1.1K.DHARSHANA

2. SPECIAL ANTENNAS Long wire antenna V antenna Rhombic antenna Yagi-Uda antenna Turnstile antenna Helical antenna Biconical antenna Log Periodic Dipole array antenna Spiral antenna Microstrip Patch 3. HF Antennas Frequency Range: 3 to 30 MHz ( 10 to 100 meters; in wavelength) For medium- and long- distance communications and broadcoasting 4. Frequency range: 3 MHz to 30 MHz Wavelength : 100m 10m Antenna-comparable in size with the wavelength Directional properties In HF band: ground - more losses Ionosphere - favorable conditions for HF broadcasting High Frequency Antenna 5. Types (1) Resonant or Periodic (2) Non-resonant or A-periodic Resonant Antennas resonant transmission line, exact no. of half wavelength long open at both ends Un-Terminated antennas Fixed frequency operation Standing wave, bidirectional pattern Cont 6. Non-resonant Antennas nonresonant transmission line, excited at one end terminated correctly in other end No standing waves Travelling wave antenna Wide band antenna, aperiodic Unidirectional radiation pattern Cont 7. Types Of HF Antennas: Non-Resonant HF Antennas Long-wire Antenna Vee Antenna Rhombic Antenna Resonant HF Antennas Monopole Antenna Dipoles and Slot Antennas Loop Antennas Log Periodic HF Antennas Early Log-Periodic Antenna Logarithmic Dipole Antenna Directional HF Antennas End-fire Arrays Broadside Arrays Circular Arrays 8. Non-Resonant HF Antennas: Wave propagates along the radiator in one direction only Remaining power is absorbed in a matched load TYPES Long-wire antenna Vee antenna Rhombic antenna 9. LONGWIRE ANTENNA 9 Non-resonant a-periodic antenna no reflected wave Larger Bandwidth It has End fire Pattern Uniform Current Distribution Acts as Travelling Wave Radiator RelativeCurrent RelativePhase Angle Distance along antenna Distance along antenna Wave Direction 10. Radiation Pattern 10 48 o 24 o 17 o Length of wire the lobes get closer & narrower Lobes = Equal amplitude & opposite in direction Angle of Major Lobe = Amplitude of major lobe = 1.25 to 5.8 17 68too o 11. The physical length of a long wire antenna (Length ) /2= feet if one half wavelength in length (Length)= feet if n half wavelength in length Long Wire Antennas )( )05.01(492 MHzf )( )05.0(492 MHzf n 12. The field strength E(r,,)= n is odd E(r,,)= n is even Long Wire Antennas r Irms60 sin cos 2 cos n r Irms60 sin cos 2 sin n 13. The field strength in case of non resonant antenna E(r,,)= Radiation Resistance Rr =73+69 log n Angle between Max. lobe and wire axis cos max=(n-1)/n Long Wire Antennas )cos1( sin60 r Irms cos1(sin L 14. 14 V ANTENNA V shaped bidirectional antenna Fed by Balanced Line at the apex Current out of phase Bidirectional Pattern Two Types: Resonant V antenna Non-Resonant V antenna 180o 15. 15 Radiation Pattern Add Characteristic impedance Unidirectional Pattern Lobes = 8 to 2 long Angle of Major Lobe = 36 72too o 16. Easy to Construct Cheap Both End-fire and Broad-side are constructed 16 Provides strong Minor Lobes On Light Aircraft Military appliances Commercial appliances 17. 17 RHOMBIC ANTENNA PRINCIPLE : Travelling Wave Radiator Double V or diamond shape antenna. Terminating Resistance = 600 to 800 ohms. Tilt angle = 90 180o out of phase current Horizontal & Vertical Polarization 18. Radiation Pattern Fed by BALUN (Balanced Tunable Transformer) system. Unidirectional Pattern Lobes = 2 to 8 long Directivity : 20 to 90 Power Gain : 50 to 60 value = 17 24too o 19. The tilt angle () The leg length (L) The height above the ground (h) Two designs (a) Alignment design (b) Maximum output design Design of Rhombic antenna 20. (a) Alignment design h is chosen that max of the main lobe coincides with the desired angle of elevation Design Of Rhombic Antenna sin4 h 2 sin 37.0 L 90 21. (b) Maximum field intensity design h is chosen that max filed intensity is obtained at the desired elevation angle (i) If his less than needed for these designs, alignment may be obtained by increasing length L (ii) If h is maintained and L is reduced , then alignment obtained by changing (iii) If both h and L are reduced, then alignment obtained by changing Design of Rhombic antenna 22. Design Of Rhombic Antenna The relative filed intensity E To design a rhombic antenna for specific direction of transmission )sincos1( )sincos1(sinsin 2 sincos2 2 Lh E )sincos1( )sincos1('sinsin'2sincos2 2 Lh E 23. Design of rhombic antenna h=h/ and L=L/ E=k sin(2h sin) where Also E=k [sin L (1-cos sin)]2 where )sincos1( )sincos1('sincos2 2 L k )sincos1( sin'2sincos2 ' h k 24. Design of rhombic antenna Max output sin4 ,...4,3,2,1,2/sin 2 0sin 2 cos 0 ' n h nn h h h E 25. )sincos1(2 2/1)sincos1(' 2/)sincos1(' 0)]sincos1('sin[ 0 L L L L L E Design of Rhombic Antenna =90- sin=cos L=/2sin2 meters 26. Highly directional Simple & cheap Input impedance is 26 Needs larger space for installation Both horizontal & vertical polarizations are produced Efficiency is Low HF transmission & reception Commercial Point to point communication 27. 27 YAGIUDA ANTENNA Most high gain antennas 3 elements: Driven element Parasitic element Spacing = 0.1 to 0.15 Parasitic element Director Reflectors Dipole impedance = 73ohms 28. Yagi-Uda Antenna (Yagi) One driven element and one or more parasitic elements Operates over a small range of frequencies (not a wide-band antenna) 29. Yagi Antenna 3 elements Associated radiation pattern A 3 element Yagi 30. 3 element array of yagi antenna Reflector length=500 / f (MHz) feet Driven element length=475/f (MHz)feet Director length=5 / f (MHz) feet Use of parasitic elements cause dipole impedance to fall below 73 Low as 25 use shunt feed or folded dipole Yagi uda Antenna 31. CHARACTERISTICS Length of Parasitic Element Parasitic Element Action Inductive (Director) Phases of currents will lag the induced voltage < Capacitive (Reflector) Phases of currents will lead the induced voltage / 2 / 2 Unidirectional antenna Gain = 8db & FBR = 20db Fixed frequency device 32. 3 element array beam antenna Unidirectional beam with light weight, low cost and simplicity in feed design Gain-order of 8dB Front to back ratio=20dB Super gain or Super directive antenna Fixed frequency device Yagi uda antenna-Characteristic 33. One driven element and one parasitic element V1=I1Z11+I2Z12 V2=I1Z21+I2Z22 Since Z21=Z12, Z13=Z31 , V2=0 V1=I1Z11+I2Z12 (1) 0=I1Z12+I2Z22 (2) Voltage and current relations 34. Voltage and current relations From (2) I1Z12=-I2Z22 I2=-I1(Z12/Z22) Substitute I2 in (1) V1=I1Z11- I1(Z12/Z22) Z12 22 2 12 11 1 1 Z Z Z V I 12 2211 12 1 2 Z ZZ Z V I 35. Voltage and current relations The input impedances of driven and parasitic elements 12 2211 12 2 2 2 22 2 12 11 1 1 1 Z ZZ Z I V Z Z Z Z I V Z 36. Log-Periodic Antenna Lengths of driven elements are related logarithmically The longest element has a length of the wavelength of the lowest frequency The shortest element is the wavelength of the highest frequency Advantage is very wide bandwidth 37. Ratio of two adjacent elements is held constant, usually 0.7-0.9 Log-Periodic Antenna 3 4 2 3 1 2 3 4 2 3 1 2 ... D D D D D D and L L L L L L 38. Electrical properties must repeat periodically with the logarithm of frequency Basic geometric structure that is repeated but with a changing size Structure size changes with each repetition by a constant scale factor so that the structure expand or contract Log periodic Antenna 39. LPDA All the dimensions increase in proportion to the distance from the origin Log Periodic Dipole Array 40. No. of dipoles of different lengths and spacings Fed by a balanced two wire tXion line Included angle is constant Length =L Spacing=R Scale factor or design ratio