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Antennas

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.

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Antennas

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  1. Antennas

  2. 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

  3. Half-Wave Dipole • Simplest practical antenna • Actual length is typically about 95% of a half wavelength in free space

  4. 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 70 ohms • Antennas also have actual resistance due to their conductors

  5. Antenna Efficiency

  6. 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)

  7. 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

  8. dBd/dBi Conversion • Gain (dBi) = Gain (dBd) + 2.14 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!)

  9. 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

  10. 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

  11. EIRP/ERP Conversion • EIRP = ERP + 2.14 dB • EIRP is used in all our equations • Sometimes government regulations specify ERP for transmitting installations • Conversion is easy (see above)

  12. Dipole Impedance • At resonance, Z = 70  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)

  13. Dipole Polarization • Polarization is same as axis of wire: • Vertical dipole is vertically polarized • Horizontal dipole is horizontally polarized

  14. 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

  15. 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

  16. 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

  17. 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

  18. 1/4 wave monopole with ground plane for 144 MHz

  19. AM Transmitter Tower (The tower is the antenna)

  20. 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

  21. 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

  22. Discone Antenna • Very wide bandwidth • Often used for wideband receiving applications such as scanners

  23. Discone antenna for 25-1300 MHz with whip antenna for transmitting on ham bands

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

  25. 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

  26. 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

  27. Loading Coil

  28. Antenna Arrays • Simple antennas can be combined to achieve desired directional effects • Individual antennas are called elements and the combination is an array

  29. 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

  30. 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

  31. Yagi for 14, 21, 28 MHz Amateur Bands

  32. UHF-TV Antenna: Yagi with Corner Reflector

  33. 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

  34. UHF Yagi with reflector VHF LPDA VHF/UHF TV Antenna

  35. Turnstile Antenna • 2 dipoles • 90 degrees between them • fed 90 degrees out of phase • mounted horizontally • Gives an omnidirectional pattern in horizontal plane with horizontal polarization

  36. Turnstile Antenna for FM Broadcast Band

  37. 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

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