Antenna Antenna Equivalent Circuit

ANTENNA A metallic conductor system capable of radiating and capturing electromagnetic energy.
- A transducer that converts electrical AC oscillations at an RF frequency to an electromagnetic waves of the same frequency - used to interface transmission lines to the atmosphere, the atmosphere to transmission lines, or both. -radio waves are electrical energy that has escaped into free space in the form of transverse electromagnetic waves -electrical circuits that carry AC radiate a certain amount of electrical energy in the form of EMW -amount of energy radiated is negligible unless the physical dimension of the circuit approaches the dimensions of a wavelength of a wave. Basic Antenna Operation l to frequency
Antenna Equivalent Circuit An antenna coupling system
Transmit Antenna Equivalent Circuit
low frequency waves require relatively large antennas Receiver Antenna Equivalent Circuit radiate more energy in certain directions relative to other directions a transmission line terminated in an open circuit result in some of the incident voltage being radiated not reflected back to the source.
is extremely low. ing the conductors farther apart (dipole antenna) antenna (vertical monopole or Marconi antenna) is when conductor are spread out in a straight line to a total length of quarter wavelength. -wave dipole is called Hertz antenna. Antenna Reciprocity An antenna is a reciprocal device in that the transmit and receive characteristics and performance are identical (gain, directivity, frequency of operation, bandwidth, radiation resistance, efficiency, etc.)
Duplexer utilizing a single antenna for both transmit and receive of an RF signal Diplexer utilizing a single antenna for transmitting two RF signal Active antenna is actually the combination of a passive antenna and a low-noise amplifier. Active antennas are nonreciprocal.
Relative radiation pattern radiation pattern plotted in terms of field strength or power density with respect to the value at reference point.
Near Field and Far Field Near field refers to the field pattern that is close to the antenna (also called induction field or Fresnel field)
where A area D diameter of antenna Far field refers to the field pattern that is at a great distance(also called radiation field or Fraunhofer field)
The directional characteristics of an electromagnetic wave radiated or receive by an antenna are general describe in the spherical coordinates given. The distance to any point on the surface of the sphere can be defined in respect to the antenna by using the radius of the sphere d and angles and .
where D diameter of antenna
Radiation Resistance -An AC antenna resistance and is equal to the ratio of the power radiated by the antenna to the square of the current at its feedpoint.
Radiation Pattern - is a polar diagram or graph representing field strengths or power densities at various angular positions relative to an antenna. Absolute radiation pattern the radiation pattern is plotted in terms of electric field strength and power density.
where - radiation resistance(ohms) - power radiated by the antenna (W) antenna current at the feedpoint (A) Antenna Efficiency - Is the ratio of the power radiated by the antenna to the sum of the power radiated and the power
dissipated or the ratio of the power radiated by the antenna to the total input power.
Pref P at the same point with a reference antenna(W/m2)
where antenna efficiency (percentage) - radiated power(watts) - input power(watts) - power dissipated in antenna(watts)
Power gain is the same as directive gain except that the total power fed to the antenna is used . = ()=10log( )
Ex. Calculate the efficiency of an antenna that has a radiation resistance of 72 and a loss resistance of 6, measured at the feedpoint.
Ex. Calculate the gain of a certain antenna relative to a dipole antenna with a gain of 4.5 dB with respect to an isotropic radiator. Also compute for the power gain if the antenna has an efficiency of 96%. = 10 log (1.64) =4.5 2.15 =.
Effective Isotropic Radiated Power Antenna Gain -Directive gain is the ratio of the power of the power density radiated in a particular direction to the power density radiated to the same point by a reference antenna, assuming both antennas are radiating the same amount of power. power. = where -EIRP is defined as an equivalent transmit
= where D directive gain P Power density(W/m2) where
where P power density R - distance from transmit antenna
Ex. Calculate the captured power 10km away from a half wave dipole transmitter with 10W transmit power at 150MHz for the (a) Hertzian dipole and (b) half-wave dipole.
WAVEGUIDE -consists of a hollow metallic tube of a rectangular or a circular shape used to guide an electromagnetic wave -used principally at frequencies in the microwave range -electric and magnetic fields are confined to space within the guides, thus no power is lost through radiation -no dielectric loss since guides are normally air filled -possible to propagate several modes of electromagnetic waves -has definite cut off frequency for each allowed mode Process of Solving Problems 1. The desired wave equations are written in the form of rectangular or cylindrical coordinate system. 2. The boundary conditions are then applied 3. Equations are in the form of partial differential equations in time or frequency domain. direction designate or . the number of half waves of electric or magnetic intensity in x direction
n m denotes
Waveguide modes In lossless waveguides
number of half waves in the y direction
Wave impedance -Ratio of the transverse components of electric to magnetic fields. Wave impedance for TE10 mode = / (sin) Also =120 x (/)
Waveguide Terminations To terminate a waveguide, a thin sheet of resistive material with equal wave impedance is use. Example: 1. A rectangular waveguide has a broadwall dimension of 0.9 inch, and is fed by a 10GHz carrier from a coaxial cable. Determine whether a TE10 wave will be propagated, and, if so, find its guide wavelength, phase and group velocities. = (/ ) where -terminating resistance resistivity of sheet material thickness of the sheet
Sheet resistivity = / Solution =0.9 =2.286 =22.286=4.572 = 3 108/1010= 3 Since > , TE10 wave will propagate. ( )
Circular waveguides Cut off wavelength, TE11 =1.71, d is the diameter
-Circular guides have special properties which enable them to be used for rotating joints and the TE10 mode has the unusual characteristics of attenuation becoming less as frequency is increase.

Antenna Antenna Equivalent Circuit

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Antenna Antenna Equivalent Circuit

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ANTENNA A metallic conductor system capable of radiating and capturing electromagnetic energy.

Antenna Equivalent Circuit An antenna coupling system

Transmit Antenna Equivalent Circuit

where D diameter of antenna

Pref P at the same point with a reference antenna(W/m2)

= where D directive gain P Power density(W/m2) where

where P power density R - distance from transmit antenna

Waveguide modes In lossless waveguides

number of half waves in the y direction

Circular waveguides Cut off wavelength, TE11 =1.71, d is the diameter

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