1. 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.
2. 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 \(P_{rad}=I_{rms}^2 R_{rad}\)
⇨ \(R_{rad}=\frac{P_{rad}}{I_{rms}^2} = \frac{5K}{2}=2.5k\Omega\)
3. 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.
\(I_{rms}=\frac{I_m}{\sqrt{2}}=\frac{10}{\sqrt{2}}=5\sqrt{2}\)
Power radiated Prad=Irms2 Rrad=(5√2)2×73=3650Ω=3.65kΩ
4. 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.
5. Friss transmission is applicable when same antenna is used for both transmission and reception.
a) True
b) False
Explanation: Friss transmission is used to find the receiver power in antenna when power is transmitted from another antenna. These are separated by a far zone distance.
6. What is the distance between antennas to apply the Friss transmission equation in terms of antennas largest dimension?
a) R » 2D2/λ
b) R « 2D2/λ
c) R » 2λ2/D
d) R « 2λ2/D
Explanation: The transmitting and receiving antennas are in a far zone to each other. So the separation distance between them is R » 2D2/λ.
7. Free space loss factor is given by _____
a) \(\frac{\lambda}{4\pi R}\)
b) \((\frac{\lambda}{4\pi R})^2\)
c) \(\frac{4\pi R}{\lambda}\)
d) \((\frac{4\pi R}{\lambda})^2\)
Explanation: The free space loss factor is given by \((\frac{\lambda}{4\pi R})^2\). It is used to know the amount of losses occurred due to the spreading of energy by an antenna.
8. Which of the following is the Friss transmission equation for the matched polarization of antennas?
a) \(\frac{P_r}{P_t} = \frac{G_t G_r\lambda^2}{(4πR)^2}\)
b) \(\frac{P_t}{P_r} = \frac{G_t G_r\lambda^2}{(4πR)^2}\)
c) \(\frac{P_r}{P_t} = \frac{G_t G_r\lambda^2}{4πR^2}\)
d) \(\frac{P_t}{P_r} = \frac{G_t G_r\lambda^2}{4πR^2}\)
Explanation: Friss transmission equation is used to calculate the power received by the receiving antenna when transmitted from other antenna separated by a distance R. the equation is given by \(\frac{P_r}{P_t} = \frac{G_t G_r \lambda^2}{(4πR)^2}.\)
9. If the operating frequency increases, powers received by the receiving antenna ______
a) will decrease
b) will Increase
c) is Independent of frequency
d) is not predictable
Explanation: From the Friss transmission equation, the received power depends on the wavelength which is inversely proportional to the frequency. So the power decreases as the frequency increases.
\(\frac{P_r}{P_t} = \frac{G_t G_r λ^2}{(4πR)^2} = \frac{G_t G_r c^2}{(4πRf)^2} \)
10. Power received by the antenna when one antenna is horizontally polarized and the other is vertically polarized is _______
a) 1
b) 0
c) \(\frac{P_r}{P_t} = \frac{G_t G_r λ^2}{(4πR)^2}\)
d) \(\frac{P_r}{P_t} = \frac{G_t G_r λ^2}{2(4πR)^2}\)
Explanation: When the receiving and transmitting antennas polarization is not matched, the Friss transmission equation includes a polarization loss factor given by cos2θ. Since one is vertically polarized and other is horizontally polarized, the angle difference is 900. PLF=cos2θ=0
\(\frac{P_r}{P_t} = PLF\frac{G_t G_r \lambda^2}{(4\pi R)^2} =0\)
So, no power is received.