Antennas Questions and Answers – Frequency Independent Antenna – Design of LPDA

This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Frequency Independent Antenna – Design of LPDA”.

1. What is the ratio of maximum frequency to minimum frequency if the scaling factor is 0.5 for 5 elements LPDA?
a) 16
b) 2
c) 4
d) 8
View Answer

Answer: a
Explanation: The relation between frequency ratio and the scaling factor is given by
\(\frac{f_{max}}{f_{min}} = \frac{1}{τ^{N-1}} \)
\(\frac{f_{max}}{f_{min}} =\frac{1}{τ^{N-1}} =\frac{1}{(0.5)^{5-1}}=2^4=16. \)

2. The value of periodicity factor in LPDA is _____
a) < 1
b) >1
c) ≥1
d) =0
View Answer

Answer: a
Explanation: The periodicity factor also known as the scaling factor is the ratio of the adjacent lengths of the dipole \(\frac{L_N}{L_{N+1}} = τ \,and\, L_N < L_{N+1}\) where L is the length of the dipole.

3. The longest and shortest dipole lengths are taken in the form of the wavelength of the operating frequencies in LPDA.
a) True
b) False
View Answer

Answer: a
Explanation: The longest length of the dipole is taken as the λu/2 where λu is the wavelength corresponding to upper frequency and shortest dipole length is taken as λl/2. In LPDA, the adjacent dipole spacing ratios and their length ratios are equal.

4. The relation between the spacing factor σ and the scaling factor τ is given by ____
a) \(σ=tan^{-1}(\frac{1-τ}{4σ}) \)
b) \(σ=tan^{-1}(\frac{1-τ}{2σ}) \)
c) \(σ=tan^{-1}⁡(\frac{4σ}{1-τ}) \)
d) \(σ=tan^{-1}⁡(\frac{2σ}{1-τ}) \)
View Answer

Answer: a
Explanation: In LPDA, the ratio of successive spacing of elements is equal to the ratio of adjacent dipole lengths. The spacing factor is \(σ=\frac{d_n}{2L_n} \,and\, d_n =\) spacing betwwen adjacent elements ‘n’ and ‘n+1’ and Ln is the length of nth dipole. The relation between the spacing factor σ and the scaling factor τ is \(σ=tan^{-1}(\frac{1-τ}{4σ}). \)

Sanfoundry Global Education & Learning Series – Antennas.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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