Antennas: Assignment 4:

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Common data for Questions 1 to 5: A 6-element linear array of isotropic antennas with equal spacing of 0.55λ_o is fed with equal amplitude and same phase. For this array,

1. The approximate directivity is:

 3.8 dBi

 5.6 dBi

 7.4 dBi

 10.4 dBi

Ans: c

2. The approximate HPBW is:

 24.6°

 18.5°

 15.4°

 9.2°

Ans: b

3. The direction of first null from the array axis is:

 38.4°

 52.6°

 66.8°

 72.4°

Ans: d

4. The approximate direction of first side lobe level from the array axis is:

 78°

 63°

 51°

 40°

Ans: b

5. The magnitude of first side lobe level is:

 -13.2dB

 -16.5dB

 -18.2dB

 -19.6dB

Ans: a

6. An array fed with equal amplitude and progressive phase shift is designed to scan at 20° from the array axis. To avoid grating lobes, the maximum allowed inter-element separation is:

 0.915λ

 0.710λ

 0.516λ

 0.425λ

Ans: c

Common data for Questions 7 and 8: A square array of isotropic antennas with inter-element spacing of 0.6λ_o is designed to realize broadside radiation pattern with a gain of 22dBi. (Assume efficiency is 0.85 due to losses in the feed network)

7. The minimum number of elements in the array should be:

 5 x 5

 8 x 8

 12 x 12

 17 x 17

Ans: b

8. Approximate HPBW in both the E- and H- planes is:

 21°

 17°

 13°

 5°

Ans: c

9. A broadside square array of isotropic elements provides 25 dBigain. If this array is used to  scan at an angle 60° from the broadside using progressive phase difference, the gain of the array in dBi will be:

 28

 25

 22

 19

Ans: c

10. A rectangular array is designed at 2.45 GHz using 12 elements with spacing = 6 cm in x-direction and 10 elements with spacing = 8 cm in the y-direction. It is fed with equal amplitude and same phase. Directivity of the array is:

 21 dBi

 26 dBi

 29 dBi

 34 dBi

Ans: b