1. Oct. 16, 2006 Midterm Next Class Assignment #4 is Marked
Problem session in lab today – review for midterm
More Antennas

2. Midterm Overview Midterm Info: Sample questions:
Sample questions:

3. Antenna Characteristics
Gain and Directivity
Radiation Pattern
Polarization
Input Impedance
Antenna Efficiency
Bandwidth
Reciprocity

4. Bandwidth
Range of frequencies over which antenna can transmit or receive signals effectively
Can apply to a certain antenna
Measurement is (Upper Frequency-Lower Frequency)/(Upper Frequency)
e.g. 2.4GHz Dipole antenna may be able to transmit signals from 2.28 to 2.52GHz without significant loss of signal power. BW = ( )/2.4 = 10%
Can apply to a certain type of antenna
E.g. Half-wave dipole antennas in general have a bandwidth of 6GHz (0-6GHz) as long as the specific dipole size is set to the half-wave length

5. Antenna Input Impedance
Want input impedance to match impedance of incoming cable
Minimize reflections
Maximize signal in to antenna
Eliminate VSWR
Impedance mismatch will reduce the antenna efficiency

6. Reciprocity An antenna can be used for transmitting and receiving
If you know the transmit properties of an antenna, you know the receive properties of an antenna…they are the same
Applies to: Gain, Radiation Pattern, Polarization, Input Impedance, Efficiency, and Bandwidth

7. Cutoff for Midterm Topics

8. Types of Antennas Omnidirectional Directional Half-wave dipole
Quarter-wave monopole
Directional
Yagi-Uda antenna
Patch antenna
Parabolic Dish
Phased Array
Sector Antennas

9. Omnidirectional Antennas
Ideally, radiates energy equally in all directions
Isotropic antenna
In reality, radiates energy equally in one plane, but not in other plane
Omnidirectional in one plane

10. Half-Wave Dipole
Dipole antenna consists of two conductors which stretch out in opposite directions
Best dipole is when length is equal to half the wavelength of EM being radiated

11. Half Wave Dipole Characteristics
Gain: 2dBi
Typical Beamwidth: 80° x 360°
Omnidirectional in horizontal plane
Beamwidth in vertical plane can vary
Polarization: linear, and perpendicular to antenna
Bandwidth: 10%.
Effective from 0-6GHz

12. Half Wave Dipole Radiation Pattern

13. Quarter Wave Monopole
One conductor carrying the signal referenced to a ground plane
Virtual mirror image of conductor created on opposite side of plane
2 x λ/4 = λ/2. This is effectively a half-wave dipole

14. λ/4 Monopole Characteristics
Gain: 2-6dBi
Typical Beamwidth: 45° x 360°
Omnidirectional in horizontal plane
Beamwidth in vertical plane can vary
Polarization: linear, and perpendicular to antenna
Bandwidth: 10%.
Effective from 0 to very high

15. λ/4 Monopole Radiation pattern

16. Directional Antennas Focus coverage more in one direction than others
Refers to all antennas that are not omnidirectional. Wide range of degrees of directionality
Semidirectional: smaller gain, wider coverage region
Highly directional: high gain, focused strongly in one direction, long distance range

17. Yagi Antenna
One radiating dipole, one reflector to reflect signal in opposite direction, several directors to focus energy
Size and number of directors determines directivity and gain

18. Yagi Antenna Characteristics
Gain: 5-15dBi
Typical Beamwidth: 50° x50°
Polarization: Horizontal
Bandwidth: 5%
Effective from 50MHz to 2.5GHz

19. Yagi Antenna Radiation Pattern

20. Patch or Panel Antenna
Consists of metallic plate and base sandwiching an insulating material
Easy to manufacture
Easy to mount
Similar to Yagi Antennas in terms of performance
Better than Yagis for cost, ease of use and aesthetics

21. Patch/Panel Antenna Characteristics
Gain: 5-18dBi
Typical Beamwidth: 50° x50°
Polarization:
Bandwidth:
Effective from XXXHz to XXX Hz

22. Patch/Panel Antenna Radiation Pattern

23. Parabolic Dish or Grid Antenna
Highly directional antennas, beamwidth can be as small as a couple of degrees
Used for fixed location, long distance communications
E.g. satellite, microwave towers
Parabolic dish or grid focuses energy in one direction.

24. Dish Antenna Characteristics
Gain: 20-30dBi
Beamwidth: 1-10 degrees
Polarization: can be linear or circular
Bandwidth: 30%
Effective from 400MHz to 13GHz

25. Parabolic Dish Radiation Pattern

26. Phased Array Antennas Group of independently fed active elements
Input signals can be controlled to cause required constructive/deconstructive waveforms
Result: antenna can be aimed without any moving parts

27. Phased Array Antenna Characteristics
Gain: Varies widely
Beamwidth: Varies Widely
Polarization: depends on array.
Bandwidth: depends on array.
Effective from 10MHz to 10GHz
Radiation Pattern: depends on array

28. Sector Antennas Generate “Pie Slice” radiation patterns
“Pie Slice” width depends on particular antenna
Because of predictability of slice, multiple antennas may be mounted together to give larger coverage area
E.g. want 270° of coverage, but have only 90° sector antennas, what do you do.

29. Sector Antenna Characteristics
Gain: 10-21dBi.
A 90° sector antenna can still have variable gain by changing the vertical pattern
Beamwidth: Depends on Pie Slice size
Polarization: linear
Bandwidth:
Effective from

30. Sector Antenna Radiation Pattern

31. Increase coverage area by joining three 120° antennas together.
Omnidirectional in horizontal plane, controllable in vertical plane.

32. Antenna Gain (dBi) Beamwidth Polarization Bandwidth Lower F Upper F
λ/2 Dipole 2
80x360
Linear
10%
None
6GHz
λ/4 Monopole
2-6
45x360
Parabolic
20-30
1-10
33%
400MHz
13GHz
Yagi
5-15
50x50 5%
50MHz
2GHz
Patch
8-18
Varies
Array
varies
10MHz
10GHz
Sector
8-21

33. Review What are lobes and nulls?
If you have a right handed circularly polarized transmission antenna, what is the best kind of antenna to use as the reception antenna?
If the gain of an antenna is 5dBi for transmission, and its beamwidth for transmission is 90°x45°, what would the gain and beamwidth be if it was used for reception? What is this principle called?
A 1 watt signal is input to an antenna, but 200mW is reflected. The gain of the antenna is 10dBi, and the loss of power due to resistance is 50mW. What is the efficiency of this antenna?

34. Review
What happens if there is an impedance mismatch between the antenna, and the cable coming into the antenna?
The center frequency of an antenna is 104.7MHz, and at 104.7MHz it radiates 50 Watts of power. If it radiates a 100MHz signal, it can only output 12.5W of power, and at 110MHz, it can only output 12.5W of power. What is the bandwidth of this antenna?

35. Review In terms of directionality, what kind of antenna is this ?
What are the horizontal and vertical beamwidths?
In the vertical plane, what is the signal strength at 45° if the strength at 90° is 50mW?

36. Review
Describe at least two ways to provide full wireless coverage to a large rectangularly shaped warehouse. Explain the types of antennas you would use and why.
If you have need for long distance point to point communications, what kind of antenna would you use.
What would you use a Yagi antenna for?

37. Resources http://www.arrl.org/tis/info/whyantradiates.html