# Antenna Parameters Questions and Answers – Radiation Resistance

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This set of Antenna Parameters Interview Questions and Answers for freshers focuses on “Radiation Resistance”.

1. What is the radiation resistance of an antenna if it radiates 1kW and current in it is Irms=10A?
a) 0.1Ω
b) 1Ω
c) 10Ω
d) 100Ω

⇨ Rrad=$$\frac{P_{rad}}{I_{rms}^2} = \frac{1000}{10×10}=10\Omega$$

2. What is the radiation resistance of an antenna if input power to it is 1KW and current in it is 10A having a power loss of 200W?
a) 10Ω
b) 2Ω
c) 12Ω
d) 8Ω

Explanation: Input power Pin=Prad+Ploss
⇨ Rrad=$$\frac{P_{rad}}{I_{rms}^2} = \frac{800}{10×10}$$=8Ω

3. What is the radiation resistance of a short dipole of length L?
a) 20π2 $$(\frac{L}{\lambda})^2$$
b) 80π2 $$(\frac{l}{\lambda})^2$$
c) 40π2 $$(\frac{l}{\lambda})^2$$
d) 160π2 $$(\frac{l}{\lambda})^2$$

Explanation: Radiation resistance of a Hertzian dipole of length l is given by R=80π2 $$(\frac{l}{\lambda})^2$$
Then short dipole is of length l/2, so the radiation resistance is given by R=80π2$$(\frac{L/2}{\lambda})^2=20\pi^2 (\frac{L}{\lambda})^2.$$
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4. If the length of the dipole decreases, then the radiation resistance will________
a) increase
b) decrease
c) depends on current distribution
d) not change

Explanation: Since the radiation resistance of a Hertzian dipole is R=80π2 $$(\frac{l}{\lambda})^2$$, the radiation resistance will decrease as the length of the dipole decreases. It is directly proportional to the square of the length of the dipole.

5. For a half-wave dipole with length λ/12, what is the antenna efficiency if the Radiation resistance is 2Ω?
a) 0.73
b) 0.073
c) 0.37
d) 0.78

Explanation: Radiation resistance $$R_{rad}=80π^2(\frac{l}{\lambda})^2=80π^2 (\frac{\lambda/12}{\lambda})^2$$=5.48Ω
Antenna efficiency $$\eta=\frac{R_{rad}}{R_{rad}+R_{loss}}=\frac{5.48}{5.48+2}=0.73$$
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6. Find the radiation resistance of a Hertzian dipole of length 1m and operating at a frequency 1MHz?
a) 0.08Ω
b) 8.8mΩ
c) 8.8Ω
d) 0.88Ω

Explanation: $$λ=\frac{c}{f}=\frac{3×10^8}{1×10^6}=300m$$
Radiation resistance of a Hertzian dipole is given by $$R=80π^2(\frac{l}{\lambda})^2=80π^2 (\frac{1}{300})^2$$=0.0088Ω

7. Radiation resistance doesn’t depend on direction of power radiated but depends on the frequency.
a) True
b) False

Explanation: Radiation resistance doesn’t depend on the direction in which the power is radiated. It depends on frequency through which we will find the wavelength and thereby the radiation resistance.

8. What is the radiation resistance of the antenna radiating at 5kW and having maximum current 2A?
a) 25kΩ
b) 2.5kΩ
c) 0.25kΩ
d) 2.5Ω

Explanation: $$I_{rms}=\frac{I_m}{\sqrt{2}}=\frac{2}{\sqrt{2}}=\sqrt{2}$$
Power radiated $$P_{rad}=I_{rms}^2 R_{rad}$$
⇨ $$R_{rad}=\frac{P_{rad}}{I_{rms}^2} = \frac{5K}{2}=2.5k\Omega$$

9. Power radiated by half-wave dipole with maximum current amplitude 10A is ______
a) 3.65kΩ
b) 3.650Ω
c) 0.365kΩ
d) 36.50Ω

Explanation: For a half-wave dipole the radiation resistance is 73Ω.
$$I_{rms}=\frac{I_m}{\sqrt{2}}=\frac{10}{\sqrt{2}}=5\sqrt{2}$$

10. The radiation resistance dissipates same amount of power as it radiated by the antenna.
a) True
b) False

Explanation: The radiation resistance is defined as the equivalent resistance that dissipates equal amount of power that is radiated by the antenna. The power radiated by the radiation resistance is given by Prad=Irms2Rrad. Radiation resistance doesn’t depend on direction of power radiated but it depends on the frequency.

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