Microwave Millimetric AntennasAbstract— This paper provides a discussion on the different microwave antennas, including reflectors, refractors, radiators and hybrids. Their uses will be discussed together with their characteristics such as focus distance, gain and bandwidth. This paper will also provide an overview on multiple antenna techniques, MIMO, beam forming and diversity. Keywords— microwave millimetric antennas, reflector antenna, refractor antenna, radiator antenna, MIMO, beam forming I. INTRODUCTION With the continually increasing demand for bandwidth, and the development of components for higher and higher frequencies, millimeter-wave systems are finding numerous applications of a commercial nature rather than being limited to military and scientific applications only. Since active device performance deteriorates with increasing frequency, the performance of the antenna becomes critical as we go higher into the millimeter-wave band. [1] UHF and microwave antennas are the types of antenna that use this millimeter-wave band. Microwave transmission refers to the technology of transmitting information or energy by the use of electromagnetic waves whose wavelengths are conveniently measured in small numbers of centimetre; these are called microwaves. This part of the radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30 centimeters down to 1.0 cm. Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can. [2] II. DIFFERENT MICROWAVE ANTENNAS A) Reflectors An antenna reflector is a device that reflects electromagnetic waves. Antenna reflectors can exist as a standalone device for redirecting radio frequency (RF) energy, or can be integrated as part of an antenna assembly. The function of a standalone reflector is to redirect electro-magnetic (EM) energy, generally in the radio wavelength range of the electromagnetic spectrum. Reflector antennas are typically used when very high gain (e.g. satellite transmission or reception) or a very narrow main beam (e.g. secure communication) is required. Gain is improved and the main beam narrowed with increase in the reflector size. Large reflectors are however difficult to simulate as they become very large in terms of wavelengths. [3] Common reflectors include parabolic and cassegrain antenna. A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis. The parabolic reflector transforms an incoming plane wave traveling along the axis into a spherical wave converging toward the focus. Conversely, a spherical wave generated by a point source placed in the focus is reflected into a plane wave propagating as a collimated beam along the axis. [4] Figure 1. Parabolic Antenna Cassegrain antenna, on the other hand, is a parabolic antenna in which the feed antenna is mounted at or behind the surface of the concave main parabolic reflector dish and is aimed at a smaller convex secondary reflector suspended in front of the primary reflector. The beam of radio waves from the feed illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam. The Cassegrain design is widely used in parabolic antennas, particularly in large antennas such as those in satellite ground stations, radio telescopes, and communication satellites. [5] It is used in radar and metering equipment that operates in the centimeter wavelength range. and as directive antennas for such devices as radar guns. The shape of the lens depends on the refractive index n (the ratio of the phase velocity of propagation of a radio wave in a vacuum to that in the lens). Unlike lenses or curved mirrors however. directing them in a beam in one direction. something like a guitar's sound box. Axial choke horn antenna with lens A zone plate is a device used to focus light or other things exhibiting wave character. low standing wave ratio (SWR). The zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from an opaque disc. zone plates use diffraction instead of refraction or reflection. Based on analysis by Augustin-Jean Fresnel. strengthening the antenna's radiation in the desired direction. [6] Figure 3. Cassegrain antenna B) Refractors Refractors are antennas that causes refraction to the radiowaves. A zone plate consists of a set of radially symmetric rings. broad bandwidth. is one for which n > 1. The waves from the different antenna elements interfere. A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Light hitting the zone plate will diffract around the opaque zones. and parasitic elements. Zone Plate C) Radiators In a radio antenna. An accelerating lens antenna (without an optical analogy) is one for which n < 1.transparent. Their advantages are moderate directivity. absorbing the radio waves from the nearby driven element and re-radiating them again with a different phase. The feed is usually a horn antenna that generates a spherical wave front or an antenna array that produces a cylindrical wave front. which is not electrically connected to anything else. automatic door openers. Horns are widely used as antennas at UHF and microwave frequencies. which alternate between opaque and Figure 4. known as Fresnel zones. they are sometimes called Fresnel zone plates in his honor. as standard calibration antennas to measure the gain of other antennas. The zones can be spaced so that the diffracted light constructively interferes at the desired focus. which are not. [8] Common radiators include. above 300 MHz. A lens antenna consists of a lens proper and of a feed. and simple construction and adjustment. Multielement antennas such as the Yagi-Uda antenna typically consist of a "driven element" which is connected to the radio receiver or transmitter through a feed line. increasing the antenna's directivity (gain). a passive radiator or parasitic element is a conductive element. Lens antenna is an antenna whose directivity pattern is a result of the difference between the phase velocity of propagation of an electromagnetic wave in air and that in the lens material. as in optics. They are used as feeders (called feed horns) for larger antenna structures such as parabolic antennas. and cancelling out the waves in undesired directions. [7] Figure 2. A decelerating lens antenna. Common refractors include lens and Fresnel zone plate. A parasitic element does this by acting as a passive resonator. typically a metal rod. horn. creating an image there. . and microwave radiometers. cavity and slot. The purpose of the parasitic elements is to modify the radiation pattern of the radio waves emitted by the driven element. On one end. so the device is equivalent to a parabolic antenna fed off-axis. The advantage of this design over a standard parabolic antenna is that the horn shields the antenna from radiation coming from angles outside the main beam axis. Also. is the use of multiple antennas at both the transmitter and receiver to improve communication performance. [10] Figure 6. The exciting monopole antenna is shown in green. The walls are metallic (electrically conducting). Rectangular Slot antenna with dimensions a and b. Hogg at Bell labs in 1961. The slot size. multiple-input and multiple-output. the aperture isn't partially obstructed by the feed and its supports. The reflector is a segment of a parabolic reflector. a slot is cut out. so its radiation pattern has very small sidelobes. Multiple antennas may be used to perform smart antenna functions such as spreading the total transmit power over the antennas to achieve an array gain that incrementally improves the spectral efficiency (more bits per second per hertz of bandwidth. as with ordinary frontfed parabolic dishes. The disadvantage is that it is far larger and heavier for a given aperture area than a parabolic dish. or MIMO (pronounced as "my-moh" or "me-moh"). Beck and Harald T. It is also referred to as the "sugar scoop" due to its characteristic shape. The cavity is typically excited by a probe antenna in the interior of the cavity. which typically is modelled as a monopole antenna. The polarization of the slot antenna is linear. [9] D) Hybrids Hybrid antennas are antenna that combine different kind of antenna to form a new one. The basic cavity-backed slot antenna is shown in Figure 7 (in a rectangular cube of size A*B*C). Cavity-backed slot antenna In radio. Pyramidal microwave horn antenna Slot antennas are used typically at frequencies between 300 MHz and 24 GHz. invented by Alfred C. It consists of a horn antenna with a reflector mounted in the mouth of the horn at a 45 degree angle so the radiated beam is at right angles to the horn axis.) or achieving a diversity gain .Figure 5. and Dielguide. and the inside is hollow. allowing it to achieve aperture efficiencies of 70% as opposed to 55-60% for front-fed dishes. as we'll see). Friis in 1941 and further developed by David C. A type of antenna that combines a horn with a parabolic reflector is the Hogg or horn-reflector antenna. Holmdel hog-horn antenna III. MULTIPLE ANTENNA TECHNIQUES Figure 7. and the focus of the reflector is at the apex of the horn. [9] Figure 8. The slot antenna is popular because they can be cut out of whatever surface they are to be mounted on. and must be mounted on a cumbersome turntable to be fully steerable. Common hybrid antennas include Hogg Horn. It is usually made to improve the performance of an antenna especially its gain and directivity. shape and what is behind it (the cavity) offer design variables that can be used to tune performance. Cas-Horn. A practical slot antenna is the cavity-backed slot antenna. and have radiation patterns that are roughly omnidirectional (similar to a linear wire antenna. ) or both. and diversity coding. The scheduling of receivers with different spatial signatures allows good separability. In (singlestream) beamforming. 3GPP Long Term Evolution.11ac (Wi-Fi). However. and biomedicine. Beamforming can be used for radio or sound waves. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). precoding. and precoding with multiple streams is often beneficial.g. seismology. but the signal is coded using techniques called space-time coding. which are mainly characterized by multipath propagation.and to reduce the multipath fading effect. MIMO has been applied to power line communications for 3-wire installations as part of standard ITU G. the same signal is emitted from each of the transmit antennas with appropriate phase and gain weighting such that the signal power is maximized at the receiver input. Figure 9. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Adaptive beamforming is used to detect and estimate the signal-of-interest at the output of a sensor array by means of optimal (e. it is considered to be all spatial processing that occurs at the transmitter. sonar. known as space-division multiple access or multi-user MIMO. it can separate these streams into (almost) parallel channels. More recently. Because there is no channel knowledge. Note that precoding requires knowledge of channel state information (CSI) at the transmitter and the receiver. Example of an antenna for LTE with 2 ports antenna diversity Beamforming or spatial filtering is a signal processing technique used in sensor arrays for directional signal transmission or reception. In diversity methods. IEEE 802. there is no beamforming or array gain from diversity coding. It has found numerous applications in radar.by making signals emitted from different antennas add up constructively . Spatial multiplexing requires MIMO antenna configuration. Diversity coding can be combined with spatial multiplexing when some channel knowledge is available at the transmitter.hn and specification HomePlug AV2. but can be combined with precoding if CSI is available. [11] Diversity Coding techniques are used when there is no channel knowledge at the transmitter. The improvement compared with omnidirectional reception/transmission is known as the receive/transmit gain (or loss). a single stream (unlike multiple streams in spatial multiplexing) is transmitted. WiMAX and HSPA+. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.. wireless communications. This is achieved by combining elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference.11n (WiFi). In line-of-sight propagation. MIMO can be sub-divided into three main categories. beamforming results in a well-defined directional pattern. 4G. Spatial multiplexing can be used without CSI at the transmitter. However. Precoding is multi-stream beamforming. in the narrowest definition. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures and the receiver has accurate CSI. least-squares) spatial filtering and interference rejection. In more general terms. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers. acoustics. spatial multiplexing or SM. today the term “MIMO” usually refers to a method for multiplying the capacity of a radio link by exploiting multipath propagation.[1] This modern MIMO is an essential element of wireless communication standards such as IEEE 802. Beam Froming . The signal is emitted from each of the transmit antennas with full or near orthogonal coding. [12] Figure 10.that improves the link reliability (reduces fading. conventional beams are not a good analogy in cellular networks. The benefits of beamforming are to increase the received signal gain . a high-rate signal is split into multiple lower-rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. When the receiver has multiple antennas. radio astronomy. in which case CSI is required at the transmitter. In spatial multiplexing. The maximum number of spatial streams is limited by the lesser of the number of antennas at the transmitter or receiver. the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas.