ECE 5318/6352 Antenna Engineering. Dr. Stuart Long. Chapter 15. Reflector Antennas

Transcription

1 ECE 5318/6352 Antenna Engineering Dr. Stuart Long Chapter 15 Reflector Antennas 1

2 GEOMETRICAL CONFIGURATIONS large flat sheet small flat sheet (wide freq. range) corner reflector (narrow freq. range) parabolic reflector 2

3 FLAT REFLECTOR Image Theory Analysis image d driven element Source polarization and spacing used to control radiating properties 3

4 FLAT REFLECTOR Gain relative to a λ/2 dipole in free space G 2 1 imperfect conductors losses decrease gain at small spacing larger spacing less gain more bandwidth d/λ.5λ λ 4

5 CORNER REFLECTOR Electrostatic Imaging for 180 = n 2 α φ #14, 4 1 #23, 3 images in phase in phase but negative 3 E φ 2πd cos λ 2πd cosφ cos λ sinφ 5

6 CORNER REFLECTOR Gain region w/ gain < 0 db main lobe in another direction 6

7 CORNER REFLECTOR Multilobes d = 0.5λ G = 3.1 if spacing gets too large, multilobe patterns in real plane d = 1.0λ G = 3.8 d = 1.5λ G = 4.4 7

8 CORNER REFLECTOR Multilobes appear when α (multilobe) 0.3λ 0.7λ 0.95λ 1.2λ 2.5λ 1 st. max. at d= 0.2λ 0.5λ 0.65λ 0.85λ 1.2λ 8

9 Design and Geometry CORNER REFLECTOR NOTE point A is 1.4 S from corner the only waves reflected from an infinite case but not from the finite case are those radiated into sector η 9

10 CORNER REFLECTOR Design and Geometry absence of reflector beyond point B does not have a large effect -slightly larger beamwidth -null not exactly at 45 (at a somewhat larger φ) L 2 d is good practical size 10

11 Design and Geometry CORNER REFLECTOR NOTE L 1.73d α=60 d A B η φ for smaller α gain goes up; but need to use larger d and point A is now at 1.73 d from corner larger reflector size needed only small increase in gain 11

12 Practical Design CORNER REFLECTOR 2d 0.35λ 0.7λ 1.0 λ L= 2d d = 0.35λ aperture of 1.0λ 12

13 Practical Design CORNER REFLECTOR 2d 0.5λ 1.0λ 1.4 λ L= 2d d = 0.5λ aperture of 1.4λ 13

14 Wire-grid arrangement CORNER REFLECTOR 2d 2d h to reduce wind resistance and weight a grid of wires can be used for the reflector g g < 0.1λ h > 0.6λ (to prevent spill-over ) 14

15 PARABOLIC REFLECTOR aperture given a point source want to produce a plane wave front over an aperture want path lengths from source to reflector to aperture plane to be equal 15

16 PARABOLIC REFLECTOR Design and Geometry R cosθ want 2L = R (1 + cosθ ) A Q P B S R path FPB path FOF 2L = (1 + cosθ ) O L R θ F eqn. of a parabola with focus at F B A 16

17 PARABOLIC REFLECTOR Design and Geometry B A R cosθ Q P S also means that distance from point P on the parabola to the focus at F is equal to the distance to a fixed line directrix O L R θ F B A 17

18 PARABOLIC REFLECTOR Design and Geometry PF PF = PQ PS = QS QP + PS = PF + QS PQ = QS Q P B A R cosθ S - thus all waves from an isotropic source at the focus, reflected from a parabola arrive at line AA with equal phase O L R θ F - wave AA appears to have come from a plane wave at the directrix Q B A 18

19 Design and Geometry PARABOLOID (PARABOLA OF REVOLUTION) a portion A of the source radiation is intercepted by the paraboloid and reflected as a plane wave of circular cross-section 19

20 Design and Geometry PARABOLOID feed source nλ choose L = with n odd 4 so direct radiation from feed source reinforces central region of reflected waves 20

21 Design and Geometry PARABOLOID feed source L for small, outer edges of D paraboloid get smaller illumination for more uniform, make illumination smaller by increasing L and keeping D the same L D large 21

22 Design and Geometry PARABOLOID feed source for optical telescopes D L F L D is very large 22

23 Design and Geometry PARABOLOID feed source Polarization depends on primary feed typically horn linearly polarized presence of primary source in path of reflected waves -cause mismatch -obstruction (blockage) Can displace feed 23

24 Feeds PARABOLOID feed source front-fed Cassegrain-fed 24

25 CORNER REFLECTOR VS. PARABOLA feed source NOTE wave from corner reflector travels shorter distance by (OO ) for AF = 2λ OO' = 0.5λ different patterns for AF λ =.35λ OO' = same patterns 12 25

26 CORNER VS. PARABOLA feed source if AF is small, the exact shape of the reflector is unimportant practical advantage of corner is its simplicity and ease of construction 26

27 YAGI-UDA ARRAYS sr sd lr l ld reflector driven element directors 27

28 YAGY UDA Characteristics Forward gain G Input impedance Z Typical G = 8 12 [ db] Front to back ratio F-B F to B = [ db] Sidelobe ratio SLR SLR = 10 [ db] 28

29 YAGY UDA Design l λ = 2 ( slightly less) s r doesn' t affect gain much l d = λ s r does affect Z and F-B l affects Z but not gain l r = λ s d and l d affect gain and impedance s d = λ s r = 0.25λ 29