Ham Radio General Class Test Antennas and Feed Lines Questions and Answers

Question 1

An amateur operator is installing a new HF station and wants to feed a center-fed dipole antenna with 50-ohm coaxial cable. What is the primary function of a balun in this setup?

Accepted Answer

To convert the balanced antenna feedpoint to the unbalanced coaxial cable

Answer

A balun, which stands for 'balanced to unbalanced', is a device used to interface a balanced antenna (like a dipole) with an unbalanced feed line (like coaxial cable). It prevents the shield of the coax from becoming part of the antenna and radiating, which can distort the radiation pattern and cause RFI.

Question 2

A ham is using an SWR meter to check a newly installed antenna system. The reading at the low end of the 20-meter band is 2.5:1, and at the high end, it is 1.5:1. What does this indicate about the antenna's length?

Accepted Answer

The antenna is too short for the band.

Answer

When the SWR is lower at the higher frequency end of the band and higher at the lower frequency end, it indicates that the antenna is electrically short for the desired frequency range. Lengthening the antenna would shift the point of lowest SWR to a lower frequency.

Question 3

Which of the following feed lines generally has the lowest loss per unit of length at HF frequencies, especially when dealing with a high SWR?

Accepted Answer

450-ohm ladder line

Answer

450-ohm ladder line, a type of parallel-conductor feed line, has significantly lower loss at HF frequencies compared to common coaxial cables. Its air dielectric and wide spacing between conductors contribute to this efficiency, making it particularly advantageous in systems with a high SWR, as it does not dissipate the reflected power as heat to the same extent as coax.

Question 4

A General Class operator wants to build a half-wave dipole antenna for the 40-meter band, centered on 7.150 MHz. Using the standard formula, what is the approximate total length of the antenna in feet?

Accepted Answer

65.5 feet

Answer

The common formula to calculate the approximate length of a half-wave dipole in feet is 468 divided by the frequency in MHz. For 7.150 MHz, the calculation is 468 / 7.150 = 65.45 feet.

Question 5

What is the primary advantage of a 3-element Yagi antenna compared to a simple dipole antenna for the same frequency?

Accepted Answer

It provides forward gain and directivity.

Answer

A Yagi antenna uses parasitic elements (a reflector and one or more directors) to concentrate radiated power in a specific direction. This results in forward gain and a directional radiation pattern, allowing for stronger signals to and from the desired direction while reducing interference from other directions.

Ham Radio General Class Test Practice Test

Ham Radio General Class Test Practice Test

Ham Radio General Class Test Antennas and Feed Lines Questions and Answers