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

5. 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

6. Antenna Efficiency

7. 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)

10. 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

12. 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!)

13. 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

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

15. 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)

16. 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)

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

18. 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

19. 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

21. 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

23. 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

24. 1/4 wave monopole with ground plane for 144 MHz

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

27. 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

29. 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

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

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

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

33. 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

34. 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

35. Loading Coil

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

37. 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

38. 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

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

42. UHF-TV Antenna: Yagi with Corner Reflector

43. 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

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

46. 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

48. Turnstile Antenna for FM Broadcast Band

49. 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

52. Collinear Array All elements along same axis
Used to provide an omnidirectional horizontal pattern from a vertical antenna
Concentrates radiation in horizontal plane

54. Broadside Array Bidirectional Array
Uses Dipoles fed in phase and separated by 1/2 wavelength

57. End-Fire Array
Similar to broadside array except dipoles are fed 180 degrees out of phase
Radiation max. off the ends

59. Stacked Yagis
Stacking in-phase Yagis with half-wavelength vertical spacing
Reduces radiation above and below horizon
Increases gain in plane of the antenna

61. 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

63. Corner Reflector More focused radiation than plane reflector
Often used with UHF TV antennas

65. UHF-TV Antenna: Yagi with Corner Reflector

66. 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

70. Parabolic Reflector Beamwidth
Beamwidth at half-power points

71. Parabolic Reflector Gain
As a power ratio (not dB)
With respect to isotropic