Adv Topics Electrical Engineer
Adv Topics Electrical Engineer ECE 992
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This 3 page Class Notes was uploaded by Miss Felicita Stiedemann on Thursday October 29, 2015. The Class Notes belongs to ECE 992 at University of New Hampshire taught by Staff in Fall. Since its upload, it has received 13 views. For similar materials see /class/231689/ece-992-university-of-new-hampshire in Computer Engineering at University of New Hampshire.
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Date Created: 10/29/15
Antenna Arrays An array exists when 2 antennas elements in close proximity radiate at the same frequency at the same time Cancellation and reinforcement from those antennas will create an array pattern in space analogous to an antenna pattern For example consider 2 isotropic antennas point sources separated by a distance d transmitting inphase The received signal in this case will be J S MS ESS2Ene We S1 S2 Which can be made more meaningful after applying the farfield approximation AR ER 9 e JJR quotS e JAR quotSJ 2E 1J 605 sin 9 This will give the expected maximum at 0 since the signal from both elements will be inphase at that angle Antenna Arrays 2 If d12 this equation becomes AR 2E J cossini9 12 is the most commonly used element spacing for arrays By adding additional elements to the array more directional patterns can be obtained 39 For Example the array pattern for a 4element array with 12 element spacing is ERi9 2 elements 4 elements E R H Rcos quotAsin 6 cos 3 sin 6 These inphase elements form a broadside array Antenna Arrays 3 Changing the phase and amplitude relationships between the elements of an array will change the shape and direction ofthe radiation pattern Source For example an antenna over a ground plane and its image form an end re array In this case the two array elements are out of phase and the pattern is given by 392E Outof Phase ER t9 J Oe MR Sln h sm 9 Source Image R lfhi4 boresightwill be at 690 In general array patterns can be quotsteeredquot and shaped by manipulating element phase and amplitude relationships Arrays that are varied in this manner are called phased arrays Pattern Multiplication Enables us to account for the pattern of the elements in an array plus the pattern ofthe array itself Because isotropic antennas don t exist any antenna that is part ofan array will have an antenna pattern Ee 9 note the pattern is not a function ofR For example Ee 9 for a halfwave dipole is given by 0037ysin 9 a The subscript e stands for E 9 element 3 sing quot The effect ofthe element pattern on the pattern of the array is given by the product of the antenna pattern and the array factor ETotal a EA 92 Ee 92 The array factor is the pattern generated by the array of point sources Pattern Multiplication Example Find the radiation pattern of a verticallyoriented halfwave dipole at a height h above a perfectlyconducting ground plane The array factor is J EA9 2sin h sin 9 quot x And the element pattern is a r 639 A 80 the resultant pattern Ema will be Zoos 7139 sin 6 Em 0 sinb h sin 9 Array Example In order to reduce multipath interference and to increase the signal in the direction ofthe approaching aircraft an array is often used for the instrument landing system ILS glide slope The antennas in the array are 22 dipoles separated by 0951 Find the pattern for this array ifall elements are fed equalmagnitude inphase signals Solution Using the center antenna as the phase center ofthe array we can write the array pattern as 1 7 71270 935mg 12mm 95mm IEAnay 6I 3 1e 6 I 1 2cos197rsin6 Problem The first null occurs at a 6 corresponding to an aircraft at runway threshold That happens when cos197r sini9 5gti9 20540