1 / 71

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.

boone
Télécharger la présentation

Antennas

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related