TC303 Antenna & Propagation Lecture 2 Introduction to Antenna 1 RS Reference: Antenna Theory, 3 rd edition, by Constantine A. Balanis

 Antenna history  General properties  Radiation mechanisms  Radiation fields and patterns  Antenna performance Introduction to Antenna 2 RS

 A structure designed for radiating and receiving EM energy in a prescribed manner.  The importance of the shape and size of the structure  the efficiency of the radiation  the preferential direction of the radiation What is antenna? 3 RS

What are not antennas? RS 4 compass only receiving magnetic field Benjamin Franklin’s kite experiment Current path depends on the medium.

What about human eyes? RS 5 The human eye of course receives high frequency electromagnetic waves but cannot transmit waves. Therefore, eyes are not the antenna.

Coupling of electricity and magnetism RS 6 Michael Faraday He slid a magnetic around the coils of a wire attached to a galvanometer. In moving the magnet, he was creating a time-varying magnetic field, which as a result (from Maxwell's Equations), must have had a time-varying electric field. Click here to watch Faraday’s experiment

The coil acts as a loop antenna. RS 7 It receives the EM radiation which is detected by the ampere meter.

Hertz’s wireless communication system RS 8 Heinrich Hertz He observed an electrical spark in the gap of a dipole antenna. When using a loop antenna as a receiver, he observed a similar disturbance. Click at the image

Dipole antenna RS 9 pole

 Complex antenna impedance Z ant needs to be matched to the system impedance. Generic Antenna network 10 RS

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 Wire antennas  Aperture antennas  Microstrip antennas  Array antennas  Reflector antennas  Lens antennas Types of Antennas RS 12

Wire antennas 13 RS

 Horn antennas have a directional radiation pattern with a high antenna gain, dB is typical.  Horn antennas are also often used to feed a dish antenna, or as a "standard gain" antenna in measurements.  The popular design is pyramidal horn. Aperture antennas RS 14

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Microstrip or patch antennas 16 RS

Antenna array 17 RS

Yagi-Uda antenna invented 1920s RS 18 The Yagi antenna consists of a single 'feed' or 'driven' element, typically a dipole antenna. simple construction, high gain over 10 dB. reflectorfeederdirector

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Reflector antennas RS 20

Lens antennas RS 21

Radiation mechanisms: Microscopic view of charge acceleration RS 22 Consider this negative charge and its electric field lines Electric field lines

Charge acceleration RS 23 What if someone moves the charge off of its original position? Electric field lines will try to realign themselves.

RS 24 If all accelerating electric charges radiate, then the wires that connect my computer to the wall should be antennas, correct? Answer is yes. However, they are very poor antennas as the radiation is cancelled over two wires carrying current in the opposite directions.

RS 25 If it is so simple, then everything could be an antenna. Why don't we just use a metal paper clip as an antenna, hook it up to the receiver ? Answer: A paper clip could act as an antenna for some given conditions. The impedance controls how much power the receiver or transmitter could deliver to the paper clip. Impedance depends on the operating frequency.

Radiation mechanisms RS 26 How is the radiation accomplished in a single wire?

 Assume  v is an electric volume charge density (C/m 3 ) and is uniformly distributed in a circular wire of a cross- sectional A and volume V.  The current density J z =  v v z A/m 2 is called “convection current.”  The current density J s on the surface is given by J s =  s v z where  s is the surface charge density (C/m 2 ). If the wire is very thin, then the current can be represented by I s =  l v z, where  l is the charge per unit length (C/m). Single wire RS 27

Single wire RS 28 To create radiation, there must be a time-varying current or an acceleration (or deceleration) of charge).

Radiation facts RS 29 1.If a charge is not moving, current is not created and there is no radiation. 2.If charge is moving with a uniform velocity: a)There is no radiation if the wire is straight, and infinite in extent. b)There is a radiation if the wire is curved, bent, discontinuous, terminated, or truncated. 3.If charge is oscillating in a time-motion, it radiates even if the wire is straight.

Wire configurations for radiation 30 RS

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Charge acceleration due to an exciting electric field and deceleration due to impedance discontinuities or smooth curves of the wire are mechanisms responsible for electromagnetic radiations. Radiation mechanisms RS 36 Click on the image