Question

Find the number of turns in the helical antenna with directivity 120 and Cλ=1 and pitch angle 6°.

(a) 100

(b) 50

(c) 120

(d) 12

I have been asked this question in class test.

This intriguing question comes from Frequency Independent Antenna in portion Frequency Independent Antenna of Antennas

Answer 1

Right choice is (a) 100

The best explanation: Directivity of helical antenna D = 12CλnSλ

Pitch angle \(α=tan^{-1}⁡\frac{S_λ}{C_λ}\)

⇨ Sλ=Cλ tan α=1*0.1=0.1

D = 12CλnSλ

120=12×1×n×0.1

n=100 turns