Half-Wave Dipole. Radiation Resistance. Antenna Efficiency

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1 Antennas Simple Antennas 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 Half-Wave Dipole Simplest practical antenna Actual length is typically about 95% of a half wavelength in free space L = f Radiation Resistance Antenna Efficiency 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 70 ohms Antennas also have actual resistance due to their conductors η = P r = P T R R r T 1

2 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) Antenna Gain Specifications 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 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!) 2

3 Gain and Directivity Directivity is a theoretical value ignoring losses Gain includes losses As a ratio, gain = directivity efficiency Specifications give gain, but computer models often find directivity 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 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) Dipole Impedance At resonance, Z = 70 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) Dipole Polarization Polarization is same as axis of wire: Vertical dipole is vertically polarized Horizontal dipole is horizontally polarized 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 3

4 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 Folded Dipole Antenna 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 Monopole Antenna Vertical 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 1/4 wave monopole with ground plane for 144 MHz 4

5 AM Transmitter Tower (The tower is the antenna) Loop Antennas 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 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 Discone Antenna Very wide bandwidth Often used for wideband receiving applications such as scanners 5

6 Helical Antenna Used to produce circular polarization Several turns of tubing, usually with a reflector A variant is used for FM broadcasting Discone antenna for MHz with whip antenna for transmitting on ham bands 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 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 This is often done with handheld transceivers Antenna Arrays Simple antennas can be combined to achieve desired directional effects Individual antennas are called elements and the combination is an array Loading Coil 6

7 Types of Arrays Broadside: maximum radiation at right angles to main axis of antenna End-fire: maximum radiation along the main axis of antenna Phased: all elements connected to source Parasitic: some elements not connected to source They re-radiate power from other elements Yagi-Uda Array Often called Yagi array Parasitic, end-fire, unidirectional One driven element: dipole or folded dipole One reflector behind driven element and slightly longer One or more directors in front of driveh element and slightly shorter Yagi for 14, 21, 28 MHz Amateur Bands UHF-TV Antenna: Yagi with Corner Reflector 7

8 Log-Periodic Dipole Array Multiple driven elements (dipoles) of varying lengths Phased array Unidirectional end-fire Noted for wide bandwidth Often used for TV antennas Turnstile Antenna VHF/UHF TV Antenna UHF Yagi with reflector VHF LPDA 2 dipoles 90 degrees between them fed 90 degrees out of phase mounted horizontally Gives an omnidirectional pattern in horizontal plane with horizontal polarization Turnstile Antenna for FM Broadcast Band 8

9 Monopole Array Vertical monopoles can be combined to achieve a variety of horizontal patterns Patterns can be changed by adjusting amplitude and phase of signal applied to each element Not necessary to move elements Useful for AM broadcasting Collinear Array All elements along same axis Used to provide an omnidirectional horizontal pattern from a vertical antenna Concentrates radiation in horizontal plane Broadside Array Bidirectional Array Uses Dipoles fed in phase and separated by 1/2 wavelength 9

10 End-Fire Array Similar to broadside array except dipoles are fed 180 degrees out of phase Radiation max. off the ends Stacked Yagis Stacking in-phase Yagis with halfwavelength vertical spacing Reduces radiation above and below horizon Increases gain in plane of the antenna 10

11 Plane Reflector Mount antenna 1/4 wavelength from flat metallic surface Reflected wave and direct wave are in phase along normal to survace Increases radiation in that direction Corner Reflector More focused radiation than plane reflector Often used with UHF TV antennas Parabolic Reflector All radiation emitted at focus emerges in a beam parallel to the axis Gives a narrow beam Suitable mainly at microwave frequencies because it must be large compared with the wavelength UHF-TV Antenna: Yagi with Corner Reflector 11

12 Parabolic Reflector Beamwidth Beamwidth at half-power points 70λ θ = D Parabolic Reflector Gain As a power ratio (not db) With respect to isotropic G = 2 2 π D 2 λ 12