Electromagnetic theory miscellaneous Easy Questions and Answers | Page - 33

Electromagnetic theory miscellaneous

The line of sight communication requires the transmit and receive antenna to face each other. If the transmit antenna is vertically polarized for best reception the receive antenna should be—

1. Horizontally polarized
1. Vertically polarized
1. At 45º with respect to horizontal polarization
1. At 45º with respect to vertical polarization

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For the best reception the receiver antenna should be in vertical polarization so that there is no phase reversal and fade of field between them.

Correct Option: B

For the best reception the receiver antenna should be in vertical polarization so that there is no phase reversal and fade of field between them.

In a uniform linear array four isotropic radiating elements are spaced λ/4 apart. The progressive phase shift between the elements required for forming the main beam at 60º off the end fire is—

1. – π radians
1. –λ/2 radians
1. – π/4 radians
1. – π/8 radians

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For λ/4 separated isotropic source, phase is given by
ψ = β d cos φ + α
for end fire array α = – βd
so we get
ψ = βd (cos φ – 1)
where,
φ = 60º
d = λ , β = 2π
4 λ

d = 2π , λ = (0.5 - 1)
4 4

= π
4

Correct Option: C

For λ/4 separated isotropic source, phase is given by
ψ = β d cos φ + α
for end fire array α = – βd
so we get
ψ = βd (cos φ – 1)
where,
φ = 60º
d = λ , β = 2π
4 λ

d = 2π , λ = (0.5 - 1)
4 4

= π
4

A medium wave radio transmitter operating at a wavelength of 492 m has a lower antenna height 124 m. What is the radiation resistance of the antenna?

1. 25 Ω
1. 36.5 Ω
1. 50 Ω
1. 73 Ω

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Radiation resistance for monopole radiator is given by relation
R_r = 400 1 ²
λ

⇒ 400 124 ² = 25Ω
492

Correct Option: A

Radiation resistance for monopole radiator is given by relation
R_r = 400 1 ²
λ

⇒ 400 124 ² = 25Ω
492

An uniform plane wave incident normally on a plane surface of dielectric material is reflected with a VSWR of 3 what is the percentage of incident power that is reflected?

1. 10%
1. 25%
1. 50%
1. 75%

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Percentage of reflected power given by
P– R = | Γ |² Pi⁺
Γ = 3 – 1 ⇒ 2 = 1
3 + 1 4 2

PR– = 1 ² Pi⁺
2

so percentage reflected power
PR^– × 100 = 1 × 100 = 25%
Pi 4

Correct Option: B

Percentage of reflected power given by
P– R = | Γ |² Pi⁺
Γ = 3 – 1 ⇒ 2 = 1
3 + 1 4 2

PR– = 1 ² Pi⁺
2

so percentage reflected power
PR^– × 100 = 1 × 100 = 25%
Pi 4

A material has conductivity of 10^–2 mho/m and a relative permittivity of 4 the frequency at which conduction current in the medium is equal to the displacement current is—

1. 45 MHz
1. 90 MHz
1. 450 MHz
1. 900 MHz

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As hints are provided in the synopsis that displacement current is the rate of change of electric flux density, so we get
J_C = σ E, J_D = ∂D ⇒ ωD = ω ε₀ε_r E
∂t

so when both currents are equal
J_C = J_D ⇒ σE = ω ε₀ ε_r E
ω = σ
ε₀ ε_r

f = σ
2π × 8.85 × 10^–12 × 4

= 4.49 × 1010^–3 × 10¹⁰ = 45 MHz

Correct Option: A

As hints are provided in the synopsis that displacement current is the rate of change of electric flux density, so we get
J_C = σ E, J_D = ∂D ⇒ ωD = ω ε₀ε_r E
∂t

so when both currents are equal
J_C = J_D ⇒ σE = ω ε₀ ε_r E
ω = σ
ε₀ ε_r

f = σ
2π × 8.85 × 10^–12 × 4

= 4.49 × 1010^–3 × 10¹⁰ = 45 MHz