How To Know What An Antenna Is For A Microwave Antenna

Transcription

1 Microwave Antennas and Radar Maria Leonora Guico Tcom nd Sem Lecture 8

2 G P P directional isotropic Antenna Basics Isotropic Dipole High gain directional 0 db i 2.2 db i 14 db i

3 Antenna performance half-power beam width Sample calculation Parabolic antenna for sat com Beam Width 3dB BW 70 D[ m] [ m]

4 Microwave Antennas Conventional antennas can be adapted to microwave use The small wavelength of microwaves allows for additional antenna types e.g. high-gain antennas of reasonable size The parabolic dish is a reflector not an antenna but we saw that it is most practical for microwaves. Most common feed antenna for use with parabolic dish is the horn.

5 Horn Antennas Not practical at low frequencies because of size Can be E-plane, H-plane, pyramidal or conical Moderate gain, about 20 dbi Horn antennas have excellent gain and directivity The longer the horn, the greater the gain and directivity

6

7 Types of Horn Antennas Flared in one dimension Sectoral Flared in two dimensions Pyramidal Conical

8 Gain and directivity are a function of the horn s dimensions Most important are the length, aperture area and flare angle The length is usually 2λ to 15λ, at the operating frequency The greater the aperture area, the higher the gain and directivity Flare angles range from 20 to 60 Horn Antennas

9 Slot Antenna Slot in the wall of a waveguide acts as an antenna Slot should have length g /2 Slots and other basic antennas can be combined into phased arrays with many elements that can be electrically steered

10 Low Frequency Antennas At frequencies less than 2 GHz, standard antennas are used (Dipole, Yagi, etc.) One variation is to use a corner reflector Better reflection than a rod reflector (Yagi) Gain is db

11 Beam Width Beam width can be measured horizontally or vertically. Horizontal Beam Width Vertical Beam Width

12 Gain The Gain of a Pyramidal Horn (Power Ratio): G = 7.5d E d H λ 2 G = gain as a power ratio,wrt an isotropic radiator d E = E-plane aperture d H = H-plane aperture λ = wavelength To find the gain in decibels use db = 10 Log P

13 Beam Width (H-plane) Beam Width for a Pyramidal Horn is different in two directions In the H-plane, it is: θ H = 70 λ d H θ H = H-plane beamwidth in degrees λ = wavelength of operating frequency d H = H-plane aperture For a Pyramidal or Circular Horn, the vertical and horizontal beam widths are about the same

14 Beam Width (E- plane) Beam Width for a Pyramidal Horn is different in two directions In the E-plane, it is: θ E = 56 λ d E θ E = E-plane beamwidth in degrees λ = wavelength of operating frequency d E = E-plane aperture For a Pyramidal or Circular Horn, the vertical and horizontal beam widths are about the same

15 Example 1 A pyramidal horn antenna has an aperture (opening) of 58 mm in the E plane and 78 mm in the H plane. The operating frequency is 10 GHz. Calculate : a) its gain in dbi b) the beamwidth in the H plane c) the beamwidth in the E plane

16 Solution to Example 1 a) At 10 GHz the wavelength is: λ = c/f = 3 x 10 8 /10 GHz = 0.03 m G = 7.5d E d H = 7.5 x x = 37.7 λ G (in dbi) = 10 Log 37.7 = 15.8 dbi b) θ H = 70 λ d H = 70 x 0.03/0.078 = c) θ E = 56 λ d E = 56 x 0.03/0.058 = 29 0

17 Bandwidth Horns operate over a wide frequency range The bandwidth of a typical horn is approximately 10% of the operating frequency At 10 GHz, the bandwidth is approximately 1 GHz This is a very large bandwidth and can accommodate almost any modulating signal

18 Parabolic Antennas Using a horn in conjunction with a parabolic reflector provides higher gain and directivity Energy radiated by the horn is focused by the reflector into a narrow beam Beam widths of only a few degrees are typical with a parabolic reflector

19 Can be used for transmit and receive Any common antenna type (dipole, etc) can be used with a parabolic reflector Most parabolic reflectors are designed so that the diameter is no less than λ at the lowest operating frequency Parabolic Antennas

20 Beam Width The Beam Width of a Parabolic Reflector is: B = 70 D/λ (degrees) D = diameter of reflector (m) λ = wavelength of operating frequency Beam Width is inversely proportional to diameter

21 Gain The Gain of a Parabolic Reflector (Power Ratio): G = 6 D λ [ ] 2 G = gain, expressed as a power ratio D = diameter of dish, m λ = wavelength, m To find the gain in decibels use db = 10 Log P

22 Radar Radar stands for Radio Detection And Ranging Two main types Pulse radar locates targets by measuring time for a pulse to reflect from target and return Doppler radar measures target speed by frequency shift of returned signal It is possible to combine these 2 types

23 Radar Cross Section Indicates strength of returned signal from a target Equals the area of a flat conducting plate facing the source that reflects the same amount of energy to the source

24 Radar Equation Expression for received power from a target P R 2 P G T r 4 where P R = received power in watts λ = free-space wavelength P T = transmitted power in watts G = antenna gain as a power ratio σ = radar cross section of the target in square meters r = range (distance to the target) in meters

25 Pulse Radar Direction to target found with directional antenna Distance to target found from time taken for signal to return from target R = ct 2 where R = distance to the target c = velocity of light t = time taken for the echo to return

26

27 Maximum Range Limited by pulse period If reflection does not return before next pulse is transmitted the distance to the target is ambiguous R max = ct 2

28

29 Minimum Range If pulse returns before end of transmitted pulse, it will not be detected R min = ct P 2 A similar distance between targets is necessary to separate them

30 Doppler Radar Motion along line from radar to target changes frequency of reflection Motion toward radar raises frequency Motion away from radar lowers frequency

31

32 Doppler Effect The equation for Doppler effect is: where f D = Doppler shift in hertz f D 2v r c f i v r = relative velocity of the source and target in meters per second along a line between them (positive if the two are getting closer ) f i = incident frequency in hertz c = velocity of light in m/s

33 Limitations of Doppler Radar Only motion towards or away from radar is measured accurately If motion is diagonal, only the component along a line between radar and target is measured

34 Stealth Used mainly by military planes, etc to avoid detection Avoid reflections by making the aircraft skin absorb radiation Scatter reflections using sharp angles