Common antenna types, feed lines, and propagation concepts for amateur radio operators, beyond the license exam question pools.
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- What is the physical length of a half-wave dipole antenna for a given frequency?
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Approximately 468 divided by the frequency in megahertz, in feet (or 143 divided by frequency in MHz for meters).- How does a Yagi antenna achieve gain over a simple dipole?
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By using a driven element, reflector, and one or more director elements that focus radiation into a narrow beam through constructive and destructive interference.- What is antenna gain measured in?
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Decibels (dB), typically referenced to an isotropic radiator (dBi) or a half-wave dipole (dBd), where dBi is approximately 2.15 dB higher than dBd.- What does an antenna's radiation pattern show?
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The relative power radiated or received in different directions from the antenna, often displayed as polar plots showing the azimuth and elevation lobes.- What does Standing Wave Ratio (SWR) measure?
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The ratio of the maximum voltage (or current) to minimum voltage (or current) on a transmission line due to impedance mismatch between the antenna and feed line.- Why is SWR important in a ham radio station?
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High SWR causes power loss in the feed line, can damage the transmitter's output stage, and reduces transmitted and received efficiency.- What is the purpose of an antenna tuner or impedance matching network?
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To transform the antenna's impedance to match the transmitter's output impedance (typically 50 ohms), reducing SWR and minimizing power loss.- What are the main layers of coaxial cable from inside to outside?
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Center conductor, dielectric insulator, outer shield (usually braided copper), and outer jacket.- What is an advantage of twin lead transmission line over coaxial cable?
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Twin lead has lower loss per unit length and higher impedance (typically 300 or 450 ohms), making it suitable for longer runs, but it is more susceptible to nearby interference.- When is ladder line (open-wire transmission line) typically used in amateur radio?
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For long-distance feed runs to multiband antennas, especially with tuners, because of its very low loss and ability to be fed with high SWR.- What is velocity factor in a transmission line?
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The ratio of the propagation velocity of an electromagnetic wave in the line to the speed of light in free space, typically 0.66 to 0.95 depending on the dielectric material.- At what impedance does a half-wave dipole antenna resonate?
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Approximately 72 ohms when in free space and elevated, or around 50 ohms when fed with a 50-ohm cable due to the lowering effect of the ground.- What determines the bandwidth of an antenna?
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The wire diameter, the length relative to wavelength, and the Q factor of the antenna, with thicker conductors and shorter antennas generally having wider bandwidth.- What is directivity of an antenna?
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The ability of an antenna to concentrate radiation in specific directions relative to other directions, expressed as the ratio of radiated power density in the main lobe to the average power density in all directions.- What is the difference between horizontal and vertical polarization?
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Horizontal polarization has the electric field parallel to the Earth's surface, while vertical polarization has it perpendicular to the surface; antennas must have matching polarization for efficient reception.
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