Antenna A mean for radiating and receiving radio waves Transitional structure between free-space and a guiding device. Application: Radiation

Transcription

1 Antenna A mean for radiating and receiving radio waves Transitional structure between free-space and a guiding device Application: adiation

2 Introduction An antenna is designed to radiate or receive electromagnetic energy with directional and polarization properties suitable for the intended application. Also, minimize reflection at the transmission line-antenna juncture. Impedance matching These properties are governed by Shape Size Material

3 Types of antennas Wire antennas Straight wire (dipole) Example: FM radio receiver, TV, Mobile phone Loop Example: AM radio receiver, Octopus Helix Example: Satellite 3

4 Types of antennas Aperture antennas Example: aircraft, spacecraft and satellite receiver Microstrip antennas Example: mobile phone 4

5 eflector antennas Example: satellite receiver Types of antennas 5

6 Hertzian dipole (A very small wire antenna) It is more convenient to use magnetic vector potential A to solve radiation problems. µ J(r A ( r ) 4π vol H A µ o E H jωε o j r r ' ' ) e r r ' J(r dv ' ) r ' r r ' r A (r) 6

7 7 Hertzian dipole I ( ) j e j j Idl H φ π + sin 4 and then dl ( ) ( ) ( ) ( ) 0 sin 4 cos φ η π η π E e j j j Idl E e j j Idl E j o j o

8 Hertzian dipole 8

9 Hertzian dipole Far field π / λ >> The region where is the far zone. Under this condition, the following terms can be neglected. and The far zone electric and magnetic fields are E H φ ( j ) ( j ) 3 Idl e j 4π Idl e j 4π j j η o sin V/m sin A/m I It is a plane wave with intrinsic impedance η o and varies inversely with the distance from the source 9

10 adiation resistance Antennas are designed for effective radiation of electromagnetic energy. Equivalent circuit of an antenna I in r input radiation resistance r epresents radiated energy input loss resistance L epresents conduction and dielectric losses of the antenna input reactance X A represents the energy stored in the field near the antenna 0

11 The power radiated is equal to: The power losses is adiation resistance W loss W If W in is the input power, the radiation efficiency is: I in L rad I in r η r W W rad in r + r L I in r

12 Directive gain, directivity and gain Stronger in some directions Same intensity for all directions Isotropic Antenna (the reference antenna)

13 Let P avg be the average Poynting vector which is the power flow density per unit area, P avg e The total power radiated W rad is then W W rad rad S S P avg ds U (, φ ) dω ( E H *) ds sin ddφ dω sin ddφ where U(,φ) is the power flow through a unit solid angle, and is called the radiation intensity (W/sr). U Directive gain, directivity and gain (, φ ) r P avg 3

14 Directive gain, directivity and gain 4

15 Directive gain, directivity and gain Directive gain G D (,φ) atio of the radiation intensity in a particular direction(,φ) to the average radiation intensity. U (, φ ) U (, φ ) G D (, φ ) U W / 4π avg Directivity Maximum value of the directive gain in a certain direction. D Max { (, φ )} G d Power Gain atio of the radiation intensity in a given direction to the radiation intensity of a lossless isotropic radiator that has the same input power. U (, φ ) G p (, φ ) W / 4π in rad 5

16 Example Find the directive gain of a Hertzian dipole. P avg P avg e H ( E *) E H φ ( Idl ) 3 π r U P avg r η o ( Idl ) 3 π I sin η o sin H φ E 6

17 7 Example and then ( ) π φ φ φ π π 0 0 sin 3 4 / sin sin sin ), ( ), ( d d U U G avg D

18 Example 8

19 Example Directive gain and the directivity of the Hertzian dipole G D (, φ ) D G d 3 sin 3 ( π /, φ ).76 db I Suppose the radiation efficiency is 46%, G p 0.46 D db ( W rad / W in 0.46) 9

20 Example Find the radiation resistance of a Hertzian dipole P r π 0 0 π P ( dl ) avg I η o 3π I ( dl ) η o π I dl 80π λ dl 80π λ sin ddφ dl Suppose, Ω Poor radiator!! λ r 0 r π 0 0 π sin 3 ddφ 0

21 Linear dipole antenna Knowing the current distribution I(z), we can sum up the fields due to the infinitesimal segments on the antenna using the results of the Hertzian dipole. η ˆ j o I m e E π ˆ ji m e H φ π cos F ( ) jk jk F ( ) F ( ) a ( h cos ) φ sin cos h h I F() is called the pattern function

22 Linear dipole antenna Antenna pattern E-plane pattern (pattern function versus for a constant φ) H-plane pattern (pattern function versus φ for a constant π/)

23 Linear dipole antenna At certain dipole lengths ( λ/, λ ) called resonant lengths, the input impedance is purely resistive. For half-wavelength dipole, Z in r 73 Ω The pattern pattern for a half-wavelength dipole is cos ( h cos ) cos h F ( ) sin π cos cos sin D.64 3

24 Effective Area and Friis Equation Effective Area The effective area A e of a receiving antenna is the ratio of the time-average power received to the time-average power density of the incident wave at the antenna. A P / e L P avg It may be shown that is A e related to the directive gain as: A λ 4π e G D (, φ ) 4

25 Friis Equation Effective Area and Friis Equation Consider two antennae separated by a distance r. The transmitting antenna transmits a total power P t. A e, G D, P t A e, G D, P L r The time-average power density at the receiving antenna is P avg P t 4πr G D 5

26 6 Effective Area and Friis Equation The power received to the load is (Friis Equation) ( ) t D D avg D e avg L P G G r P G A P P 4 4 π λ π λ ( ) 4 D D t L G G r P P π λ