B-006
Subelement B-006
Section B-006-010
Section B-006-010
How do you calculate the approximate length in metres of a quarter-wavelength antenna for use on frequencies below 30 MHz?
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Divide 468 by the operating frequency in MHz
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Correct AnswerDivide 71.3 by the operating frequency in MHz
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Divide 143 by the operating frequency in MHz
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Divide 300 by the operating frequency in MHz
Key words: QUARTER-wavelength. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: 95 % of one quarter wavelength in free space = '300 / 4 * 0.95' divided by frequency in megahertz = 71.3 divided by frequency in megahertz.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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If you made a quarter-wavelength vertical antenna for 21.125 MHz, approximately how long would it be?
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3.55 metres
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7.10 metres
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6.77 metres
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Correct Answer3.37 metres
Key words: QUARTER-wavelength. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: 95 % of one quarter wavelength in free space = '300 / 4 * 0.95' divided by frequency in megahertz = 71.3 divided by frequency in megahertz. In this example, '300 / 21.125 MHz / 4 * 0.95' = 3.37 metres.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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If you made a half-wavelength vertical antenna for 223 MHz, approximately how long would it be?
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Correct Answer67 cm
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105 cm
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32 cm
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135 cm
Key words: HALF-wavelength. The ARRL Antenna Book, 24th ed. (2019), section 2.1.1, Effects of Conductor Diameter: "However, at VHF and UHF where the wavelength is short and antennas are several wavelengths above ground, the free-space dipole length formulas are accurate and useful." Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: one half wavelength in free space = '300 / 2' divided by frequency in megahertz = 150 divided by frequency in megahertz. In this example, '300 / 223 MHz / 2' = 0.67 metres.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Why is a five-eighths wavelength vertical antenna better than a quarter-wavelength vertical antenna for VHF or UHF mobile operations?
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Because it can handle more power
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Because it has a high radiation angle
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Because it does not require impedance matching
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Correct AnswerBecause it has more gain
The 'five eights' wavelength antenna focuses energy somewhat better towards the horizon (lower radiation angle) than a regular quarter-wave antenna.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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If a quarter-wavelength vertical antenna is placed on the roof of a car, in what direction does it send out radio energy?
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Most of it goes vertically straight-up
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Most of it goes in one direction
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Correct AnswerIt goes out equally well in all horizontal directions
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Most of it goes equally in two opposite directions
What is an advantage of downward sloping radials on a ground plane antenna?
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It increases the radiation angle
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It lowers the radiation angle
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It brings the feed point impedance closer to 300 ohms
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Correct AnswerIt brings the feed point impedance closer to 50 ohms
Radials are the three or four rods simulating ground at the base of an elevated vertical antenna (ground plane antenna). Sloping radials (lower than 90 degrees) BRING up the impedance from about 30 ohms to 50 ohms for a better direct match to coaxial cable.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What configuration of radials will match an elevated quarter-wave vertical antenna to a 50-ohm coaxial cable?
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Horizontal half-wave radials
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Upward sloping half-wave radials
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Horizontal quarter-wave radials
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Correct AnswerDownward sloping quarter-wave radials
Radials are the three or four rods simulating ground at the base of an elevated vertical antenna (ground plane antenna). Sloping radials (lower than 90 degrees) BRING up the impedance from about 30 ohms to 50 ohms for a better direct match to coaxial cable.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Which of the following transmission lines will give the best match to the base of a quarter-wave ground-plane antenna?
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300-ohm balanced transmission line
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75-ohm coaxial cable
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Correct Answer50-ohm coaxial cable
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75-ohm balanced transmission line
How can a vertical antenna, 2 metres in length, be made to resonate in the 80-metre band for mobile use?
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Correct AnswerInstall an inductor in series with the antenna
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Connect the transmission line shield to the vehicle's chassis
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Use a solid radiating element instead of tubing
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Install a capacitor in series with the antenna
A series inductance in an antenna is termed a "loading coil". It makes the antenna appear LONGER electrically than its physical size. Making the antenna longer brings down the resonant frequency.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Why is a loading coil often used with an HF mobile vertical antenna?
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Correct AnswerTo tune out capacitive reactance
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To filter out electrical noise
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To lower the Q
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To lower the losses
Short answer: a coil (inductor) has a behaviour totally opposite to capacitors; 'cancelling reactive capacitance' makes sense. A short antenna (e.g., 2.5 m) operated on HF frequencies (wavelengths of 10 to 80 metres) looks like an antenna operated well below its natural resonant frequency. If you think of an ideal antenna as a resonant circuit where capacitive and inductive reactances cancel each other, you'll note that CAPACITIVE reactance ( XC = 1 over '2 * PI * f * C' ) grows below the resonant frequency. A "loading coil" cancels out that capacitive reactance.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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When using a ground mounted vertical HF antenna, what can you do to reduce ground losses?
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Raise the antenna one eighth of a wavelength
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Use an insulator with a higher breakdown voltage
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Improve the impedance match at the base
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Correct AnswerInstall a wire ground system (radials) at the antenna base
A quarter-wave ground mounted HF antenna resembles half of a dipole antenna. Such a quarter-wave antenna uses ground conductivity as a return circuit for RF currents. A ground system ensures much better conduction.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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