Fundamentals of Antennas. Prof. Ely Levine

Transcription

1 Fundamentals of Antennas Prof. Ely Levine 1

2 Chapter 3 Wire Antennas 2

3 Types of Antennas 3

4 Isotropic Antenna Isotropic radiator is the simplest antenna mathematically Radiates all the power supplied to it, equally in all directions Theoretical only, can t be built Useful as a reference: other antennas are often compared with it 4

5 Half-Wave Dipole Simplest practical antenna Actual length is typically about 95% of a half wavelength in free space L = 0.48 λ 5

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7 Radiation Resistance Signal radiated into space appears as loss from the antenna Electrically this translates into a resistance For a half-wave dipole fed in the center the radiation resistance is approximately 73 ohms Antennas also have actual resistance due to their conductors 7

8 Antenna Efficiency η= P r = P T R R r T 8

9 Directional Characteristics All real antennas transmit more power in some directions than in others Two, two-dimensional diagrams are generally used to show radiation patterns Distance from the center represents radiation in different directions Calibration may be in db relative to max. for that antenna, or relative to isotropic (dbi) or half wave dipole (dbd) 9

10 Dipole Pattern 10

11 Dipole Pattern 11

12 Dipole Gain dbi means decibels with respect to an isotropic radiator dbd means decibels with respect to an ideal half-wave dipole in its direction of maximum radiation The gain of a dipole is 2.14 dbi 12

13 dbd/dbi Conversion Gain (dbi) = Gain (dbd) db Use dbi in Friis s Formula Use dbi when it is necessary to find gain as a power ratio compared with isotropic: Gain (ratio) = antilog (dbi/10) Antennas may be specified either way in catalogs, etc. (check!) 13

14 EIRP and ERP EIRP = effective isotropic radiated power Equal to the amount of power that would have to be applied to an isotropic radiator to give the same power density at a given point ERP = effective radiated power Equal to the amount of power that would have to be applied to a half-wave dipole, oriented in direction of maximum gain, to give the same power density at a given point 14

15 EIRP/ERP Conversion EIRP = ERP db EIRP is used in all our equations Sometimes government regulations specify ERP for transmitting installations Conversion is easy (see above) 15

16 Dipole Impedance At resonance, Z = 73 Ω resistive if fed in center Above resonant frequency: inductive Below resonant frequency: capacitive Impedance can be raised by moving feedpoint out towards ends (delta match) 16

17 Antenna Matching Antennas usually are resistive at only one frequency Even then, resistance may not match feedline impedance Any of the matching schemes discussed previously can be used 17

18 Antenna Loading Coil When an antenna is too short an inductance can be added to increase its electrical length Loading coils often used at base or center of a vertical monopole The whole antenna can also be wound into a coil 18

19 Loading Coil 19

20 Dipole Polarization Polarization is same as axis of wire: Vertical dipole is vertically polarized Horizontal dipole is horizontally polarized 20

21 Ground Effects Effect of ground near antenna is important when antenna is within a few wavelengths of ground Very important up to and including HF, usually less important for VHF and up Effect of ground depends on ground characteristics and distance of antenna from ground 21

22 Reflection from Ground Phase shift at ground of 180 degrees Perfectly conductive ground would reflect all the power that hits it Real ground is not perfectly conductive conductivity depends largely on moisture content Effect of combinining reflected and direct signals depends on distance from ground 22

23 Ground Effects 23

24 Ground Effects 24

25 Folded Dipole Same length as half wave dipole Uses 2 conductors Impedance 4 times that of normal dipole Approximately 300 ohms at resonance Bandwidth is greater than single-conductor dipole 25

26 Folded Dipole 26

27 Two Folded Dipoles Turnstile Antenna for FM Broadcast Band 27

28 Monopole Vertically Mounted Half the length of a dipole (one-quarter wave approximately) Ground supplies the other half If installed above ground, a ground plane can be used instead For a car antenna, the car is the ground plane Input impedance half that of a dipole, about 35 ohms. Gain 2+3 = 5 dbi 28

29 Monopole 1/4 wave monopole with ground plane 29

30 AM Transmitter Tower (The tower is the antenna) 30

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32 5/8 Wavelength Antenna Lower radiation angle and higher impedance than 1/4 wave antenna Can be used without an efficient ground because of the high impedance 32

33 Discone Very wide bandwidth Often used for wideband receiving applications such as scanners 33

34 Discone antenna for MHz with whip antenna for transmitting on ham bands 34

35 Loop Usually small in comparison with wavelength Used in AM receivers and direction finders May be air-wound or wound on a ferrite rod Bidirectional as shown on next slide 35

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37 Helical Used to produce circular polarization Several turns of tubing, usually with a reflector A variant is used for FM broadcasting 37

38 Helical 38

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