Antenna Frequency

Frequency

Frequency, term used in the physical sciences to denote the number of times that any regularly recurring phenomenon occurs in one second. Frequency is important in many fields of science, such as mechanics, and the study of sound waves.
Frequencies of oscillating objects can cover a wide range of values. The tremors of earthquakes may have a frequency of less than 1, while the rapid electromagnetic oscillations of gamma rays may have frequencies of 1020 or more. In almost all forms of mechanical vibration, a relationship exists between frequency and the physical dimensions of the vibrating object. Thus, the time required by a pendulum to make one complete swing is partly determined by the length of the pendulum; the frequency or speed of vibration of a string of a musical instrument is partly determined by the length of the string. In each instance, the shorter the object, the higher the frequency of vibration.
In wave motion of all kinds, the frequency of the wave is usually given in terms of the number of wave crests that pass a given point in a second. The velocity of the wave and its frequency and wavelength are interrelated. The wavelength (the distance between successive wave crests) is inversely proportional to frequency and directly proportional to velocity. In mathematical terms, this relationship is expressed by the equation V = ? f, where V is velocity, f is frequency, and ? is wavelength. From this equation any one of the three quantities can be found if the other two are known.
Frequency is expressed in hertz (Hz); a frequency of 1 Hz means that there is 1 cycle or oscillation per second. The unit is named in honor of the German physicist Heinrich Rudolf Hertz, who first demonstrated the nature of electromagnetic wave propagation. Kilohertz (kHz), or thousands of cycles per second, megahertz (MHz), or millions of cycles per second, and gigahertz (GHz), or billions of cycles per second, are employed in describing certain high-frequency phenomena, such as radio waves. Radio waves and other types of electromagnetic radiation may be characterized either by their wavelengths, or by their frequencies. Electromagnetic waves of extremely high frequencies, such as light and X rays, are usually described in terms of their wavelength measure, which is often expressed in angstrom units (Å; hundred-millionths of a cm). An electromagnetic wave that has a wavelength of 1 Å has a frequency of about 3 billion GHz.
See Sound; Ultrasonics; Wave Motion.

Antenna Electromagnetic Radiation

Electromagnetic Radiation

1. INTRODUCTION

Electromagnetic Radiation, energy waves produced by the oscillation or acceleration of an electric charge. Electromagnetic waves have both electric and magnetic components. Electromagnetic radiation can be arranged in a spectrum that extends from waves of extremely high frequency and short wavelength to extremely low frequency and long wavelength (see Wave Motion). Visible light is only a small part of the electromagnetic spectrum. In order of decreasing frequency, the electromagnetic spectrum consists of gamma rays, hard and soft X rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.

2. PROPERTIES

There are three phenomena through which energy can be transmitted: electromagnetic radiation, conduction, and convection (see Heat Transfer). Unlike conduction and convection, electromagnetic waves need no material medium for transmission. Thus, light and radio waves can travel through interplanetary and interstellar space from the sun and stars to the earth. Regardless of the frequency, wavelength, or method of propagation, electromagnetic waves travel at a speed of 3 × 10¹⁰ cm (186,272 mi) per second in a vacuum. All the components of the electromagnetic spectrum, regardless of frequency, also have in common the typical properties of wave motion, including diffraction and interference. The wavelengths range from millionths of a centimeter to many kilometers. The wavelength and frequency of electromagnetic waves are important in determining heating effect, visibility, penetration, and other characteristics of the electromagnetic radiation.

3.THEORY

British physicist James Clerk Maxwell laid out the theory of electromagnetic waves in a series of papers published in the 1860s. He analyzed mathematically the theory of electromagnetic fields and predicted that visible light was an electromagnetic phenomenon.

Physicists had known since the early 19th century that light is propagated as a transverse wave (a wave in which the vibrations move in a direction perpendicular to the direction of the advancing wave front). They assumed, however, that the wave required some material medium for its transmission, so they postulated an extremely diffuse substance, called ether, as the unobservable medium. Maxwell's theory made such an assumption unnecessary, but the ether concept was not abandoned immediately, because it fit in with the Newtonian concept of an absolute space-time frame for the universe. A famous experiment conducted by the American physicist Albert Abraham Michelson and the American chemist Edward Williams Morley in the late 19th century served to dispel the ether concept and was important in the development of the theory of relativity. This work led to the realization that the speed of electromagnetic radiation in a vacuum is an invariant.

4. QUANTA OF RADIATION

At the beginning of the 20th century, however, physicists found that the wave theory did not account for all the properties of radiation. In 1900 the German physicist Max Planck demonstrated that the emission and absorption of radiation occur in finite units of energy, known as quanta. In 1904, German-born American physicist Albert Einstein was able to explain some puzzling experimental results on the external photoelectric effect by postulating that electromagnetic radiation can behave like a particle (see Quantum Theory).

Other phenomena, which occur in the interaction between radiation and matter, can also be explained only by the quantum theory. Thus, modern physicists were forced to recognize that electromagnetic radiation can sometimes behave like a particle, and sometimes behave like a wave. The parallel concept—that matter also exhibits the same duality of having particlelike and wavelike characteristics—was developed in 1923 by the French physicist Louis Victor, Prince de Broglie.

HARMONIC OPERATION

HARMONIC OPERATION

If there is reflection from the end of, a wire, the number of standing waves on the wire will be equal to length of the wire divided by a half wavelength. Thus,if the wire is two half wave long there will be two standing waves, and so on. There longer wires,each multiples of a half wave in length,will also be resonant,therefore,at the same frequency as the single half-- wire. When an antenna is two or more half waves in length at the operation frequency it is said to be harmonically resonant,or to operate at a harmonic. The number of the harmonic is the number of standing waves on the wire. For example, a wire two half waves long is said to be operating on its second harmonic ; one three half waves long on its third harmonic, and so on.

Harmonic operation is often utilized in antenna work because it permits operating the same on several harmonically related amateur bands. It is also an important principle in the operation of certain types of directive antenna.

Fig.2. Harmonic operation of along enough to contain several half waves. The current and voltage curves cross.

The heavy line representing the wire to indicate that there is reversal in the direction of the current,and a reversal, in the polarity of the voltage,at intervals of a half wavelength. The reversals or current and voltage do not coincide,but occur at point a quarter wavelength a part.

CURRENT AND VOLTAGE DISTRIBUTION

CURRENT AND VOLTAGE DISTRIBUTION

If the wire in the first illustration had been infinitely long the charge (voltage) and the current (an electric current is simply a charge in motion ) would both slowly decrease would result from dissipation of energy in the form of radio waves and in heating the wire because of its resistance. However , when the wire is short the charge is reflected when it reaches the far end, just as the ball bounced back from the barrier. With radio –frequency exitantion of a half –wave antenna, there is of course not just a single charge but a continuous supply of energy, varying in voltage according to a sine—wive cycle. We might consider this a series of charge, each of slightly different amplitude than the preseding one. When a charge is now haveling in the opposite direction. However, the next charge is just reaching the end of the antenna, so we have two current of practically the same amplitude flowing in opposite direction. The resultant current at the end of the antenna there fore is zero. As we move father back from the end of the antenna the magnetudes of the out going and returning current are no longer the same because the charge causing them have been supplied to the antenna at different part of the Rf cycle. There is cancellation, therefore,and a measurable current exist. The greatest different – that is, the largest resultant current--- Will be found to exist a quarter wavelength away from the end of antenna. as we move back still father from this point the current will decrease until, a half wavelength away from the antenna, it will reach zero again. Thus, in a half-- wave antenna the current is zero at the end and maximum at the center. This current distribution along a half-- wave wire is shown in Fig.1.

Fig.1.Current and voltage distribution on a half--wave wire . In this conventional representation the distance at any point ( x , for instance ) from the wire , represented be heavy line, to the curve gives the relative intensity of current or voltage at the point. The relative direction of current flow ( or polarity of voltage ) is indicated by drawing the curve either above or bellow the line that represent the antenna. The curve above, for example, show that the instantaneous polarity of the voltage in one half of the antenna is opposite to that in the other half .
this distance measured vertically from the antenna wire to the curve marked “ current “, at any point a long the wire represent the relative amplitude of the current as measured by an ammeter at the point . This is called a standing wave of current. The instantaneous value of current of at any point varies sinusoidally at the applied frequency, but is amplitude is different at every point along the wire as shown by the curve. The standing wave curve it self has the shape of a half sine wave , at least to a good approximation.

The voltage along the wire will behave differently ; it is obviously greatest at the end since at this point we have two practically equal charge adding. As we move back along the wire , however,the out going and returning charges are not equal are their sum is smaller. At the quarter-wave point the returning charge is of equal magnitude but of opposite sign to the out going charge, since at this time the polarity of the voltage wave from the source has reversed ( one-half cycle ). The two voltage therefore cancel each other end the resultant voltage is zero . Beyond the quarter- wave point, away from the end of the wire,the voltage again increases,but this time with the opposite polarity.

It will be observed, therefore, that the voltage is maximum at every point wire the current is minimum,and vise versa. The polarity of the current or voltage reverses every half wavelength along the wire , but the reverses do not occur, at the same point for both current and voltage ; the respective reversals do not occur, in fact, at point a quarter wave a part .A maximum point on standing wave is called a loop ( or antinode ) ;a minimum point is called a node.

RESONANCE IN LINIEAR CIRCUITS

RESONANCE IN LINIEAR CIRCUITS

The shortest length of wire that will resonate to a given frequency is one just long enough to permit an electric charge to travel from one end RF cycle. If the speed at which the charge travel is equal to the velocity of light,approximately 300,000,000 meters per second, the distance it will cover in one cycle or period will be equal to this velocity divided by the frequency in hertz, or = 300,000,000/f

note: ==> wavelength

f ==> frequency in meter

Since the charge traverses the wire twice , the length of wire needed to permit the charge to travel a distance in one cycle is / 2 , or one—half wavelength. Therefore the shortest resonant were will be a half wavelength long.

The reason for this length can be made clear by a simple example. Imagine a trough with barriers at each end. If an elastic ball is started along the trough from one end,it will strike the far barrier,bounce again,and continue until the energy imported to it originally is all dissipated. if however,whenever it returns to the near barrier it is given a new push just as it starts a way, its back-and-forth motion can be kept up indefinitely. The impulses, however must be timed properly ; in other words, the rate or frequency of the impulses and the speed of the ball are fixed, the length of the trough must be adjusted to “ Fit “.

In the case of the antenna, the speed is essentially constant, so we have the alternatives of adjusting the frequency to a given length of wire,or the length of wire to a given operating frequency. The latter is usually practical condition.

By changing the units in the equation just given and dividing by 2, the formula : l =492/f(MHz) is obtained. In this case “ l “ is the length in feet of a half wavelength for a frequency “ f “ , given in megahertz, when the wave travels with length in antenna work are developed. It represents the length of a half wavelength in space, When no factors that modify the speed of propagation exist.

To determine a half wavelength in meters.

The relationship is ; l =150/ f (Mhz)

ANTENNA FUNDAMENTAL

ANTENNA FUNDAMENTALS

An antenna is an electric circuit of a special kind. In the ordinary type of circuit the dimension of coils,capacitors and connections usually are small compared with the wavelength that corresponds to the frequency in use.when this is the case most of the electromagnetic energy escapes by radiation in the form of electromagnetic waves. when the circuit is intentionally designed so that the major portion of the energy is radiated,we have an antenna.
Usually,the antenna is a straight section of conductor,or a combination of such conductor.Very frequently the conductor is a wire,although rods and tubing also are used .in this chapter we shall use the term “WIRE”to mean any type of conductor having across section that is small compared with its length.

The strength of the electromagnetic field radiated from a section of wire carrying radio-frequency current depends on the length of the wire and the amount of current flowing.( it would also be true to say that the field strength depend on the voltage across the section of wire, but it is generally more convenient to mean sure current. The electromagnet field, consist of both magnetic and electric energy, but it is generally more convenient to mean sure current. The electromagnetic filed consist of both magnetic and electric energy, with the total energy equally divided between the two one cannot exist without the other in a electromagnetic waves, and the voltage in an antenna is just as much a mean sure of the field intensity as the current ). other things being equal,the field strength will be directly proportional to the current .It is therefore desirable to make the current as large as possible,considering the power available. In any circuit that contains both resistance and reactance , the largest current will flow ( for given a mount of power ) when the reactance is “ Tuned out “-- in other words, when the circuit in made resonant at the operating frequency. So it is with the common type of antenna ; the current in it will be largest, and the radiation therefore greatest,when the antenna is resonant.
In an ordinary circuit the inductance is usually concentrated in a coil, the capacitance in a capacitor , and the resistance is principally concentrated in resistors, although some may be distributed around the circuit wiring and coil conductor. Such circuits are said to have lumped constant. In an antenna, on the other hand, the inductance, capacitance,and resistance are distributed along the wire. Such a circuit is said to have distributed constant. Circuits with distributed constant are so frequently straight-line conductor that they are customarily called linear circuits.