General exam: antennas and feed lines
Feed lines, SWR, dipoles, verticals, Yagis, loops and specialized HF antennas. 46 questions, 10 at a time, starting with ones you haven't seen.
Rules to remember10 key antennas and feed lines rulesShow the rules
- Power is reflected at the feed point when the antenna's impedance does not match the feed line's characteristic impedance. (p. 74)
- Coaxial cable loss rises as frequency goes up, which is why the same cable loses more signal at VHF/UHF than on HF. (p. 74)
- Feed line loss is a power ratio, so it is given in decibels, conventionally quoted per 100 feet of line. (p. 74)
- A quarter-wave ground-plane vertical radiates equally in all compass directions, so its pattern is omnidirectional in azimuth. (p. 75)
- The driven element of a Yagi is essentially a half-wave dipole, so it is about 1/2 wavelength long. (p. 77)
- Adding directors on a longer boom concentrates more energy forward, so the main effect is higher gain. (p. 78)
- Front-to-back ratio compares the power radiated in the main lobe with the power radiated in the opposite direction, off the back of the beam. (p. 78)
- The main lobe of a directional antenna is the direction in which it radiates its strongest field. (p. 78)
- Properly stacking two identical antennas doubles the effective aperture, which in theory adds about 3 dB of gain over a single Yagi. (p. 79)
- A log-periodic antenna's main advantage is working over a wide frequency range with fairly constant performance. (p. 80)
Taken from the explanations to checked practice questions; page numbers link to the official handbook.
Sample antennas and feed lines questions
1.What is the common name of a dipole with a single central support?
- AInverted L
- BSloper
- CInverted V
- DLazy H
Show the answer
✓ C. Inverted V
A dipole hung from one central support with its ends sloping down forms an upside-down V, so it is called an inverted V.
2.What is a beta or hairpin match?
- AA 1/4 wavelength section of 75-ohm coax in series with the feed point of a Yagi to provide impedance matching
- BA section of 300-ohm twin-lead transmission line used to match a folded dipole antenna
- CA series capacitor selected to cancel the inductive reactance of a folded dipole antenna
- DA shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
Show the answer
✓ D. A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching
A beta or hairpin match is a short, shorted transmission-line stub at the Yagi's feed point that acts as an inductance to match the driven element to the feed line.
3.Which of the following is a characteristic of using a gamma match with a Yagi antenna?
- AIt does not require the driven element to be insulated from the boom
- BIt is useful for matching multiband antennas
- CIt does not require any inductors or capacitors
- DAll these choices are correct
Show the answer
✓ A. It does not require the driven element to be insulated from the boom
A gamma match taps the driven element away from its center, so the element can be mounted directly on a metal boom without insulating it.
4.How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?
- AIt peaks at about 1/8 wavelength from the end
- BIt steadily increases
- CIt steadily decreases
- DIt is unaffected by the location of the feed point
Show the answer
✓ B. It steadily increases
Along a half-wave dipole, current is highest at the center and voltage highest at the ends, so feed point impedance rises steadily as the feed point moves toward an end.