INTRODUCTION: An antenna (or aerial) is an electrical device which couples radio waves in free space to an electrical current used by a radio receiver or transmitter. In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage that the radio receiver can amplify. Alternatively, a radio transmitter will produce a large radio frequency current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic wave (radio wave) radiated into free space. Antennas are thus essential to the operation of all radio equipment, both transmitters and receivers. They are used in systems such as radio and television broadcasting, two-way radio, wireless LAN, mobile telephony, radar, and satellite communications.

Typically an antenna consists of an arrangement of metallic conductors (or "elements") with an electrical connection (often through a transmission line) to the receiver or transmitter. A current forced through such a conductor by a radio transmitter will create an alternating magnetic field according to Ampère's law. Or the alternating magnetic field due to a distant radio transmitter will induce a voltage at the antenna terminals, according to Faraday's law, which is connected to the input of a receiver. In the so-called far field, at a considerable distance away from the antenna, the oscillating magnetic field is coupled with a similarly oscillating electric field; together these define an electromagnetic wave which is capable of propagating great distances.

Light is one example of electromagnetic radiation, along with infrared and x-rays, while radio waves differ only in their much lower frequency (and much longer wavelength). Electronic circuits can operate at these lower frequencies, processing radio signals conducted through wires. But it is only through antennas that those radio frequency electrical signals are converted to (and from) propagating radio waves. Depending on the design of the antenna, radio waves can be sent toward and received from all directions ("omnidirectional"), whereas a directional or beam antenna is designed to operate in a particular direction.

Current research on antenna involves metamaterials (materials that have engineered dielectric and magnetic constants, that can be simultaneously negative, allowing for interesting properties like a negative index of refraction). Current research focuses on making antennas smaller, particularly in communications for personal wireless communication devices (e.g. cell phones). A lot of work is being performed on numerical modeling of antennas, so that their properties can be predicted before they are built and tested.

HISTORICAL ASPECT: The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early

radio apparatuses in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless equipment. A 2.5 meter long

pole, along which was carried a wire, was used as a radiating and receiving aerial element. In Italian a tent pole is known as l'antenna centrale, and the pole with a

wire alongside it used as an aerial was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals. Marconi's use of the word antenna (Italian for pole) would become a

popular term for what today is uniformly known as the antenna.

The first antennas were built in 1888 by Heinrich Hertz (1857–1894) in his pioneering experiments to prove the existence of electromagnetic waves predicted by the theory of James Clerk Maxwell. Hertz placed dipole antennas at the focal point of parabolic reflectors for both transmitting and receiving. He published his work and installation drawings in Annalen der Physik und Chemie.

In the 1890s, there were only a few antennas in the world. These rudimentary devices were primarly a part of experiments that demonstrated the transmission of electromagnetic waves. By World War II, antennas had become so ubiquitous that their use had transformed the lives of the average person via radio and television reception. The number of antennas in the United States was on the order of one per household, representing growth rivaling the auto industry during the same period.

A rough outline of some major antennas and their discovery/fabrication dates are listed:

Yagi-Uda Antenna , 1920s Horn antennas, 1939. Interesting, the early antenna literature discussed waveguides as "hollow metal pipes". Antenna Arrays, 1940s Parabolic Reflectors, late 1940s, early 1950s? Patch Antennas, 1970s. PIFA, 1980s.

RADIO SPECTRUM: Radio spectrum refers to the part of the electromagnetic spectrum(range of all possible frequencies of electromagnetic

radiation) corresponding to radio frequencies – that is, frequencies lower than around 300 GHz (or, equivalently, wavelengths longer than about 1 mm).

Different parts of the radio spectrum are used for different radio transmission technologies and applications. Radio spectrum is typically government regulated in developed countries, and in some cases is sold or licensed to operators of private radio transmission systems (for example, cellular telephone operators or broadcast television stations). Ranges of allocated frequencies are often referred to by their provisioned use (for example, cellular spectrum or television spectrum)

IEEE US(SPECTRUM NOMENCLATURE)

Band Frequency range Origin of name[2]

HF band 3 to 30 MHz High FrequencyVHF band 30 to 300 MHz Very High FrequencyUHF band 300 to 1000 MHz Ultra High FrequencyL band 1 to 2 GHz Long waveS band 2 to 4 GHz Short waveC band 4 to 8 GHz Compromise between S and XX band 8 to 12 GHz Used in WW II for fire control, X for cross (as in crosshair)Ku band 12 to 18 GHz Kurz-underK band 18 to 27 GHz German Kurz (short)Ka band 27 to 40 GHz Kurz-aboveV band 40 to 75 GHzW band 75 to 110 GHz W follows V in the alphabetmm   band 110 to 300 GHz

OVERVIEW: Antennas are required by any radio receiver or transmitter in order to couple its electrical connection to the electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at almost the speed of light with almost no transmission loss.

According to their applications and technology available, antennas generally fall in one of two catagories:

1. Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions. These are employed when the relative position of the other station is unknown or arbitrary. They are also used at lower frequencies where a directional antenna would be too large, or simply to cut costs in applications where a directional antenna isn't required.

2. Directional or beam antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern.

An antenna lead-in is the transmission line (or feed line) which connects the antenna to a transmitter or receiver. The antenna feed may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called aperture antenna, such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system (normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in lieu of a (conductive) transmission line.An antenna counterpoise or ground plane is a structure of conductive material which improves or substitutes for the ground. It may be connected to or insulated from the natural ground.

BASIC ANTENNA TERMINOLOGY: There are several critical parameters affecting an antenna's performance that can be adjusted during the design processThese are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth..The brief description of each of these is as follows:

RESONANT FREQUENCY: Many types of antenna are tuned to work at one particular frequency, and are effective only over a range of frequencies centered on this frequency, called the resonant frequency. These are called resonant antennas. The antenna acts as an electrical resonator. When driven at its resonant frequency, large standing waves of voltage and current are excited in the antenna elements. These large currents and voltages radiate intense electromagnetic waves, so the power radiated by the antenna is maximum at the resonant frequency.

In antennas made of thin linear conductive elements, the length of the driven element(s) determines the resonant frequency. To be resonant, the length of a driven element should typically be either half or a quarter of the wavelength at that frequency; these are called half-wave and quarter-wave antennas. The length referred to is not the physical length, but the electrical length of the element, which is the physical length divided by the velocity factor (the ratio of the speed of wave propagation in the wire to c0, the speed of light in a vacuum). Antennas are usually also resonant at multiples (harmonics) of the lowest resonant frequency.

ANTENNA GAIN:The term Gain describes how much power is transmitted in the direction of peak radiation to that of an isotropic source. Gain is more commonly quoted in a real antenna's specification sheet because it takes into account the actual losses that occur.

A gain of 3 dB means that the power received far from the antenna will be 3 dB (twice as much) higher than what would be received from a lossless isotropic antenna with the same input power.

Gain is sometimes discussed as a function of angle, but when a single number is quoted the gain is the 'peak gain' over all directions. Gain (G) can be related to directivity (D) by:

[Equation 3]

The gain of a real antenna can be as high as 40-50 dB for very large dish antennas (although this is rare). Directivity can be as low as 1.76 dB for a real antenna, but can never theoretically be less than 0 dB. However, the peak gain of an antenna can be arbitrarily low because of losses or low efficiency. Electrically small antennas (small relative to the wavelength of the frequency that the antenna operates at) can be very inefficient, with gains lower than -10 dB (even without accounting for impedance mismatch loss).

ANTENNA EFFICIENCY:The efficiency of an antenna relates the power delivered to the antenna and the power radiated or dissipated within the antenna. A high efficiency antenna has most of the power present at the antenna's input radiated away. A low efficiency antenna has most of the power absorbed as losses within the antenna, or reflected away due to impedance mismatch.

The losses associated within an antenna are typically the conduction losses (due to finite conductivity of the antenna) and dielectric losses (due to conduction within a dielectric which may be present within an antenna).

The antenna efficiency (or radiation efficiency) can be written as the ratio of the radiated power to the input power of the antenna:

[Equation 1]

Efficiency is ultimately a ratio, giving a number between 0 and 1. Efficiency is very often quoted in terms of a percentage; for example, an efficiency of 0.5 is the same as 50%. Antenna efficiency is also frequently quoted in decibels (dB); an efficiency of 0.1 is 10% or (-10 dB), and an efficiency of 0.5 or 50% is -3 dB.

Equation [1] is sometimes referred to as the antenna's radiation efficiency. This distinguishes it from another sometimes-used term, called an antenna's "total efficiency". The total efficiency of an antenna is the radiation efficiency multiplied by the impedance mismatch loss of the antenna, when connected to a transmission line or receiver (radio or transmitter). This can be summarized in Equation [2],

where is the antenna's total efficiency, is the antenna's loss due to

impedance mismatch, and is the antenna's radiation efficiency.

[Equation 2]

Since is always a number between 0 and 1, the total antenna efficiency is always less than the antenna's radiation efficiency. Said another way, the radiation efficiency is the same as the total antenna efficiency if there was no loss due to impedance mismatch.

Efficiency is one of the most important antenna parameters. It can be very close to 100% (or 0 dB) for dish, horn antennas, or half-wavelength dipoles with no lossy materials around them. Mobile phone antennas, or wifi antennas in consumer electronics products, typically have efficiencies from 20%-70% (-7 to -1.5 dB). The losses are often due to the electronics and materials that surround the antennas; these tend to absorb some of the radiated power (converting the energy to heat), which lowers the efficiency of the antenna. Car radio antennas can have a total antenna efficiency of -20 dB (1% efficiency) at the AM radio frequencies; this is because the antennas are much smaller than a half-wavelength at the operational frequency, which greatly lowers antenna efficiency. The radio link is maintained because the AM Broadcast tower uses a very high transmit power.

Improving impedance mismatch loss is discussed in the Smith Charts and impedance matching section. Impedance matching can greatly improve the efficiency of an antenna.

IMPEDANCE: An antenna's impedance relates the voltage to the current at the input to the antenna

BANDWIDTH: Bandwidth is another fundamental antenna parameter. This describes the range of frequencies over which the antenna can properly radiate or receive energy. Often, the desired bandwidth is one of the determining parameters used to decide upon an antenna. For instance, many antenna types have very narrow bandwidths and cannot be used for wideband operation.

Bandwidth is typically quoted in terms of VSWR. For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, the return loss S11=20*log10(0.2)=-13.98 dB.

Note that the above does not imply that 96% of the power delivered to the antenna is transmitted in the form of EM radiation; losses must still be taken into account.

Also, the radiation pattern will vary with frequency. In general, the shape of the radiation pattern does not change radically.

There are also other criteria which may be used to characterize bandwidth. This may be the polarization over a certain range, for instance, an antenna may be described as having circular polarization with an axial ratio <3dB from 1.4-1.6 GHz. This polarization bandwidth sets the range over which the antenna's operation is roughly circular.

The bandwidth is often specified in terms of its Fractional Bandwidth (FBW). The antenna Q also relates to bandwidth.

DIRECTIVITY: Directivity is a fundamental antenna parameter. It is a measure of how 'directional' an antenna's radiation pattern is. An antenna that radiates equally in all directions would have effectively zero directionality, and the directivity of this type of antenna would be 1 (or 0 dB).

An antenna's normalized radiation pattern can be written as a function in spherical coordinates:

[Equation 1]

A normalized radiation pattern is the same as a radiation pattern, just scaled in magnitude such that the peak (maximum value) of the magnitude of the radiation pattern (F in equation [1]) is equal to 1. Mathematically, the formula for directivity (D) is written as:

This equation for directivity might look complicated, but the numerator is the maximum value of F, and the denominator just represents the "average power radiated over all directions". This equation then is just a measure of the peak value of radiated power divided by the average, which gives the directivity of the antenna.

The general rule in Antenna Theory is that you need an electrically small antenna to produce low directivity.

Radiation Resistance VS Antenna Resistance:

The terms 'radiation resistance' and 'antenna resistance' are sometimes used interchangeably, but they shouldn't be, because they have distinctly different meanings.

• Radiation resistance is equal to the power radiated by an antenna divided by the square of the rms (effective) antenna current at a specified point in an antenna. That point usually is the point where RF power is supplied.

• In contrast, Antenna Resistance is equal to the power supplied to an entire antenna circuit divided by the square of the rms (effective) antenna current at a specified point. The difference between the power-values used in the two calculations is the loss power, because the antenna resistance calculation includes not only radiated power, but also power lost in RF conductor resistance, eddy current loss, insulator leakage loss, dielectric loss, corona loss, ground resistance loss, and any other power loss.

TYPES OF ANTENNAS:

1.DIPOLE ANTENNA:

The short dipole antenna is the simplest of all antennas. It is simply an open-circuited wire, fed at its center as shown in Figure 1.

Figure 1. Short dipole antenna of length L.

The words "short" or "small" in antenna engineering always imply "relative to a wavelength". So the absolute size of the above dipole antenna does not matter, only

the size of the wire relative to the wavelength of the frequency of operation. Typically, a dipole is short if its length is less than a tenth of a wavelength:

If the short dipole antenna is oriented along the z-axis with the center of the dipole at z=0, then the current distribution on a thin, short dipole is given by:

The current distribution is plotted in Figure 2. Note that this is the amplitude of the current distribution; it is oscillating in time sinusoidally at frequency f.

Figure 2. Current distribution along a short dipole antenna.

The fields radiated from the short dipole antenna in the far field are given by:

The above equations can be broken down and understood somewhat intuitively.

First, note that in the far-field, only the and fields are nonzero. Further,

these fields are orthogonal and in-phase. Further, the fields are perpendicular to the

direction of propagation, which is always in the direction (away from the antenna).

Also, the ratio of the E-field to the H-field is given by (the intrinsic impedance of free space). This indicates that in the far-field region the fields are propagating like a plane-wave.

Second, the fields die off as 1/r, which indicates the power falls of as

Third, the fields are proportional to L, indicated a longer dipole will radiate more power. This is true as long as increasing the length does not cause the short dipole assumption to become invalid. Also, the fields are proportional to the current

amplitude , which should make sense (more current, more power).

The exponential term:

describes the phase-variation of the wave versus distance. Note also that the fields are oscillating in time at a frequency f in addition to the above spatial variation.

Finally, the spatial variation of the fields as a function of direction from the antenna

are given by . For a vertical antenna oriented along the z-axis, the radiation will be maximum in the x-y plane. Theoretically, there is no radiation along the z-axis far from the antenna.

In the next section further properties of the short dipole will be discussed.

Directivity, Impedance and other Properties of the Short Dipole Antenna

The directivity of the center-fed short dipole antenna depends only on the component of the fields. It can be calculated to be 1.5 (1.76 dB), which is very low for realizable (physical or non-theoretical) antennas. Since the fields of the short dipole antenna are only a function of the polar angle, they have no azimuthal variation and hence this antenna is characterized as omnidirectional. The Half-Power Beamwidth is 90 degrees.

The polarization of this antenna is linear. When evaluated in the x-y plane, this antenna would be described as vertically polarized, because the E-field would be vertically oriented (along the z-axis).

We now turn to the input impedance of the short dipole, which depends on the radius a of the dipole. Recall that the impedance Z is made up of three components, the radiation resistance, the loss resistance, and the reactive (imaginary) component which represents stored energy in the fields:

The radiation resistance can be calculated to be:

The resistance representing loss due to the finite-conductivity of the antenna is given by:

In the above equation represents the conductivity of the dipole (usually very high, if made of metal). The frequency f come into the above equation because of the skin effect. The reactance or imaginary part of the impedance of a dipole is roughly equal to:

As an example, assume that the radius is 0.001 and the length is 0.05 . Suppose further that this antenna is to operate at f=3 MHz, and that the metal is copper, so that the conductivity is 59,600,000 S/m.

The radiation resistance is calculated to be 0.49 Ohms. The loss resistance is found to be 4.83 mOhms (milli-Ohms), which is approximatley negligible when compared to the radiation resistance. However, the reactance is 1695 Ohms, so that the input

resistance is Z=0.49 + j1695. Hence, this antenna would be very difficult to have proper impedance matching. Even if the reactance could be properly cancelled out, very little power would be delivered from a 50 Ohm source to a 0.49 Ohm load.

For short dipole antennas that are smaller fractions of a wavelength, the radiation resistance becomes smaller than the loss resistance, and consequently this antenna can be very inefficient.

The bandwidth for short dipoles is difficult to define. The input impedance varies wildly with frequency because of the reactance component of the input impedance. Hence, these antennas are typically used in narrowband applications.

2.LOOP ANTENNAS:

The small loop antenna is a closed loop as shown in Figure 1. These antennas have low radiation resistance and high reactance, so that their impedance is difficult to match to a transmitter. As a result, these antennas are most often used as receive antennas, where impedance mismatch loss can be tolerated.

The radius is a, and is assumed to be much smaller than a wavelength (a<< ). The loop lies in the x-y plane.

Figure 1. Small loop antenna.

Since the loop is electrically small, the current within the loop can be approximated as

being constant along the loop, so that I= .

The fields from a small circular loop are given by:

The variation of the pattern with direction is given by , so that the radiation pattern of a small loop antenna has the same power pattern as that of a short dipole. However, the fields of a small dipole have the E- and H- fields switched relative to that of a short dipole; the E-field is horizontally polarized in the x-y plane.

The small loop is often referred to as the dual of the dipole antenna, because if a small dipole had magnetic current flowing (as opposed to electric current as in a regular dipole), the fields would resemble that of a small loop.

While the short dipole has a capacitive impedance (imaginary part of impedance is negative), the impedance of a small loop is inductive (positive imaginary part). The radiation resistance (and ohmic loss resistance) can be increased by adding more turns to the loop. If there are N turns of a small loop antenna, each with a surface area S (we don't require the loop to be circular at this point), the radiation resistance for small loops can be approximated (in Ohms) by:

For a small loop, the reactive component of the impedance can be determined by finding the inductance of the loop, which depends on its shape (then X=2*pi*f*L). For a circular loop with radius a and wire radius p, the reactive component of the impedance is given by:

Small loops often have a low radiation resistance and a highly inductive component to their reactance. Hence, they are most often used as receive antennas. Exaples of their use include in pagers, and as field strength probes used in wireless measurements.

YAGI-UDA ANTENNAS:

INTRODUCTION:The Yagi-Uda antenna or Yagi Antenna is one of the most brilliant antenna designs. It is simple to construct and has a high gain, typically greater than 10 dB. The Yagi-Uda antennas typically operate in the HF to UHF bands (about 3 MHz to 3 GHz), although their bandwidth is typically small, on the order of a few percent of the center frequency. You are probably familiar with this antenna, as they sit on top of roofs everywhere.

The Yagi antenna was invented in Japan, with results first published in 1926. The work was originally done by Shintaro Uda, but published in Japanese. The work was presented for the first time in English by Yagi (who was either Uda's professor or colleague, my sources are conflicting), who went to America and gave the first English talks on the antenna, which led to its widespread use. Hence, even though the antenna is often called a Yagi antenna,

GEOMETRY: The basic geometry of a Yagi-Uda antenna is shown below in Figure 1.

Figure 1. Geometry of Yagi-Uda antenna.

The Yagi antenna consists of a single 'feed' or 'driven' element, typically a dipole or a folded dipole antenna. This is the only member of the above structure that is actually excited (a source voltage or current applied). The rest of the elements are parasitic - they reflect or help to transmit the energy in a particular direction. The length of the feed element is given in Figure 1 as F. The feed antenna is almost always the second from the end, as shown in Figure 1. This feed antenna is often altered in size to make it resonant in the presence of the parasitic elements (typically, 0.45-0.48 wavelengths long for a dipole antenna).

The element to the left of the feed element in Figure 1 is the reflector. The length of this element is given as R and the distance between the feed and the reflector is SR. The reflector element is typically slightly longer than the feed element. There is typically only one reflector; adding more reflectors improves performance very slightly. This element is important in determining the front-to-back ratio of the antenna.

Having the reflector slightly longer than resonant serves two purposes. The first is that the larger the element is, the better of a physical reflector it becomes.

Secondly, if the reflector is longer than its resonant length, the impedance of the reflector will be inductive. Hence, the current on the reflector lags the voltage induced on the reflector. The director elements (those to the right of the feed in Figure 1) will be shorter than resonant, making them capacitive, so that the current leads the voltage. This will cause a phase distribution to occur across the elements, simulating the phase progression of a plane wave across the array of elements. This leads to the array being designated as a travelling wave antenna. By choosing the lengths in this manner, the Yagi-Uda antenna becomes an end-fire array - the radiation is along the +y-axis as shown in Figure 1.

The rest of the elements (those to the right of the feed antenna as shown in Figure 1) are known as director elements. There can be any number of directors N, which is typically anywhere from N=1 to N=20 directors. Each element is of length Di, and separated from the adjacent director by a length SDi. As alluded to in the previous paragraph, the lengths of the directors are typically less than the resonant length, which encourages wave propagation in the direction of the directors.

The above description is the basic idea of what is going on with the Yagi-Uda antenna. Yagi antenna design is done most often via measurements, and sometimes computer simulations. For instance, let's look at a two-element Yagi antenna (1 reflector, 1 feed element, 0 directors). The feed element is a half-wavelength

dipole, shortened to be resonant (gain = 2.15 dB). The gain as a function of the separation is shown in Figure 2.

Figure 2. Gain versus separation for 2-element Yagi antenna.

The above graph shows that the gain is increased by about 2.5 dB if the separation SD is between 0.15 and 0.3 wavelengths. Similarly, the gain for this Yagi antenna can be plotted as a function of director spacings, or as a function of the number of directors used. Typically, the first director will add approximately 3 dB of overall gain (if designed well), the second will add about 2 dB, the third about 1.5 dB. Adding an additional director always increases the gain; however, the gain in directivity decreases as the number of elements gets larger. For instance, if there are 8 directors, and another director is added, the increases in gain will be less than 0.5 dB.

Yagi-antenna radiation patterns, a 6-element Yagi antenna (with axis along the +x-axis) is simulated in FEKO (1 reflector, 1 driven half-wavelength dipole, 4 directors). The resulting antenna has a 12.1 dBi gain, and the plots are:

Figure 1. E-plane gain of Yagi antenna.

Figure 2. H-Plane gain of Yagi-Uda antenna.

The above plots are just an example to give an idea of what the radiation pattern of the Yagi-Uda antenna resembles. The gain can be increased (and the pattern made more directional) by adding more directors or optimizing spacing (or rarely, adding another refelctor). The front-to-back ratio is approximately 19 dB for this antenna, and this can also be optimized if desired.

REFLECTOR ANTENNAS: The most well-known reflector antenna is the parabolic reflector antenna, commonly known as a satellite dish antenna.

Parabolic reflectors typically have a very high gain (30-40 dB is common) and low cross polarization. They also have a reasonable bandwidth, with the fractional bandwidth being at least 5% on commercially available models, and can be very wideband in the case of huge dishes (like the Stanford "big dish" above, which can operate from 150 MHz to 1.5 GHz).

The smaller dish antennas typically operate somewhere between 2 and 28 GHz. The large dishes can operate in the VHF region (30-300 MHz), but typically need to be extremely large at this operating band.

The basic structure of a parabolic dish antenna is shown in Figure 3. It consists of a feed antenna pointed towards a parabolic reflector. The feed antenna is often a horn antenna with a circular aperture.

Figure 3. Components of a dish antenna.

Unlike resonant antennas like the dipole antenna which are typically approximately a half-wavelength long at the frequency of operation, the reflecting dish must be much larger than a wavelength in size. The dish is at least several wavelengths in diameter, but the diameter can be on the order of 100 wavelengths for very high gain dishes (>50 dB gain). The distance between the feed antenna and the reflector is typically several wavelenghts as well. This is in contrast to the corner reflector, where the antenna is roughly a half-wavelength from the reflector.

The fields across the aperture of the parabolic reflector is responsible for this antenna's radiation. The maximum possible gain of the antenna can be expressed in terms of the physical area of the aperture:

The actual gain is in terms of the effective aperture, which is related to the physical area by the efficiency term ( ). This efficiency term will often be on the order of 0.6-0.7 for a well designed dish antenna:

Understanding this efficiency will also aid in understanding the trade-offs involved in the design of a parabolic reflector. The efficiency can be written as the product of a series of terms:

In this section, the 3d radiation patterns are presented to give an idea of what they look like. This example will be for a parabolic dish reflector with the diameter of the dish D equal to 11 wavelengths. The F/D ratio will be 0.5. A circular horn antenna will be used as the feed.

The maximum gain from the physical aperture is ; the actual gain is 29.3 dB = 851, so we can conclude that the overall efficiency is 77%. The 3D patterns are shown in the following figures.

MICROSTRIP ANTENNAS: Microstrip or patch antennas are becoming increasingly useful because they can be printed directly onto a circuit board. Microstrip antennas are becoming very widespread within the mobile phone market. Patch antennas are low cost, have a low profile and are easily fabricated.

Consider the microstrip antenna shown in Figure 1, fed by a microstrip transmission line. The patch antenna, microstrip transmission line and ground plane are made of high conductivity metal (typically copper). The patch is of length L, width W, and sitting on top of a substrate (some dielectric circuit board) of thickness h with permittivity. The thickness of the ground plane or of the microstrip is not critically important. Typically the height h is much smaller than the wavelength of operation, but not much smaller than 0.05 of a wavelength.

The frequency of operation of the patch antenna of Figure 1 is determined by the length L. The center frequency will be approximately given by:

The above equation says that the microstrip antenna should have a length equal to one half of a wavelength within the dielectric (substrate) medium.

The width W of the microstrip antenna controls the input impedance. Larger widths also can increase the bandwidth. For a square patch antenna fed in the manner above, the input impedance will be on the order of 300 Ohms. By increasing the width, the impedance can be reduced. However, to decrease the input impedance to 50 Ohms often requires a very wide patch antenna, which takes up a lot of valuable space. The width further controls the radiation pattern. The normalized radiation pattern is approximately given by:

In the above, k is the free-space wavenumber, given by . The magnitude of the fields, given by:

The fields of the microstrip antenna are plotted in Figure 2 for W=L=0.5 .

Figure 2. Normalized Radiation Pattern for Microstrip (Patch) Antenna.

The directivity of patch antennas is approximately 5-7 dB. The fields are linearly polarized, and in the horizontal direction when viewing the microstrip antenna as in Figure 1a (we'll see why in the next section). Next we'll consider more aspects involved in Patch (Microstrip) antennas.

METHOD OF FEEDING ANTENNAS:

Inset Feed

Previously, the patch antenna was fed at the end as shown here. Since this typically yields a high input impedance, we would like to modify the feed. Since the current is

low at the ends of a half-wave patch and increases in magnitude toward the center, the input impedance (Z=V/I) could be reduced if the patch was fed closer to the center. One method of doing this is by using an inset feed (a distance R from the end) as shown in Figure 1.

Figure 1. Patch Antenna with an Inset Feed.

Since the current has a sinusoidal distribution, moving in a distance R from the end will increase the current by cos(pi*R/L) - this is just noting that the wavelength is 2*L, and so the phase difference is 2*pi*R/(2*L) = pi*R/L.

The voltage also decreases in magnitude by the same amount that the current increases. Hence, using Z=V/I, the input impedance scales as:

In the above equation, Zin(0) is the input impedance if the patch was fed at the end. Hence, by feeding the patch antenna as shown, the input impedance can be decreased. As an example, if R=L/4, then cos(pi*R/L) = cos(pi/4), so that [cos(pi/4)]^2 = 1/2. Hence, a (1/8)-wavelength inset would decrease the input impedance by 50%. This method can be used to tune the input impedance to the desired value.

Fed with a Quarter-Wavelength Transmission Line

The microstrip antenna can also be matched to a transmission line of characteristic impedance Z0 by using a quarter-wavelength transmission line of characteristic impedance Z1 as shown in Figure 2.

Figure 2. Patch antenna with a quarter-wavelength matching section.

The goal is to match the input impedance (Zin) to the transmission line (Z0). If the impedance of the antenna is ZA, then the input impedance viewed from the beginning of the quarter-wavelength line becomes

This input impedance Zin can be altered by selection of the Z1, so that Zin=Z0 and the antenna is impedance matched. The parameter Z1 can be altered by changing the width of the quarter-wavelength strip. The wider the strip is, the lower the characteristic impedance (Z0) is for that section of line.

Coaxial Cable or Probe Feed

Microstrip antennas can also be fed from underneath via a probe as shown in Figure 3. The outer conductor of the coaxial cable is connected to the ground plane, and the center conductor is extended up to the patch antenna.

Figure 3. Coaxial cable feed of patch antenna.

The position of the feed can be altered as before (in the same way as the inset feed, above) to control the input impedance.

The coaxial feed introduces an inductance into the feed that may need to be taken into account if the height h gets large (an appreciable fraction of a wavelength). In addition, the probe will also radiate, which can lead to radiation in undesirable directions.

Coupled (Indirect) Feeds

The feeds above can be altered such that they do not directly touch the antenna. For instance, the probe feed in Figure 3 can be trimmed such that it does not extend all the way up to the antenna. The inset feed can also be stopped just before the patch antenna, as shown in Figure 4.

Figure 4. Coupled (indirect) inset feed.

The advantage of the coupled feed is that it adds an extra degree of freedom to the design. The gap introduces a capacitance into the feed that can cancel out the inductance added by the probe feed.

Aperture Feeds

Another method of feeding microstrip antennas is the aperture feed. In this technique, the feed circuitry (transmission line) is shielded from the antenna by a conducting plane with a hole (aperture) to transmit energy to the antenna, as shown in Figure 5.

Figure 5. Aperture coupled feed.

The upper substrate can be made with a lower permittivity to produce loosely bound fringing fields, yielding better radiation. The lower substrate can be independently made with a high value of permittivity for tightly coupled fields that don't produce spurious radiation. The disadvantage of this method is increased difficulty in fabrication