What is an antenna？

Antennas have been widely used since the turn of the last century. Ever since, this field has undergone extensive research, resulting in a wide body of experimental and theoretical knowledge alongside numerous designs and applications.

The earliest antenna was introduced in the late 19th century by the German physicist Heinrich Hertz. Hertz’s work was followed by a great theoretical investigation of the subject during the early to mid 20th century. This investigation proceeded with the development of computer aided design (CAD) tools during the 1970s-2000s, made possible by the development of powerful yet affordable computer technology.

Antenna applications are vast and diverse. These include: television and radio broadcasting, RADAR, wireless computer communication, Bluetooth enabled devices, military personal communication, satellite communication, cell phones, RFID tags and much more.

This paper intends to cover the basic concepts behind antennas’ operation and performance. The paper’s main aims are to inform the reader on both, the physical mechanisms governing antenna’s operation, and the various parameters comprising antenna specifications. Proper appreciation of these concepts will ensure a suitable and informed choice of products for all potential applications.

The paper begins with a short physical introduction to the subject followed by a more comprehensive and detailed review of the various antenna parameters.

The rigorous treatment of this subject requires an extensive mathematical background, and is beyond the scope of this paper. When writing this paper, mathematical complexity was avoided in favor of a more straight forward approach. Generally speaking, things were kept as simple as possible while assuring no loss of validity.

This paper addresses all professional audiences related to this field, including marketing personnel, system engineers, managers, designers and all potential users. The paper was composed such that no special professional training nor past knowledge of the subject is required.

It is both my wish and intention that this paper will be as comprehensive and informative as possible. I wish you an enjoyable reading that will hopefully provide you some insight into this fascinating subject.

PHYSICAL INTRODUCTION

An Antenna is an electrical device designated to radiate or capture electromagnetic (EM) waves. In order to properly appreciate this definition, and the physical operation of antennas as a whole, we will have to familiarize the reader with some basic electromagnetic concepts.
The physical laws governing all classical electromagnetic phenomena are Maxwell’s equations. First introduced by the Scottish scientist James Clark Maxwell, in his famous article: "A Dynamical Theory of the Electromagnetic Field“, in 1864. These four equations provide us with an almost complete mathematical description of the way electric and magnetic fields are generated and altered by each other, as well as by charges and currents.

The electric and magnetic fields are represented as vectors, having both magnitude (strength) and orientation (direction). The fields vary in magnitude and orientation depending on both the location and time, at which they are measured.

Maxwell’s equations imply that the sources of all EM fields are electric charges and currents. As one may expect, different charge or current distributions give rise to different EM fields.

One particular case of interest is that of an accelerating electric charge. The acceleration of  an electric charge produces an EM field which propagates in a wavelike manner, referred to as an  EM Wave. EM waves propagate at the speed of light and in an outward direction with respect to their origin. The process mentioned above is referred to as EM Radiation.

It is therefore clear, that in order to produce EM radiation, we must introduce a device capable of holding an alternating electric current. This device is known as an antenna.

A. The Transmission Mode and The Reception Mode

By definition, an antenna can be used in one of two operation modes. These are known as the transmission mode and the reception mode (Tx mode and Rx mode). When operating in the transmission mode, an oscillating RF signal incidents upon the antenna input terminals. This signal is then converted into an alternating electric current, which in turn radiates en EM wave. This EM wave can be then captured by other antennas. In the reception mode, an EM wave incident upon the antenna induces an electric current on its input terminals which can then be converted back into a RF signal. The device operation in these two modes is completely equivalent. This property is known as reciprocity.

Antenna designs are vast and diverse, depending on the desired application. It is therefore clear, that we must establish means for the quantitative description of an antennas’ performance. This of course, requires the definition of clear mathematical quantities  (antenna parameters) dedicated to that purpose. These will be introduced and discussed next.


B. Field Regions

The EM fields generated by an antenna display different characteristics depending on the distance from the antenna at which they are measured. It is customary to divide the space surrounding the antenna into three zones, in which the EM fields posses different distinguishable properties.

In the immediate vicinity of the antenna, the fields are purely reactive. This indicates that EM energy is completely stored. This region is referred to as the Reactive Near-Field. Mathematically speaking, the electric and magnetic fields are out of phase, similar to the voltage and current on reactive lumped elements in an AC circuit (such as a capacitor or an inductor).

As the distance from the antenna increases, the EM fields become less reactive, i.e. a portion of the EM energy is converted into radiation. This region is referred to as the Radiating Near-Field.

Sufficiently far from the antenna, the reactive fields become negligible and the radiating fields dominate. This region is known as the Far-Field. Furthermore, the electric and magnetic fields in this region are perpendicular, in-phase, and the ratio between their magnitudes become constant (locally plane waves).

The radiated fields vary in magnitude, depending on both the direction of observation and the distance from the antenna. Nevertheless, the fields’ general pattern remains the same in the far-field, regardless of the distance from the antenna.

This does not imply that the fields are independent of the distance from the antenna, but rather that they decay uniformly in all directions. In a more precise manner, the magnitude of the radiated fields decays proportionally to one over the distance from the antenna, in the far field.

Both the antenna size and wavelength are required for numerically determining the boundary between the different regions. These are denoted below, and illustrated in figure 1.

Where  r  is the distance from the antenna,  D  is the maximum antenna dimension, and  λ   is the wavelength.

A. Radiation Intensity

First, we shall introduce one important figure of merit describing antennas radiation properties, and from which other antenna parameters are derived - the Radiation Intensity.

The EM wave radiated by the antenna carries EM power. The radiated power varies in magnitude, depending on both the direction of observation and the distance from the antenna. As mentioned earlier, the EM power’s general pattern is maintained in the far-field, regardless of the distance from the antenna. Therefore, we may introduce a normalized EM power density that will be independent of the distance from the antenna in the far-field. This is known as the radiation intensity.

The radiation intensity is a mathematical description of the angular radiated power distribution in the far field (for a given polarization). Or in simpler terms - how much power is radiated by the antenna in a certain direction in the far field (using proper normalization with respect to the distance from the antenna).

In order to mathematically describe the radiation intensity, we have to define a way for representing directions. We will associate two angles with each direction that uniquely define it - an azimuth angle denoted by φ, and an elevation angle denoted by θ. The elevation angle is used to describe the antenna tilt relative to the horizon while the azimuth angle is used to describe the antenna traverse in a zero tilt state. A graphical illustration of these angles is demonstrated in figure 2.

B. Radiation Patterns

As mentioned, the radiation intensity is a function of two variables: the azimuth and elevation angles.It is sufficient in many practical cases to consider only two 2D cuts of this 3D graph in order to properly describe the antenna radiation properties. The two cuts are made along two perpendicular planes, called the Principal Planes, as demonstrated in figure 4.The cutting procedure leaves us with two 2D graphs of the antenna radiation pattern. these graphs are denoted by:

In one of the principal planes, the azimuthangle is fixed and elevationangle varies. This is referredto as the elevationplane. In the other plane, the elevationangle is fixed and azimuthangle varies. This is referred to as the azimuthplane.The cutting procedure results in a significant reduction of antenna measurement time since only two 2D cuts are needed to be measured instead of many.A typical directional antenna RP is presented in figure 5. As one may observe, the radiation pattern is comprised out of lobes. These lobes are classified as follows:The lobe containing the direction of maximum radiation is referred to as the Major Lobe, or the Main Beam. All other lobes are referred to as Minor Lobes.The main beam often represents the angular sector wherein the majority of the radiated power is intended to lay. The minor lobes therefore represent radiation in undesired directions, and should be kept as low as possible.The minor lobes are classified as well. The highest minor lobe is referred to as the Side Lobe. The side lobe is often adjacent to the main lobe, as illustrated in figure 5. The minor lobe containing the direction opposite to that of the main beam is referred to as the Back Lobe.The RP is usually plotted in logarithmic scale (decibels). This is done in order to sharpen the more subtle characteristics of the graph.

C. Beam Width

Another important parameter used to describe the angular width of the main beam is the antenna Beam Width. The extent of this angular sector determines the coverage region of the antenna. The beam width can be defined in several ways: Half Power Beam Width (HPBW) is defined as the angular difference between the points where the radiation intensity reaches half of its maximal value (3 dB difference in decibels). First Null Beam Width (FNBW) is defined as the angular difference between the two nulls enclosing the main beam.

D. Side Lobe Level

The Side Lobe Level (SLL) is a parameter used to describe the level of side lobe suppression. As previously mentioned, high side lobes are often not desired, since they represent radiation outside the main beam sector. The side lobe level is defined as the difference in decibels between the main beam peak value and the side lobe peak value.


 E. Front to Back Ratio

The Front to Back Ratio (F/B Ratio) is a parameter designated to describe the extent of backward radiation. That is to say, the radiation in the direction opposite to that of the main beam. The F/B ratio is defined as the difference in decibels between the value of the radiation pattern in the direction of maximum radiation (front direction) and the value of the radiation pattern in the opposite direction (back direction).


F. Radiation Patterns Types

Radiation patterns may be classified into three main categories:
 1.Directional Radiation Pattern : A pattern containing one clear main beam in both azimuth and elevation planes.
2.Isotropic Radiation Pattern : Constant pattern in both azimuth and elevation planes.
3.Omni Directional Radiation Pattern : A pattern containing one clear main beam at only one plane and a constant pattern in the other.
 The physical meaning of an Isotropic Antenna is that the antenna radiates equally in all directions. This type of antenna is not physically realizable, but is a convenient mathematical reference antenna.

G. Directivity

An antenna’s Directivity is defined as the ratio between the radiation intensity and the total radiated power by the antenna, divided by 4 pi.

In a more physical insightful manner, it can be alternatively defined as : The ratio between the radiation intensity of the antenna and the radiation intensity, assuming we spread all the of the radiated power isotropically. In directions wherein the directivity is low valued, the radiated power represents a small portion of the total radiated power. Similarly, in directions wherein the directivity is high valued, the radiated power represents a significant portion of the total radiated power.

The general idea behind this particular definition is to compare the antenna to a hypothetical source which radiates power equally in all directions (isotropic source). It then follows that the directivity of an isotropic equals unity.

As stated above, the directivity is proportional to the radiation intensity, and as the later is a function of both the azimuth and elevation angles. If the direction is not stated it should be understood that the direction of maximum radiation is implied.

The directivity is often measured in logarithmic scale (dBi isotropic decibels). A directional antenna’s directivity graph is given in figure 6. The graph corresponds to one of the antenna’s principal planes. An equivalent isotropic source’s directivity is also plotted for comparison.

H. Efficiency

In reality, not all of the EM power delivered to the antenna is converted into radiation, i.e.

 
There are several inherent loss mechanisms responsible for the dissipation of the incident power. These include: dielectric losses, conduction losses, and reflection losses.
Conductor losses and dielectric losses are caused due to the finite conductivity of the antenna’s conductors and dielectrics. This means that some power is always dissipated as heat on those materials. Reflection losses are caused due to an impedance mismatch between the antenna and its driving transmission line. This would be discussed later in more detail.The antenna Efficiency is defined as the ratio, in percent, between the radiated power and the incident power:

 

It is clear that the radiated power must be smaller than the incident power, since part of the later is always dissipated or reflected. Therefore, the efficiency will be less than 100%. An efficient antenna will radiate the majority of the incident power upon it, so its efficiency will approach 100% (minor dissipations and reflections). The antenna efficiency can be further represented as a multiplication of three sub-efficiencies, each accounts for different loss mechanism. This is denoted below and illustrated in figure 7.

I. Gain

The antenna’s directivity does not provide us with any information about the antenna’s efficiency, but merely on its radiation pattern’s directive properties. This is the main reason for introducing a new concept called antenna Gain. The antenna Gain is defined as:

As one may observe, the definition is similar to that of directivity, but rather then considering the radiated power, the input power is considered. The antenna gain takes into account the antenna efficiency since it is a measure of how much power the antenna radiates in a certain direction, relative to how much power was incident upon the antenna.Antenna’s directivity and gain relate via：

In order to the fully appreciate the meaning of this concept, it may helpful to think of the antenna as an input/output (I/O) system. In the discussed system, the input is represented by the antenna’s input power and the output is represented by the radiated power in a certain direction (which is available for reception by other antennas). The system’s output is nothing but its input multiplied by some constant number. This constant number is proportional to the antenna gain. In that sense, the term gain fits with the terminology used for amplifiers or attenuators.


J. Input Impedance and VSWR

Another eminent parameter describing antennas is their input impedance, i.e. the ratio between the voltage and the current at their terminals. EM power is delivered to an antenna via a transmission line or a waveguide - devices used to guide EM waves from the transmitter to the antenna. In this process EM waves can be attenuated or reflected. In order to avoid reflections of EM waves back to the transmitter, the antenna input impedance should match that of the driving transmission line (usually 50 ohm).

Nevertheless, the antenna’s input impedance varies with frequency, and could not be equal to that of the transmission line at all frequency points. This indicates that some reflections are unavoidable. The Voltage Standing Wave Ratio (VSWR) is a measure for how much power is reflected. A low valued VSWR indicates that the majority of the incident power is delivered to the antenna and reflections are nearly avoided.


K. Polarization

The polarization of an antenna is defined as the polarization of the EM wave it radiates in the far field. The EM wave radiated by the antenna is a mixture of electric and magnetic fields. If we were to track the curve traced by the tip of the electric field vector, in some fixed location in space, we would get, as time varies, a curve referred to as the polarization ellipse . Note, that for each fixed location we would generally get different curves, that is to say : the antenna polarization is dependent upon the direction of observation. The curve is referred to as the polarization ellipse, since it forms an ellipse for an arbitrarily polarized antenna.

Polarization may be classified as linear, circular or elliptical depending on the properties of the polarization ellipse. If the ellipse has equal minor and major axis it transforms into a circle. In that case we say that the antenna is circularly polarized. If the ellipse has no minor axis it transforms into a a straight line, In that case we say that the antenna is linearly polarized. The various polarization types are graphically demonstrated in figure 8.

Each polarization has a sense. For a linearly polarized antenna it is defined by the tilt angle of  the polarization ellipse, denoted by τ. Linear polarizations are classified by that sense (90º vertical, 0º horizontal,  ± 45º slant). For a circularly polarized antennas the sense is given by the nature of movement of the electric field vector tip: clockwise or counterclockwise (RHCP for clockwise, LHCP for counterclockwise). An illustration is given in figure 10.

L. Cross-Polarization and Co-Polarization

As mentioned above, the different polarizations form many orthogonal pairs.Co-Polarization is defined as the polarization the antenna was meant to radiate, while Cross-Polarization is defined as its orthogonal pair. A purely polarized antenna will have low cross polarized radiation. A measure of how purely polarized an antenna is, is the cross polarization level. It is defined as the difference in decibels between the maximum radiation intensity of the co and cross polarizations respectively.Antennas must operate in similar polarizations in order to ensure optimal performance.Antennas operating in orthogonal polarizations will not perform at all due to significant polarization losses.

M. Axial Ratio

This parameter is majorly used to describe the polarization nature of circularly polarized antennas. The Axial Ratio (AR) is defined as the ratio between the minor and major axis of the polarization ellipse. Recall that if the ellipse has en equal minor and major axis it transforms into a circle, and we say that the antenna is circularly polarized. In that case the axial ratio is equal to unity (or 0 dB). The axial ratio of a linearly polarized antenna is infinitely big since one of the ellipse axis is equal to zero. For a circularly polarized antenna, the closer the axial ratio is to 0 dB, the better.

N. Polarization Diversity and Isolation

Some antennas may offer polarization diversity, that is to say that they are designated to operate in different polarizations. These antennas posses several ports, each permits the transmission of different wave polarization. The different ports are often intended to operate independently. Therefore, it is clear that we require a measure describing how much these ports are isolated. The Isolation between the two ports is defined as the ratio between the power incident upon one port and the power delivered to another port, when it is terminated by a matched load. Good isolation will promise uncorrelated transmission of electric signals on both ports.


O. Power Handling

This is defined as the maximum input power the antenna can handle while working properly.

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