Extra exam: antennas and transmission lines
Antenna gain and patterns, matching, transmission lines and the Smith chart. 85 questions, 10 at a time, starting with ones you haven't seen.
Rules to remember10 key antennas and transmission lines rulesShow the rules
- The first Fresnel zone's radius shrinks as wavelength gets shorter, so the highest frequency listed, 5.8 GHz, has the smallest zone. (p. 100)
- With too few segments per half wavelength, the current distribution is poorly approximated and the computed feed point impedance can be wrong. (p. 103)
- A folded dipole is a half-wave dipole with a second parallel conductor joining its two ends, forming a narrow loop. (p. 104)
- A Zepp antenna is a half-wave dipole fed at one end, originally through tuned feeders on airships. (p. 105)
- A beta or hairpin match needs a split driven element, so the element must be insulated from the boom. (p. 108)
- A Wilkinson divider splits power equally into two 50-ohm loads while keeping a 50-ohm input and isolating the outputs. (p. 109)
- The two families on a Smith chart are resistance circles and reactance arcs. (p. 112)
- A Beverage is a long, low wire receiving antenna that works best when it is at least one wavelength long. (p. 114)
- On 160 and 80 meters, atmospheric noise is so strong that a receiving antenna's directivity matters far more than its efficiency. (p. 114)
- A terminated single-turn loop such as a pennant has a cardioid pattern with one deep null toward the rear. (p. 115)
Taken from the explanations to checked practice questions; page numbers link to the official handbook.
Sample antennas and transmission lines questions
1.How is a Smith chart normalized?
- AReassign the prime center to the reactance axis
- BReassign the resistance axis with reactance values
- CReassign the prime center's impedance value
- DReassign the reactance axis with resistance values
Show the answer
✓ C. Reassign the prime center's impedance value
Normalizing a Smith chart means assigning the line's impedance, such as 50 ohms, to the prime center point and scaling everything to it.
2.Which of the following is used to shunt feed a grounded tower at its base?
- ABeta or hairpin match
- BDouble-bazooka match
- CGamma match
- DAll these choices are correct
Show the answer
✓ C. Gamma match
A grounded tower can be shunt-fed with a gamma match: the coax shield goes to the base and the center conductor taps the tower partway up.
3.What is a disadvantage of decreasing the number of wire segments in an antenna model below 10 segments per half-wavelength?
- AThe computed feed point impedance may be incorrect
- BThe antenna will become mechanically unstable
- CGround conductivity will not be accurately modeled
- DThe resulting design will favor radiation of harmonic energy
Show the answer
✓ A. The computed feed point impedance may be incorrect
With too few segments per half wavelength, the current distribution is poorly approximated and the computed feed point impedance can be wrong.
4.What type of analysis is commonly used for modeling antennas?
- AGraphical analysis
- BMutual impedance analysis
- CMethod of Moments
- DCalculus differentiation with respect to physical properties
Show the answer
✓ C. Method of Moments
Most antenna modeling programs, such as NEC-based software, use the Method of Moments.