e.g., 200 ohms). To avoid impedance mismatch, sections of quarter-wavelength- long impedance transformers [55] can be used to transform a large input impedance to a 50-ohm line. With this feed approach, an array of patch elements and their microstrip power division lines can all be designed and chemically etched on the same substrate with relatively lower fabrication cost per element. However, the leakage radiation of the transmission lines, in some cases, may be large enough to raise the sidelobe or cross-polarization levels of the array radiation.
4.4.4Proximity-Coupled Microstrip Line Feed
An open-ended microstrip line can be used to feed a patch radiator through proximity coupling. For example, the open end of a 100-ohm line can be placed underneath the patch at its 100-ohm location as shown in Figure 4.16. The open-ended microstrip line can also be placed in parallel and very close to the edge of a patch, as shown in Figure 4.17, to achieve excitation through fringe-field coupling [59]. Both these methods will avoid any soldering connection, which in some cases could achieve better mechanical reliability.
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Patch radiator |
Microstrip line |
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FIGURE 4.16 Non contact proximity feed from underneath the patch.
178 MICROSTRIP ANTENNAS: ANALYSIS, DESIGN, AND APPLICATION
Patch radiator
Side view
Microstrip line
Top
view Patch radiator
FIGURE 4.17 Non contact proximity feed from edge of patch.
4.4.5Aperture-Coupled Feed
An open-ended microstrip line or stripline transmission line can be placed on one side of the ground plane to excite a patch radiator situated on the other side through an opening slot in the ground plane. This slot-coupling or aperture-coupling technique [12], as shown in Figure 4.18, can be used to avoid a soldering connection, as well as to avoid leakage radiation of the lines that interferes with the patch radiation. In addition, this feed method allows the patch to achieve wide bandwidth (>10%) with a thick substrate or extremely wide bandwidth (>30%) with stacked parasitic patches [60, 61]. The extra bandwidth achieved by this method when compared to the coax probe feed is generated by the coupling slot, which is also a resonator and a radiator. When two resonators (slot and patch) have slightly different sizes, a wider bandwidth is achieved. Another advantage of the noncontacting feeds (proximity-coupled and aperture-coupled) is the reduction
Ground plane
Patch radiator
Microstrip line
Slot in ground plane
FIGURE 4.18 Patch fed by aperture-coupling slot.
4.5 DUAL-POLARIZATION AND CIRCULAR-POLARIZATION TECHNIQUES |
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of passive intermodulation distortion due to other harmonic frequencies created by nonlinear devices present in the circuit.
4.4.6COAX PROBE WITH A U-SLOT FEED
A unique combination of a regular coax probe and a U-shaped slot on the patch has been developed to achieve very wide bandwidth (>30%). This U-slot feed [25, 26] as illustrated in Figure 4.19 is designed with a certain slot width to provide the needed capacitance for canceling the inductance introduced by the relatively thick substrate. The U-slot also provides two different resonant patch sizes to achieve the very wide bandwidth.
4.5DUAL-POLARIZATION AND CIRCULAR-POLARIZATION TECHNIQUES
4.5.1DUAL POLARIZATION
For either a square patch or a circular patch with conventional thickness and fundamental mode, the two orthogonal points on the patch (along the two cross center lines of a square patch) are generally isolated from each other. This can be explained by referring to Figure 4.4, where the field at the orthogonal region of the feed is always zero. Thus a second feed probe can be placed at the orthogonal region of the first probe without encountering significant field coupling. This, also explained in Section 4.3.1.2, is the basic reason why a single square or circular patch can be excited at its two orthogonal locations, as indicated in Figure 4.20, to achieve dual-linear polarization. Most of the excitation techniques presented in Section 4.4 with two orthogonal feeds can be used here to achieve dual polarization. The two orthogonal feeds do not need to be the same excitation technique. For example, one excitation could use a microstrip line feed, while the orthogonal excitation could use an aperture-coupling slot feed. One must be aware that, with a square or a circular patch, two different orthogonal feeds may cause the patch to resonate at two slightly different frequencies. In other words, one may have to use a slightly rectangular or elliptical patch to achieve the same resonant frequency for two different feed techniques.
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Patch radiator
FIGURE 4.19 Patch fed by a U-slot for wide bandwidth.
180 MICROSTRIP ANTENNAS: ANALYSIS, DESIGN, AND APPLICATION
α
Patch radiator
α
Two orthogonal feeds
FIGURE 4.20 Patch with two orthogonal feeds for dual polarizations.
The above orthogonal excitation techniques for achieving dual polarization are suitable for a conventional patch with relatively small substrate thickness. For thick substrate, instead of two, four feed probes are needed with electrical phases arranged as 0◦, 180◦, 0◦, 180◦. Due to the presence of stronger higher order modes in a thick substrate, using only two orthogonal feeds will result in a large amount of mutual coupling. By using four feeds [62] as shown in Figure 4.21, one pair of oppositely located feeds with 0◦ and 180◦ phases will cause the higher order modes to cancel each other (see Figure 4.5b) while reinforcing the fundamental modes (see Figure 4.5a). In doing so, not only are the cross-polarization radiations canceled, but good isolation between the two pairs of feeds is also achieveds.
For a dual-polarization array application using a relatively thick substrate, higher order modes can be canceled by use a single feed per element. This can be achieved by having each pair of adjacent elements use oppositely located feeds with opposite phases as shown in Figure 4.22b (refer to Figures 4.5 and 4.6). It is apparent, as explained in the figure, that the cross-polarizations in the far-field distance will be canceled in both the
0°
0° |
180° |
180°
FIGURE 4.21 Four probes with 0◦, 180◦, 0◦, 180◦ phase arrangement for dual polarization with relatively thick substrate.
4.5 DUAL-POLARIZATION AND CIRCULAR-POLARIZATION TECHNIQUES |
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F = 180° |
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E10 fields reinforce |
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E02 fields cancel in
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E02 fields cancel in
E02 fields cancel in E-plane, E-plane reinforce in H-plane away
from broadside
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FIGURE 4.22 (a) Two-element subarray with conventional feed method and (b) two-element subarray feeds with 0◦ and 180◦ phases and opposite orientations to cancel cross-polarization radiation.
E - and H -planes, while the co polarization fields reinforce each other. To further illustrate the principle, a previously developed dual polarized 2 × 2 microstrip array [63] is presented in Figures 4.23 and 4.24. Both the V-port (vertical polarization) and the H-port (horizontal polarization) feed locations are offset from the centers of the feed transmission lines to achieve the required 180◦ phase differential. This 180◦ phase differential of each port is not only for the purpose of suppressing the cross-polarization as explained above; it is also for canceling the energy coupled in through the patches’ cavities from
V-port |
H-port |
Square patch
Microstrip line
FIGURE 4.23 A dual-polarized 2 × 2 microstrip array.
182 MICROSTRIP ANTENNAS: ANALYSIS, DESIGN, AND APPLICATION
FIGURE 4.24 Photo of an L-band 2 × 2 dual-polarized microstrip array with low cross-polarization and high port isolation.
its orthogonal port. This cancellation does not happen in the patch or in space but occurs in the microstrip transmission line at the input port. It is this cancellation that provides the high isolation between the two input ports. For the antenna shown in Figure 4.24, the measured port isolation is below −40 dB across the bandwidth of the antenna. The worst cross-polarization level is −28 dB below the copolarization peak. For arrays larger than 2 × 2, the above principle has also been successfully applied by several researchers [31, 64].
4.5.2CIRCULAR POLARIZATION
Circular polarization (CP) from a microstrip antenna can generally be achieved by either a square or a circular patch with two orthogonal feeds having equal amplitudes and 90◦ phase differential. As illustrated in Figure 4.25, the square patch with its two orthogonal microstrip line feeds in this case will provide a right-hand circular polarization (RHCP). Similar to the dual-linear polarization case, a CP patch can be excited by various feed
RHCP |
−90° |
patch |
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0°
FIGURE 4.25 Circularly polarized patch with two orthogonal feeds.
4.5 DUAL-POLARIZATION AND CIRCULAR-POLARIZATION TECHNIQUES |
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techniques, as presented in Section 4.3, with two orthogonal excitations. A single CP patch with relatively thick substrate can also be excited by four feeds [65] with phases arranged as 0◦, 90◦, 180◦, 270◦ to suppress cross-polarization level. A circular patch with diameter larger than a half-wavelength can be excited to radiate a higher order mode [47] with conical pattern shape (zero radiation in broadside direction). The higher the mode order, the larger the patch diameter required. In any case, four feeds arranged in special angular orientations and phases are needed to provide good CP radiation.
A CP patch can also be achieved by a single excitation with a small portion of the patch perturbed. For examples, CP can be generated by a square patch with its two diagonal corners truncated [8, 66] as shown in Figure 4.26, by a square or circular patch with a tilted center slot [8] as shown in Figure 4.27, or by a circular patch with two opposing edges indented with notches [67] as shown in Fig 4.28a or extended with pads [68] as shown in Fig 4.28b. A slight rectangular patch with a single feed located at its diagonal line [69], as shown in Figure 4.29, can also generate CP radiation. All these single-feed techniques are called the perturbation method. The small perturbation has to be just the right amount at the desired frequency to produce two orthogonal polarizations with the same amplitude but with a 90◦ phase differential. As a result, the CP bandwidths
FIGURE 4.26 Circularly polarized patch with a single feed and truncated corners.
FIGURE 4.27 Circularly polarized patch with a single feed and a slot.
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FIGURE 4.28 Circularly polarized circular patch with a single feed and (a) two indents and (b) two pads.
184 MICROSTRIP ANTENNAS: ANALYSIS, DESIGN, AND APPLICATION
B
Feed
A
A ≠ B
FIGURE 4.29 Circularly polarized patch (slightly rectangular) with a single feed.
of these single-feed methods, regardless of their impedance bandwidths, are extremely narrow (generally around 0.5%). However, these single-feed methods yield simplicity with reduced line loss.
In array application, CP performance, such as CP bandwidth and cross-polarization level, can be improved by sequentially arranging (orientations and phases) four neighboring CP elements as shown in Figure 4.30. Four single-fed CP elements can also be sequentially arranged [22] to improve the CP bandwidth as shown in Figure 4.31. This improvement of CP performance is based on the same reason as explained in Section 4.5.1 where the cross-polarization is canceled in a pair of oppositely located patches with opposite phases. In a large array, each of its four neighboring elements can even be linearly polarized but arranged sequentially [23], as shown in Figure 4.32, to provide CP radiation.
4.6BROADBAND AND DUAL-BAND TECHNIQUES
4.6.1Broadband Techniques
The microstrip antenna is basically a resonating cavity with open side walls. It is well known that a closed cavity with fixed dimensions indicates narrow bandwidth behavior. Thus the microstrip antenna also behaves as a narrow band device. Figure 4.33
F = 0°
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90°
F = 90°
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FIGURE 4.30 Circularly polarized 2 × 2 subarray with sequential rotation, Each element has two orthogonal feeds.
4.6 BROADBAND AND DUAL-BAND TECHNIQUES |
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F = 0°
F = 90°
F = 270°
F = 180°
FIGURE 4.31 Circularly polarized 2 × 2 subarray with sequential rotation, Each element is a single-feed CP patch.
F = 0°
F = 90°
F = 270°
F = 180°
FIGURE 4.32 Circularly polarized 2 × 2 subarray with sequential rotation, Each element is a linearly polarized patch.
BW (%)
10 
TM10 A = 1.5 b
T = 0.318 cm
0.159 cm ER = 2.32
0.0795 cm
0.127 cm
0.0635 cm
0.0254 cm
1
ER = 9.8
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Resonant frequency (GHz)
FIGURE 4.33 Bandwidths of rectangular microstrip patches with various substrate thicknesses and two dielectric constants. (From Ref. 11, with permission from Peter Peregrinus Ltd.)
186 MICROSTRIP ANTENNAS: ANALYSIS, DESIGN, AND APPLICATION
is a plot of bandwidth versus operating frequency [11] for a typical rectangular patch with various thicknesses of the substrates and two different relative dielectric constants (2.32 and 9.8). This figure demonstrates that the bandwidth of a single patch with nominal substrate thickness has a bandwidth of less than 5%. However, due to the patch’s open side walls, as the thickness of this cavity is increased, the bandwidth of this open cavity can be significantly increased. To illustrate this point, a rectangular patch is designed with x , y , z dimensions with x being the resonating dimension and z the cavity thickness. When the cavity thickness increases, as indicated in Figure 4.34 as well as in Figure 4.2 and 4.4, the height of the fringing field also increases. It is obvious from this figure that the degree of freedom for the resonant frequency to change is proportional to L2 — L1 , which is greater for the thicker substrate than for the shallow cavity. This bandwidth increase can also be explained from the antenna’s quality factor (Q). The Q of a rectangular patch [70] is inversely proportional to the cavity thickness (h):
Q = |
c√εe |
(4.15) |
4fr h
and the bandwidth (BW) increases as Q reduces:
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From the above equation, one can conclude that a larger BW is possible for a microstrip antenna by choosing a thicker substrate and/or by lowering the value of the dielectric constant, εr . However, as the thickness is increased, the feed line or feed probe will encounter an impedance matching issue. Generally, a large reactance (inductance) is introduced by the feed. In the case of the microstrip line feed, an impedance matching circuit [21] can be used to balance out the large reactance. In the case of a feed probe, a capacitive feed can be used to cancel the excessive inductance as discussed in Section 4.4.2.
There is a limitation to how much the BW can be increased by increasing the substrate thickness. The maximum achievable BW is about 15% with the thickness equal to about
L2
L1
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FIGURE 4.34 Primary fringing fields of a rectangular patch.