Antennas: Assignment 9

1. A helical antenna with pitch angle equal to 0° or 90° is similar to _______________ or _____________ antenna, respectively.

Loop, Linear

Loop, Slot

Linear, Loop

 Slot, Loop

Ans: a

2. As the number of turns of an axial mode helical antenna increases, its HPBW:

Increases 

Decreases

Remains almost constant 

First increases then decreases

Ans: b

3. A 4-turn helical antenna shown in Fig. 1 has diameter D = 2 cm and spacing S = 4 mm. The axial length and total wire length of the antenna are:

12 mm, 25.2 mm

12 mm, 188.9 mm

16 mm, 221.9 mm

16 mm, 251.8 mm

Ans: d

Common Data for Questions 4-6: An axial mode helical antenna is to be designed for 22 dBi gain at 500 MHz with following conditions: C_λ = 1; α = 13° and efficiency ƞ = 60%.

4. The required number of turns are:

96

48

40

20

Ans: a

 

5. The approximate HPBW is: 

5o$5^o$

8o$8^o$

11o${11}^o$

16o${16}^o$

${\mathrm{<br>}}^{}$

${\mathrm{Ans:\; c}}^{}$

6. If 2 x 2 array of helical elements is used instead of a single helix, then the number of turns per helix and the distance between the helical elements will be: 

48, 68.8 cm

24, 137.5 cm

12, 275 cm

6, 550 cm

Ans: b

7. If the wire radius of a normal mode helical antenna increases, then its bandwidth:

Decreases

Increases

Remains almost same

First increases then decreases

Ans: b

 

8. The condition to obtain circular polarization in a normal mode helical antenna is:

Cλ2=2Sλ${C_{\lambda }}^2=2S_{\lambda }$

Cλ2=2Sλ2${C_{\lambda }}^2=2{S_{\lambda }}^2$

Cλ=2Sλ$C_{\lambda }=2S_{\lambda }$

Cλ=2Sλ2$C_{\lambda }=2{S_{\lambda }}^2$

Ans: a

 

9. A normal mode helical antenna is designed with following parameters: diameter of helix (D) = 0.015λ, axial length of helix (A) = 0.15λ and number of turns (n) = 5.

The approximate axial ratio (in dB) for this antenna is:

14.3

19.2

28.6

38.4

Ans: c

 

10. For N x N array of helical antennas, if the distance between the helical elements increases then the directivity of the antenna:

Increases 

Decreases

Remains almost constant

First decreases then increases               

Ans: a