REPORT ON ANTENNA FUNDAMENTALS

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ANTENNA FUNDAMENTALS An Antenna is a circuit element that provides a transition from a guided wave on a transmission line to a free space and it provides for the collection of electromagnetic energy. Antenna is basically a component which provides link between transmitter and free space and helpful in finding characteristic of the system in which particular antenna is used. 1.1 Antenna Parameters: The Antenna parameters describe the antenna performance with respect to space distribution of the radiated energy, power efficiency, matching to the feed circuitry etc. Many of these parameters are interrelated. 1.1.1 Radiation Pattern:

This is a graph which shows the variation in actual field strength of electromagnetic field at all points which are at equal distance from the antenna. The graphical representation of radiation of an antenna as a function of direction is given by the name radiation pattern of the antenna. If the radiation from the antenna is expressed in terms of field strength, the radiation pattern is called as the field strength pattern. For radiation pattern rectangular and polar graphs. Fig 1.1.1 : Antenna pattern in polar coordinate graph Fig 1.1.2:

Antenna pattern in rectangular coordinate graph Major Lobes: It is also called the main lobe or main beam and is defines as the radiation lobe containing the direction of maximum radiation. Minor Lobe: A lobe expect-major lobe are called minor lobe. Side Lobe: It is a radiation lobe in any direction other that the intended lobe. Generally it is an adjacent to the main lobe. Back Lobe: Normally refer to a minor lobe that occupies in direction opposite to the main lobe. 1.1.2 Front to back Ratio:

The front-to-back ratio is the ratio of the energy radiated in the principal direction compared to the energy radiated in the opposite direction for a given antenna. 1.1.3 Beam Width: Antenna Beam Width is simply a way to measure the directivity. This is an angular measurement on the radiation pattern between points where the radiated power has half power value of its maximum value. This half power beam width is also known as 3 dB beam width because at this point the power is 3 dB below to the maximum power of the major lobes. 1.1.4 Aperture: The effective antenna aperture is the ratio of the available power at the terminals of the antenna to the power flux density of a plane wave incident upon the antenna, which is polarization matched to the antenna. If there is no specific direction chosen, the direction of maximum radiation intensity is implied. The gain is related to the effective area by the following relation: 1.1.5 Directivity: Directivity of an antenna in a given direction is the ratio of the radiation intensity in this direction and the radiation intensity averaged over all direction. The radiation intensity averaged over all direction is equal to the power radiated by the antenna divided by 4?. It can be also defined as the ratio of the radiation intensity of the antenna in a given direction and the radiation intensity of an isotropic radiator fed by the same amount of power. 2. PATCH ANTENNA A patch antenna (also known as a Rectangular Micro strip Antenna) is a popular antenna type. Its name is attributed to the fact that it consists of a single metal patch suspended over a ground plane. Patch antennas are simple to fabricate and easy to modify and customize,

which are a length of micro strip transmission line of approximately one-half wavelength. The radiation mechanism arises from discontinuities at each truncated edge of the microstrip transmission line. The radiation at the edges causes the antenna to be slightly larger than its physical dimension electrically. In order to obtain a resonant condition at the antenna driving point, a shorter than a one-half wavelength section of microstrip transmission line is used. A patch antenna is generally constructed on a dielectric substrate, usually employing the same sort of lithographic patterning used to fabricate printed circuit boards. 2.1 Gain: The gain of a rectangular micro strip patch antenna with air dielectric can be very roughly estimated as follows. Since the length of the patch, half a wavelength, is about the same as the length of a resonant dipole, we get about 2 dB of gain from the directivity relative to the vertical axis of the patch. If the patch is square, the pattern in the horizontal plane will be directional,

somewhat as if the patch were a pair of dipoles separated by a half-wave; this counts for about another 2-3 dB. Finally, the addition of the ground plane cuts off most or all radiation behind the antenna, reducing the power averaged over all directions by a factor of 2 (and thus increasing the gain by 3 dB). Adding this all up, we get about 7-9 dB for a square patch, in good agreement with more sophisticated approaches A typical radiation pattern for a linearly-polarized 900-MHz patch antenna is shown below. The figure shows a cross-section in a horizontal plane; the pattern in the vertical plane is similar though not identical. The scale is logarithmic, so (for example) the power radiated at 180° (90° to the left of the beam center) is about 15 dB less than the power in the center of the beam. The beam width is about 65° and the gain is about 9 dB. Fig 2.1: Radiation pattern for a linearly-polarized patch antenna 2.2 Impedance Bandwidth: The impedance bandwidth of a patch antenna is strongly influenced by the spacing between the patch and the ground plane. As the patch is moved closer to the ground plane, less energy is radiated and more energy is stored in the patch capacitance and inductance: that is, the quality factor Q of the antenna increases. A very rough estimate of the bandwidth is Where d is the height of the patch above the ground plane, W is the width (typically a half-wavelength), Z0 is the impedance of free space, and Rrad is the radiation resistance of the antenna.

The fractional bandwidth of a patch antenna is linear in the height of the antenna. The impedance of free space is approximately 377 ohms, so for the typical radiation resistance of about 150 ohms, a simplified expression can be obtained: The negative return loss for a pair of representative commercial patch antennas is shown below; both antennas are nominally designed to operate in the US Industrial, Scientific, and Medical (ISM) band from 902-928 MHz. Antenna B uses a 16-mm patch height above ground, and the measured bandwidth of about 150 MHz at 10 dB return loss is rather close to that estimated above. However, this antenna also uses a very large (30x30 cm) ground plane. Antenna A delivers similar bandwidth but at about 20x20 cm is considerably smaller and more convenient to mount and position.

Commercial antennas vary widely in performance, often due to poor centering of the band even when theoretical bandwidth is achieved. Fig 2.2: Return loss v/s Frequency Graph 3. APERATURE COUPLED By the early 1980s basic microstrip antenna elements and arrays were fairly well established in terms of design and modeling, and workers were turning their attentions to improving antenna performance features (e.g., bandwidth), and to the increased application of the technology. One of these applications involved the use of microstrip antennas for integrated phased array systems, as the printed technology of microstrip antenna seemed perfectly suited to low-cost and high-density integration with active MIC or MMIC phase shifter and T/R circuitry. The straightforward approach of building an integrated millimeter wave array (or sub array) using a single Gas substrate layer had several drawbacks. First, there is generally not enough space on a single layer to hold antenna elements, active phase shifter and amplifier circuitry, bias lines, and RF feed lines. Second, the high permittivity of a semiconductor substrate such as GaAs was a poor choice for antenna bandwidth, since the bandwidth of a microstrip antenna is best for low dielectric constant substrates. And if substrate thickness is increased in an attempt to improve bandwidth, spurious feed radiation increases and surface wave power increases. This latter problem ultimately leads to scan blindness, whereby the antenna is unable to receive or transmit at a particular scan angle. Because of these and other issues, we were looking at the use of a variety of two or more layered substrates. One obvious possibility was to use two back to back substrates with feed through pins. This would allow plenty of surface area, and had the critical advantage of allowing the use of GaAs (or similar) material for one substrate,

with a low dielectric constant for the antenna elements. The main problem with this approach was that the large number of via holes presented fabrication problems in terms of yield and reliability. We had looked at the possibility of using a two sided-substrate with printed slot antennas fed with microstrip lines, but the bidirectionality of the radiating element was unacceptable. Using a slot or aperture to couple a microstrip feed line to a resonant microstrip patch antenna. After considering the application of small hole coupling theory to the fields of the microstrip line and the microstrip antenna, we designed a prototype element for testing.

Our intuitive theory was very simple, but good enough to suggest that maximum coupling would occur when the feed line was centered across the aperture, with the aperture positioned below the center of the patch, and oriented to excite the magnetic field of the patch. The first aperture coupled microstrip antenna was fabricated and tested by a graduate student, Allen Buck, on August 1, 1984, in the University of Massachusetts Antenna Lab. This antenna used 0.062” Duroid substrates with a circular coupling aperture, and operated at 2 GHz. As is the case with most original antenna developments, the prototype element was designed without any rigorous analysis or CAD - only an intuitive view of how the fields might possibly couple through a small aperture. 3.1 BASIC OPERATION OF THE APERTURE COUPLED MICROSTRIP ANTENNA Figure 3.1.1 shows the geometry of the basic aperture coupled patch antenna. The radiating microstrip patch element is etched on the top of the antenna substrate, and the microstrip feed line is etched on the bottom of the feed substrate. The thickness and dielectric constants of these two substrates may thus be chosen independently to optimize the distinct electrical functions of radiation and circuitry. Although the original prototype antenna used a circular coupling aperture, it was quickly realized that the use of a rectangular slot would improve the coupling, for a given Aperture area, due to its increased magnetic polarizability. Most aperture coupled microstrip antennas now use rectangular slots, or variations thereof. The aperture coupled microstrip antenna involves over a dozen material and dimensional parameters, and we summarize the basic trends with variation of these parameters below: 3.1.1 Antenna substrate dielectric constant: This primarily affects the bandwidth and radiation efficiency of the antenna, with lower permittivity giving wider impedance bandwidth and reduced surface wave excitation. 3.1.2 Antenna substrate thickness Substrate thickness affects bandwidth and coupling level.

A thicker substrate results in wider bandwidth, but less coupling for a given aperture size. 3.1.3 Microstrip patch length: The length of the patch radiator determines the resonant frequency of the antenna. 3.1.4 Microstrip patch width: The width of the patch affects the resonant resistance of the antenna, with a wider patch giving a lower resistance. Square patches may result in the generation of high cross polarization levels, and thus should be avoided unless dual or circular polarization is required. 3.1.5 Feed substrate dielectric constant: This should be selected for good microstrip circuit qualities, typically in the range of 2 to 10. 3.1.6 Feed substrate thickness: Thinner microstrip substrates result in less spurious radiation from feed lines, but higher loss. A compromise of 0.01l to 0.02l is usually good. 3.1.7 Slot length: The coupling level is primarily determined by the length of the coupling slot, as well as the back radiation level. The slot should therefore be made no larger than is required for impedance matching. 3.1.8 Slot width: The width of the slot affects the coupling level, but to a much less degree than the slot width. The ratio of slot length to width is typically 1/10. 3.1.9 Feed line width: Besides controlling the characteristic impedance of the feed line, the width of the feed line affects the coupling to the slot. To a certain degree, thinner feed lines couple more strongly to the slot. 3.1.10 Feed line position relative to slot:

For maximum coupling, the feed line should be positioned at right angles to the center of the slot. Skewing the feed line from the slot will reduce the coupling, as will positioning the feed line towards the edge of the slot. Position of the patch relative to the slot: For maximum coupling, the patch should be centered over the slot. Moving the patch relative to the slot in the H plane direction has little effect, while moving the patch relative to the slot in the E-plane (resonant) direction will decrease the coupling level. Figure 3.1.1: Geometry of the basic aperture coupled microstrip antenna. 3.1.11 length of tuning stub: The tuning stub is used to tune the excess reactance of the slot coupled antenna. The stub is typically slightly less than lg/4 in length; shortening the stub will move the impedance locus in the capacitive direction on the Smith chart. Figure 3.1.2 shows a typical Smith chart plot of the impedance locus versus frequency for an aperture coupled microstrip antenna. The size of the locus is controlled primarily by the slot length; increasing the slot length increases the diameter of the circular portion of the locus.

The effect of the stub length is to rotate the entire locus up (inductive) or down (capacitive) on the chart. Thus, optimum matching, where the locus is just large enough to pass through the center of the Smith chart, can be obtained by properly adjusting the length and the stub length. Fig 3.1.2: Smith chart plot of the impedance locus v/s frequency for an aperature coupled micro strip antenna Figure3.1.3 shows that typical radiation pattern plot for an aperature coupled antenna slot.

The forward radiation patterns are typical of those obtained with microstrip antenna elements, while the back radiation lobe is caused byradiation from the coupling slot. Fig 3.1.3: Principal pattern of an Aperature coupled patch antenna 3.2 VARIATIONS ON THE APERTURE COUPLED MICROSTRIP ANTENNA 3.2.1 Radiating elements:The original aperture coupled antenna used a single rectangular patch. Since then, workers have successfully demonstrated the use of circular patches, stacked patches, parasitically coupled patches, patches with loading slots, and radiating elements consisting of multiple thin printed dipoles. Most of these modifications are intended to yield improved bandwidth, and this topic is discussed in more detail in the following section. 3.2.2 Slot shape: The shape of the coupling aperture has a significant impact on the strength of coupling between the feed line and patch. Thin rectangular coupling slots have been used in the majority of aperture coupled microstrip antennas, as these give better coupling than round apertures. Slots with enlarged ends, such as “dogbone”, bow-tie, or H-shaped apertures can further improve coupling. 3.2.3 Type of feed line: The microstrip feed line can be replaced with other planar lines, such as strip line, coplanar waveguide, dielectric waveguide, and similar. The coupling level may be reduced with such lines, however. It is also possible to invert the feed substrate, inserting an additional dielectric layer so that the feed line is between the ground plane and the patch element. 3.2.4 Polarization: Besides linear polarization, it has been demonstrated that dual polarization and circular polarization can be obtained with aperture coupled elements. This is discussed in more detail in the following section. 3.2.5 Dielectric layers:

As with other types of microstrip antennas, it is easy to add a radome layer to an aperture coupled antenna, either directly over the radiating element, or spaced above the element. It is also possible to form the antenna and feed substrates from multiple layers, such as foam with thin dielectric skins for the etched conductors. 3.3 LATER DEVELOPMENT OF THE APERTURE COUPLED MICROSTRIP ANTENNA Here we discuss in more detail, and with references, some of the significant developments in the field of aperture coupled microstrip antennas. 3.3.1 Wideband aperture coupled microstrip antennas: One of the useful features of the aperture coupled microstrip antenna is that it can provide substantially improved impedance bandwidths. While single layer probe or microstrip line-fed elements are typically limited to bandwidths of 2%-5%, aperture coupled elements have been demonstrated with bandwidths up to 10 - 15% with a single layer and up to 30-50% with a stacked patch configuration.

This improvement in bandwidth is primarily a result of the additional degrees of freedom offered by the stub length and coupling aperture size. The tuning stub length can be adjusted to offset the inductive shift in impedance that generally occurs when thick antenna substrates are used, and the slot can be brought close to resonance to achieve a double tuning effect. Use of a stacked patch configuration also introduces a double tuning effect. 3.3.2 Dual and circularly polarized aperture coupled microstrip antennas: As with other types of microstrip antennas, dual polarization capability can be obtained by using two orthogonal feeds. This was first demonstrated by Adrian and Schaubert, with two orthogonal non-overlapping slots were used to feed a square patch element. Dual linear polarization with 18 dB isolation was achieved, and circular polarization was demonstrated using an off-board 90° hybrid coupler. One problem with this approach was that the asymmetry of the slots constrained the size of the slots (and thus the coupling level that could be achieved), and also limited the isolation and polarization purity. Another approach was suggested by Tsao, Hwang, Killburg, and Dietrich, whereby a crossed slot was used to feed the patch. In this case 27 dB isolation was achieved, with very good bandwidth. The drawback in this case was the requirement for a crossover in the balanced feed lines that fed each arm of the crossed slot. This problem was solved by Targonski and Pozar, who used a different arrangement of feed lines with the crossed slot for circular polarization. This element led to a 3 dB axial ratio bandwidth of up to 50%, and a comparable impedance bandwidth. It is also possible to use a crossed slot feed with two feed lines on distinct substrate layers to avoid the crossover problem,

as demonstrated in. Circularly polarized aperture coupled elements can also be designed with a single diagonal coupling slot and a slightly nonsquare patch, similar to circularly polarized patches with a single probe feed, but the resulting axial ratio bandwidth is very narrow. Somewhat improved axial ratio bandwidth can be obtained by using a crossed slot with a single microstrip feed line through the diagonal of the cross, and a slightly non-square patch 3.3.3 Aperture coupled microstrip antenna arrays: Like other types of microstrip antennas, aperture coupled elements lend themselves well to arrays using either series or corporate feed networks. The two-sided structure of the aperture coupled element allows plenty of space for feed network layout, and this extra room is especially useful for dual-polarized or dual frequency arrays.

In addition, the ground plane serves as a very effective shield between the radiating aperture and the feed network. One drawback is that the coupling apertures will radiate a small amount of power in the back direction, but in practice a ground plane located some distance below the feed layer can be used to eliminate this radiation. Some examples of corporate-fed aperture coupled array antennas are given in references and. in addition, series fed aperture coupled arrays have been demonstrated. 3.3.4 Monolithic arrays using aperture coupled microstrip antennas: While the objective of a truly monolithic array with integrated planar antennas and phase shifters was largely the driving force behind the development of the aperture coupled microstrip element, there are only a few examples where such arrays have actually been implemented.

One of the first monolithically integrated antennas was the 40 GHz module reported by Ohmine, Kashiwa, Ishikawa, Iida, and Matsunaga. In this work, an aperture coupled antenna element was integrated with a three-stage RF amplifier and a mixer. Probably the best example of a fully integrated phased array is the 4x4 Ka-band subarray developed by Texas Instruments. This antenna used circular aperture coupled patch elements, with 164-bit MMIC phase shifters and 16 100mW MMIC power amplifiers. This work successfully demonstrated beam scanning up to 45°, with an ERP of 77 watts at 30 GHz. 3.3.5 Modeling of aperture coupled microstrip antennas: Analysis of the aperture coupled microstrip element is complicated by the presence of two dielectric layers, and the microstrip line-to-slot transition. In fact, however, the slot feed is generally easier to model in a rigorous manner than a probe or line-fed element because the patch current near the feed point is less singular. The initial report of the aperture coupled element presented only a simplified cavity-type model, and the antenna was not rigorously analyzed until Sullivan and Schaubert treated it using a full-wave moment method solution. This work also presented data showing the effect of various design parameters, such as slot position and size, on the input impedance locus of the antenna. An alternative way of treating the microstrip to slot transition was introduced by Pozar.

This technique was derived using the reciprocity theorem, and eliminates the need for brute-force modeling of the microstrip feed line and stub. Many later analyses, both moment method and cavity model, utilized this technique for treating the feed. The moment method technique has also been applied to mutual coupling between aperture coupled elements, and to infinite arrays of aperture coupled elements. Moment method analysis techniques are rigorous, highly accurate, and versatile enough to handle important practical variations such as stacked patches, patches with radome layers, and patches fed with stripline, but at the expense of relatively long computer run times. Cavity models, on the other hand, are more approximate, but require negligible computer resources. Some examples of cavity models for aperture coupled microstrip antennas include. 3.3.6 Computer aided design software for aperture coupled microstrip antennas: Computer aided design (CAD) software is one of the most pervasive subjects in the fields of microwave and antenna engineering today, probably because of the perception among engineers that such software will not only make their work easier but provide a tool solve problems that would not otherwise be possible. 3.4 APPLICATIONS OF APERTURE COUPLED MICROSTRIP ANTENNAS While most of the rapid advances in microstrip antennas and arrays that took place in the 1980s were driven by defense and space markets present applications of this technology are growing most rapidly in the commercial sector. While specifications for defense and space application antennas typically emphasize maximum performance with little constraint on cost, commercial applications demand low cost components, often at the expense of reduced electrical performance. Thus, microstrip antennas for commercial systems require low-cost materials, and simple and inexpensive fabrication techniques. Some of the commercial systems that presently use microstrip antennas are listed in the table below: 3.4.1 Application Frequency Global Positioning Satellite 1575 MHz 1227 MHz Paging 931-932 MHz Cellular Phone 824-849 MHz 869-895 MHz Personal Communication System 1.85-1.99 GHz 2.18-2.20 GHz GSM 890-915 MHz 935-960 MHz Wireless Local Area Networks 2.40-2.48 GHz 5.4 GHz Cellular Video 28 GHz Direct Broadcast Satellite 11.7-12.5 GHz Automatic Toll Collection 905 MHz 5-6 GHz Collision Avoidance Radar 60 GHz 94 GHz Wide Area Computer Networks 60 GHz 4. About the HFSS HFSS is a high-performance full-wave electromagnetic (EM) field simulator for arbitrary 3D volumetric passive device modeling that takes advantage of the familiar Microsoft Windows graphical user interface.

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