Free HAM Radio Extra Class Test Antennas and Feed Lines Questions and Answers — Questions and Answers
Question 1: What is the input impedance of a 1/2-wavelength transmission line that is shorted at the far end?
- Very high impedance
- The same as the characteristic impedance of the line
- Very low impedance (Correct answer)
- The same as the impedance of the load
Correct answer: Very low impedance
A 1/2-wavelength transmission line, when shorted at the far end, reflects the short circuit back to the input. Therefore, the input impedance is very low, appearing as a short circuit.
Question 2: In antenna modeling software like EZNEC or 4nec2, what does the term 'far-field' refer to?
- The region where the antenna's radiation pattern is independent of distance (Correct answer)
- The area immediately surrounding the antenna where stored energy is dominant
- The physical space occupied by the antenna elements
- The portion of the feed line closest to the antenna terminals
Correct answer: The region where the antenna's radiation pattern is independent of distance
The 'far-field' is the region distant from the antenna where the angular distribution of the radiated field is essentially independent of the distance from the antenna. This is the region where radiation patterns are typically measured and evaluated. The 'near-field' is the area closer to the antenna where the fields are more complex.
Question 3: A Yagi antenna has a forward gain of 10 dBi and a front-to-back ratio of 20 dB. If the main lobe has a power of 100 watts, what is the approximate power radiated from the back lobe?
- 10 watts
- 5 watts
- 20 watts
- 1 watt (Correct answer)
Correct answer: 1 watt
The front-to-back ratio is the difference in power, measured in dB, between the main (forward) lobe and the lobe in the opposite direction (back lobe). A 20 dB difference corresponds to a power ratio of 100:1. Therefore, if the forward power is 100 watts, the power from the back lobe is 100 watts / 100 = 1 watt.
Question 4: Which of the following feed lines generally offers the lowest loss per unit of length at VHF and UHF frequencies, assuming a good impedance match?
- RG-58
- RG-8X
- Air-insulated hard line (Correct answer)
- 450-ohm window line
Correct answer: Air-insulated hard line
Air-insulated hard line, also known as Heliax, has significantly lower loss compared to flexible coaxial cables like RG-58 or RG-8X, especially at VHF and UHF frequencies. While window line has low loss, it is more susceptible to weather and proximity effects, making hard line the superior choice for minimizing loss in permanent installations.
Question 5: What does a velocity factor of 0.66 for a coaxial cable signify?
- The cable can only handle 66% of its rated power.
- The signal travels at 66% of the speed of light in a vacuum. (Correct answer)
- The characteristic impedance is 66% of the ideal value.
- The SWR of the cable is 1:0.66.
Correct answer: The signal travels at 66% of the speed of light in a vacuum.
The velocity factor of a transmission line is the ratio of the speed of signal propagation in the line to the speed of light in a vacuum. A velocity factor of 0.66 means the signal travels at 66% of the speed of light. This is crucial for calculating the physical length of a line needed for a specific electrical length (e.g., a quarter-wavelength).
Question 6: A directional wattmeter in a 50-ohm line indicates a forward power of 100 watts and a reflected power of 4 watts. What is the SWR on the transmission line?
- 1.2:1
- 2.0:1
- 1.5:1 (Correct answer)
- 2.5:1
Correct answer: 1.5:1
The Standing Wave Ratio (SWR) can be calculated from the forward and reflected power. First, find the reflection coefficient (ρ) which is the square root of (P_reflected / P_forward). Here, ρ = sqrt(4/100) = sqrt(0.04) = 0.2. Then, SWR = (1 + ρ) / (1 - ρ). So, SWR = (1 + 0.2) / (1 - 0.2) = 1.2 / 0.8 = 1.5. The SWR is 1.5:1.
Free HAM Radio Extra Class Test Antennas and Feed Lines Questions and Answers
What is the input impedance of a 1/2-wavelength transmission line that is shorted at the far end?