General · correct answers only, in question-pool order.
For a parallel-conductor line, characteristic impedance is set mainly by conductor spacing relative to conductor diameter. Greater spacing raises impedance; larger conductors lower it.
Standing waves increase current and voltage at points along the line. In a lossy feed line, those larger values increase dissipation, so high SWR increases total line loss.
Common window line is a balanced parallel-conductor feed line with a nominal characteristic impedance of about 450 ohms.
Reflected power occurs when the antenna feed-point impedance does not match the feed line's characteristic impedance.
Coaxial-cable attenuation increases as frequency rises because conductor and dielectric losses become greater.
RF feed-line loss is commonly specified as decibels of attenuation per 100 feet.
Standing waves are eliminated when the antenna feed-point impedance matches the characteristic impedance of the transmission line.
A tuner at the transmitter can make the transmitter see 1:1 SWR, but it does not change the mismatch at the antenna end of the existing feed line. The line itself still has 5:1 SWR.
For a purely resistive mismatch, SWR is the larger resistance divided by the smaller. 200/50 gives 4:1.
A 10-ohm resistive load on a 50-ohm line gives an impedance ratio of 50/10, or 5:1.
Loss attenuates the reflected wave on its return trip, making the mismatch appear better at the input of a lossy line than it really is at the load.
A random wire connected directly to the transmitter can place substantial RF voltage and current on station equipment because the station and wiring may become part of the antenna system.
Sloping the radials downward on an elevated quarter-wave ground-plane vertical raises the feed-point impedance from roughly 36 ohms toward 50 ohms.
A quarter-wave ground-plane vertical radiates approximately equally in all horizontal azimuth directions.
In free space, a half-wave dipole has maximum radiation broadside to the wire and nulls off the ends, producing a figure-eight pattern in the plane of the conductor.
When a horizontal dipole is lower than about one-half wavelength, ground interaction fills in the azimuth pattern at high elevation angles, making it nearly omnidirectional.
Ground-mounted vertical radials are normally laid on the soil surface or buried shallowly. They provide the return path and ground screen for the antenna.
As a horizontal half-wave dipole is brought very close to ground, its feed-point impedance generally decreases.
A center-fed half-wave dipole has relatively low impedance at the center and very high impedance near the ends. Moving the feed point outward therefore raises impedance.
A horizontal HF antenna generally has lower ground losses than a vertically polarized antenna because it does not rely on ground currents in the same way.
The usual dipole-length approximation is 468 divided by frequency in MHz. At 14.250 MHz the result is about 33 feet.
Using 468 divided by 3.550 MHz gives about 132 feet for a half-wave dipole.
A quarter-wave vertical is approximately 234 divided by frequency in MHz. At 28.5 MHz the result is a little over 8 feet.
Larger-diameter Yagi elements have lower Q, which broadens the antenna's usable frequency range.
The driven element of a conventional Yagi is approximately one-half wavelength long.
In a three-element Yagi, the reflector is slightly longer than the driven element and the director is slightly shorter.
dBi references an isotropic radiator, while dBd references a dipole. Because a dipole has about 2.15 dB gain over isotropic, the same antenna's dBi figure is 2.15 dB larger.
Adding directors and increasing boom length generally concentrates the radiation pattern more strongly in the forward direction, increasing gain.
Front-to-back ratio compares radiation in the antenna's main forward lobe with radiation in the opposite direction.
The main lobe is the direction in which a directional antenna produces its maximum radiated field strength.
Stacking two identical Yagis vertically by about one-half wavelength typically provides about 3 dB more gain than a single antenna.
The official G9C10 source identifies boom length as one Yagi design variable that can be adjusted to optimize gain, front-to-back ratio, or SWR bandwidth.
The official G9C10 source identifies element count as one of the variables used to optimize Yagi performance.
The official G9C10 source identifies element spacing as another Yagi design variable that affects gain, front-to-back ratio, and bandwidth.
A beta or hairpin match is a shorted transmission-line stub placed at the Yagi feed point to provide impedance matching.
A gamma match can match a Yagi while leaving the driven element electrically connected to the boom, so the element does not have to be insulated from it.
NVIS requires strong high-angle radiation. A horizontal 40-meter dipole only about one-tenth to one-quarter wavelength above ground directs substantial energy nearly straight upward.
An end-fed half-wave has a current minimum and voltage maximum at its end, so the feed-point impedance there is very high.
A halo is a horizontally polarized loop-like VHF/UHF antenna whose maximum radiation is approximately omnidirectional in the plane of the halo.
A trap acts as a frequency-selective circuit that electrically isolates portions of the antenna on some bands, allowing one antenna to operate on multiple bands.
Vertically stacking horizontally polarized Yagis compresses the pattern in the vertical plane, narrowing the elevation beamwidth.
A log-periodic antenna maintains useful impedance and directional performance over a broad frequency range.
In a log-periodic antenna, element lengths and spacings change in a logarithmic progression along the boom.
A screwdriver antenna contains a movable loading coil contact. Changing the effective base-loading inductance tunes the antenna and changes its feed-point impedance.
A Beverage is a long, directional receiving antenna widely used on MF and the lower HF bands because it offers useful directivity and low-noise reception.
An electrically small loop has deep nulls broadside to the plane of the loop, which makes it useful for direction finding and interference nulling.
Multiband antennas can respond on harmonically related frequencies, so they may provide poor rejection of transmitter harmonics compared with a single-band antenna.
A dipole whose two legs slope downward from a single central support is commonly called an inverted V.