Antenna Noise Temperature

In telecommunication, antenna noise temperature is the temperature of a hypothetical resistor at the i/p of an ideal noise-free receiver that would generate the same o/p Noise power per unit BandWidth as that at the antenna output at a specified frequency. In other words, antenna noise temperature is a parameter that describes how much noise an antenna produces in a given environment. This temperature is not the physical temperature of the antenna. Antenna Noise Temperature

 The temperature depends on its gain pattern and the thermal environment that it is placed in. Antenna temperature is also sometimes referred to as Antenna Noise Temperature.  Temperature distribution (T(θ,Ф)(temperature in every direction away from the antenna in spherical coordinates)  An antenna's temperature will vary depending on whether it is directional and pointed into space or staring into the sun.  For an antenna with a Radiation Pattern R(θ,Ф), The Antenna noise temperature is mathematically defined as: Antenna Noise Temperature

 The Noise Power received from an antenna at temperature (T A ) can be expressed in terms of the bandwidth (B) the antenna (and its receiver) are operating over: P TA =K T A B k is Boltzmann’s constant (1.38×10 −23 J/K). Any object whose temperature is above the absolute zero radiates EM energy. Thus, each antenna is surrounded by noise sources, which create noise power at the antenna terminals. Two types of Antenna Noise: - Noise due to the loss resistance of the antenna itself; - Noise, which the antenna picks up from the surrounding environment.

Galactic radiation Earth heating The sunsun Electrical devices The antenna itself Galactic noise is high below 1000 MHz. At around 150 MHz, it is approximately 1000K. At 2500 MHz, it has leveled off to around 10K. Earth has an accepted standard temperature of 290˚K. The level of the sun's contribution depends on the solar flux. It is given by Sources of Antenna noise temperature: F is Solar flux

 The polarization of the radio wave can be defined by direction in which the electric vector E is aligned during the passage of at least one full cycle.  Also polarization can also be defined the physical orientation of the radiated electromagnetic waves in space. The polarization are 3 types. Elliptical polarization circular polarization Linear polarization. also, Co-Polarization (Tx & Rx Antenna polarized in same orientation) Cross Polarization Antenna Polarization

Polarisation of EM wave Electrical field, E vertical horizontal circular

What is field? If at each point in a region is a corresponding value of some physical quantity such as Potential, Temperature, E, H etc, the region is called Field. “Field is spatial distribution of Physical Quantity” What is field theory ? Field theory is describing the existence and variations of the “field” quantities such as E and H in free space Basics of “Radiation Phenomena”

 E=hf ; h = plank’s constant = 6.624x J-sec  Say the freq of operation is 3 Ghz;  E = 6.624x X 3x10 9 J;  E = 6.624x X 3 J  But, 1 eV = 1.6x J or 1 J = 1/ (1.6x ) eV  So, E = 6.624x x 3 x 1/ (1.6x ) eV E = 1.2x10 -5 eV How EM radiation is related to Energy of the EM signal?

So, for different freq, EM signal would have the following Energy

Inference: ?......

EM spectrum

 Ions are identified as Charged particls since they are electrically positive or negative ( +Ve Charge, -Ve Charge):  Atom is electrically neutral:  Charge is conserved, implies “ A disappearance of charge at one place always follows by a reappearance of charge at another place.  Charge is quantized; it exhists in integer multiple of +e or –e  The value of charge of one electron is 1.6x C  1C = 6,250,000,000,000,000,000 electrons i.e, (6.250x ) CHARGE, ELECTRON, WAVE  Electron is a packet of Energy  Electron is a source of Force  Electron is a quantification of Electricity  Electron is a Particle  Electron is a Wave Group of Electron is Volt Flow of electron is Current Transition of Electron leads to Absorbtion or Emission Oscillation of Electron leads to “Radiation ”

RADIATION FROM OSCILLATING DIPOLE  A device that uses an oscillating distribution to produce electromagnetic radiation is called an antenna.  A simple example of an antenna is an oscillating electric dipole,  “A pair of electric charges that vary sinusoidally with time such that at any instant the two charges have equal magnitude but opposite sign”.  One charge could be equal to Q sin vt and the other to −Q sin vt.  An oscillating dipole antenna can be constructed in various ways, depending on frequency.  One technique that works well for radio frequencies is to connect two straight conductors to the terminals of an ac source

 A key feature of the radiation in the far(Fraunhofer) region is that it is not a plane wave, but a wave that travels out radially in all directions from the source.  The wave fronts are not planes; in the far region they are expanding concentric spheres centered at the source.

Production of Electromagnetic Waves Oscillating charges will produce electromagnetic waves:

Production of Electromagnetic Waves Far from the source, the waves are plane waves:

Representation of the electric field (red lines) and the magnetic field (blue dots and crosses) in a plane containing an oscillating electric dipole. During one period the loop of E shown closest to the source moves out and expands to become the loop shown farthest from the source. CROSS SECTION OF THE RADIATION PATTERN AT ONE INSTANT

The Magnitudes in the Far region are :  Magnitudes are proportional to 1/r. (contrast to the E field of a stationary point charge or the B field of a point charge moving with constant velocity, both of which are proportional of 1/r 2 ), Since 1/r 2 terms become negligible at Far region.  The field magnitudes are greatest in the directions Perpendicular to the dipole, where θ = π /2;  There is no radiation along the axis of the dipole, where θ = 0˚ or π

At points very far from the oscillating dipole, E and B are perpendicular to each other, and the direction of the Poynting vector S = (E x B)/µo is radially outward from the source. Oscillating magnetic dipoles also act as radiation sources; An example is a circular loop antenna that uses a sinusoidal current. At sufficiently high frequencies a magnetic dipole antenna is more efficient at radiating energy than is an electric dipole antenna of the same overall size.

E-field (E) & M-field (B) used to determine radiation pattern E goes through antenna ends & spreads out in increasing loops B is a series of concentric circles centered at midpoint gap E B Cont.,

Azimuth Pattern Elevation Pattern Polar Radiation Pattern 3-dimensional field pattern is donut shaped -antenna is shaft through donut center radiation pattern determined by taking slice of donut - if antenna is horizontal  slice reveals figure 8 - maximum radiation is broadside to antenna’s arms Cont.,

HALF WAVE DIPOLE ANTENNA The half-wave dipole antenna is just a special case of the dipole antenna. Half-wave term means that the length of this dipole antenna is equal to a half-wavelength at the frequency of operation. The dipole antenna, is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents applied at its two terminals through a balanced transmission line.

To make it crystal clear, if the antenna is to radiate at 600 MHz, what size should the half-wavelength dipole be? One wavelength at 600 MHz is = c / f = 0.5 meters. Hence, the half-wavelength dipole antenna's length is 0.25 meters. The half-wave dipole antenna is as you may expect, a simple half-wavelength wire fed at the center as shown in Figure λ= c / f λ= (3x10 8 ) / (600x10 6 ) λ= 0.5 mts λ= 0.25 mts

Electric Current on a half-wave dipole antenna The fields from the half-wave dipole antenna are given by:  It can be noted that by reducing the length slightly the antenna can become resonant. If the dipole's length is reduced to 0.48, the input impedance of the antenna becomes Zin = 70 Ω, with no reactive component. This is a desirable property, and hence is often done in practice. The radiation pattern remains virtually the same.resonant  The above length is valid if the dipole is very thin. In practice, dipoles are often made with fatter or thicker material, which tends to increase the bandwidth of the antenna.bandwidth

 The voltage and current levels vary along the length of the radiating section of the antenna. This occurs because standing waves are set up along the length of the radiating element.  The centre point is where the current is a maximum and the voltage is a minimum, this makes a convenient point to feed the antenna as it present a low impedance.  For a dipole antenna that is an electrical half wavelength long, the inductive and capacitive reactance's cancel each other and the antenna becomes resonant. With the inductive and capacitive reactance levels cancelling each other out, the load becomes purely resistive and this makes feeding the half wave dipole antenna far easier. Coaxial feeder can easily be used as standing waves are not present

RADIATION PATTERN OF HALFWAVE DIPOLE ANTENNA

RADIATION FROM HALF WAVE DIPOLE The current distribution on a thin half-wave dipole antenna is closely approximated by a sinusoidal standing wave of the form: K o = 2π/λ

The far-zone radiated field from the half-wave dipole antenna: the unit vector Note that : The Electric field is given by:

The magnetic field is given by: The power flux per unit area is given by: The total radiated power is obtained by integrating this expression over the surface of a sphere of radius r; thus

The result of carrying out the integration is: When we equate this expression to P= (1/2)I 2 R, we find that the radiation resistance of the half-wave dipole is 2x36.56, or Ω.

Intuitive Picture of Radiation

When the size of the system is approaching half wavelength (c=f x λ), then the system becomes an efficient radiator

The folded dipole is the same length as a standard dipole, but is made with two parallel conductors, joined at both ends and separated by a distance that is short compared with the length of the antenna. The folded dipole differs in that it has wider bandwidth and has approximately four times the feed point impedance (≈300Ω) of a standard dipole FOLDED DIPOLE ANTENNA

Folded antenna is a single antenna but it consists of two elements. First element is fed directly while second one is coupled inductively at its end. Radiation pattern of folded dipole is same as that of dipole antenna i.e figure of eight (8). The folded dipole antenna is resonant and radiates well at odd integer multiples of a half-wavelength (0.5λ, 1.5λ,...), when the antenna is fed in the center.resonant FOLDED DIPOLE ANTENNA

Advantages of folded dipole antenna Input impedance of folded dipole is four times higher than that of straight dipole. Typically the input impedance of half wavelength folded dipole antenna is 288 ohm. Bandwidth of folded dipole is higher than that of straight dipole. Find F, if total length of antenna is 2 mts

The folded dipole operates as an unbalanced transmission line. The current on the folded dipole can be decomposed into two distinct modes: -An antenna mode (currents flowing in the same direction yielding significant radiation) and -Transmission line mode (currents flowing in opposite directions yielding little radiation). Radiation analysis of folded dipole

The total folded dipole input current can then be defined as the sum of the transmission line and antenna currents such that: so that the folded dipole input impedance may be written as W.k.t, The general equation for the input impedance of a transmission line of characteristic impedance Z o and length l terminated with an load impedance Z L is But for a special case,…

For the shorted line, Z L = 0 and the length is l/2 so that For the special case of a folded dipole of length l = /2,

The impedance of the half-wave folded dipole becomes: The half-wave folded dipole can be made resonant with an impedance of approximately 300 which matches a common transmission line impedance (twin-lead). Thus, the half-wave folded dipole can be connected directly to a twin-lead line without any matching network necessary. Used in domestic television and VHF FM broadcast antennas Zd is Impedance of λ/2 antenna

YAGI-UDA ANTENNA

 In the 1926, Dr. Shintaro UDA and Dr. Hidetsugu YAGI of the tohoku imperial university invented a directional antenna system consisting of an array of coupled parallel dipoles. This is commonly known as yagi-uda or simply yagi antenna.  Yagi-uda antenna is familiar as the commonest kind of terrestrial TV antenna to be found on the rooftops of houses.  It is usually used at frequencies between 30Mhz and 3Ghz and covers 40 to 60 km.

A Yagi-Uda array consists of 2 or more simple antennas (elements) arranged in a line. The RF power is fed into only one of the antennas (elements), called the driver. Other elements get their RF power from the driver through mutual impedance. The largest element in the array is called the reflector. There may be one or more elements located on the opposite side of the driver from the reflector. These are directors. construction

FIVE ELEMENT YAGI-UDA REFLECTOR DRIVER Ele ment s Gain dBi Gai n dBd

This type of Yagi-Uda array uses dipole elements The reflector is ~ 5% longer than the driver. The driver is ~ 0.5l long The first director ~ 5% shorter than the driver, and subsequent directors are progressively shorter Interelement spacings are 0.1 to 0.2 λ

Radiation pattern formed by the YAGI-UDA directional antenna

Typical yagis (6 m and 10m)

The 2 element Yagi The parasitic element in a 2- element yagi may be a reflector or director Designs using a reflector have lower gain (~6.2 dBi) and poor FB(~10 dB), but higher input Z (32+j49  ) Designs using a director have higher gain (6.7 dBi) and good FB(~20 dB) but very low input Z (10  ) It is not possible simultaneously to have good Z in, G and FB

The 3 element Yagi High gain designs (G~ 8 dBi) have narrow BW and low input Z Designs having good input Z have lower gain (~ 7 dBi), larger BW, and a longer boom. Either design can have FB > 20 dB over a limited frequency range It is possible to optimize any pair of of the parameters Z in, G and FB

Larger yagis (N > 3) There are no simple yagi designs, beyond 2 or 3 element arrays. Given the large number of degrees of freedom, it is possible to optimize BW, FBR, gain and sometimes control side lobes through proper design.

Yagi Array of Loops (quad array) This Yagi-Uda array uses rectangular loops as elements. The reflector’s perimeter is ~ 3% larger than the driver’s. The driver’s perimeter is ~ 1 The first director’s perimeter is ~ 3% smaller than the driver’s, and additional directors are progressively smaller. Interelement spacings are 0.1 to 0.2 λ.

 Advantages:  IT HAS A MODERATE GAIN OF ABOUT 7 (DB).  IT IS A DIRECTIONAL ANTENNA.  CAN BE USED AT HIGH FREQUENCY.  ADJUSTABLE FRONT TO BACK RATIO.  Disadvantages:  THE GAIN IS NOT VERY HIGH.  NEEDS A LARGE NUMBER OF ELEMENTS TO BE USED.