Free HAM Radio Extra Class Test Antennas and Feed Lines Questions and Answers

Question 1

What is the input impedance of a 1/2-wavelength transmission line that is shorted at the far end?

Accepted Answer

Very low impedance

Answer

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?

Accepted Answer

The region where the antenna's radiation pattern is independent of distance

Answer

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?

Accepted Answer

1 watt

Answer

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?

Accepted Answer

Air-insulated hard line

Answer

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?

Accepted Answer

The signal travels at 66% of the speed of light in a vacuum.

Answer

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).

HAM - Radio Extra Class Test Practice Test

HAM - Radio Extra Class Test Practice Test

Free HAM Radio Extra Class Test Antennas and Feed Lines Questions and Answers