General License Class

General License Class Chapter 6 Antennas & Transmission Lines

Antenna Basics • Review • Elements • Conducting portion of an antenna. • Radiate or receive signal. • Driven element. • Element(s) to which power is applied.

Antenna Basics • Review • Polarization • Orientation of electric field with respect to the earth. • Horizontal polarization. • Electric field parallel to surface of the earth. • Vertical polarization. • Electric field perpendicular to surface of the earth. • Same as orientation of driven element.

Antenna Basics • Review • Feed-Point Impedance • Ratio of RF voltage to RF current at antenna feedpoint. • Antenna is resonant if impedance is purely resistive.

Antenna Basics • Review • Radiation Pattern • Azimuthal pattern. • Graph of relative signal strength in horizontal directions. • Elevation pattern. • Graph of relative signal strength in vertical directions. • Lobes & nulls. • Regions where antenna is or is not radiating a signal.

Antenna Basics • Review • Isotropic Antenna. • Theoretical point radiator. • Impossible to construct. • Radiates equally in ALL directions. • Radiation pattern is perfect sphere. • Used as a reference for antenna gain. • Gain referenced to an isotropic radiator expressed as dBi.

Antenna Basics • Review • Directional Antenna. • Gain. • Antennas are passive, so POUT = PIN. • Energy is concentrated in one direction at expense of another.

Antenna Basics • Review • Directional Antenna. • Front-to-back ratio (F/B). • Ratio of signal strength in forward direction (largest lobe) to signal strength 180° from forward direction. • Front-to-side ratio (F/S). • Ratio of signal strength in forward direction to signal strength 90° from forward direction.

Dipoles, Ground-Planes, & Random Wires • Dipoles • Most basic real-world antenna. • Most other antenna designs are based on the dipole.

Dipoles, Ground-Planes, & Random Wires • Dipoles • Easily constructed. • Actual length shorter than free-space length. • Physical thickness of wire. • Thicker = shorter. • Thicker = wider bandwidth. • Proximity to nearby objects. • Closer = shorter. • Start with Length(ft) = 492 / fMHz & trim for resonance.

Dipoles, Ground-Planes, & Random Wires • Dipoles • Radiation pattern. • Toroidal (donut-shaped). • Gain = 2.15 dBi. • Also used as a reference for antenna gain. • Gain referenced to a dipole is expressed as dBd. • 0 dBd = 2.15 dBi

Dipoles, Ground-Planes, & Random Wires • Dipoles • Feedpoint Impedance. • Approximately 72Ωin free space. • Varies with height above ground. • Varies with proximity to nearby objects. • Typically closer to 50Ω in real-world installations. • Feedpoint impedance at odd harmonics will be about the same as the impedance at fundamental frequency. • A 40m dipole (7 MHz) will work well on 15m (21 MHz).

Dipoles, Ground-Planes, & Random Wires • Dipoles • Voltage distribution. • Minimum at feedpoint. • Maximum at ends.

Dipoles, Ground-Planes, & Random Wires • Dipoles • Current distribution. • Maximum at feedpoint. • Zero at ends.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • One half of a dipole with other half replaced with an electrical “mirror” called a ground plane. • Ground plane. • Earth -- metal screen, mesh, or plate -- radials. • 1/4λ long. • Length(ft) = 246 / fMHz (trim for resonance). • Feedpoint at junction of radiator & ground plane.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Radiation pattern. • Omni-directional.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Feedpoint impedance. • 35Ω with horizontal radials. • Angling radials downward (drooping) raises impedance. • At between 30° & 45° impedance equals 50Ω. • Increasing droop angle to 90° results in a half-wave dipole with an impedance of 72Ω.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Mobile HF antennas. • Usually some variation of a ground-plane antenna. • Thin steel whip mounted vertically. • Vehicle body serves as ground plane. • Full-size vertical not practical on HF bands. • Exception: 10m. • Some type of “loading” used to make short antenna resonant on desired band.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Mobile HF antennas. • Loading techniques. • Loading coil. • Adds inductance to lower resonant frequency. • Narrows bandwidth. • Adds loss. • Can be placed at bottom, middle, or top of radiator. • Can be adjustable to cover different bands.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Mobile HF antennas. • Loading techniques. • Capacitive hat. • Adds capacitance to lower resonant frequency. • Increases bandwidth. • Reduces loss. • Usually placed near the top of the radiator.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Mobile HF antennas. • Loading techniques. • Linear loading. • Part of antenna folded back on itself. • Not commonly used.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Mobile HF antennas. • Corona ball. • Small metal sphere at tip of whip. • Prevents static discharge from sharp tip of antenna.

Dipoles, Ground-Planes, & Random Wires • Ground-Planes (Verticals) • Mobile HF antennas. • Screwdriver antenna. • Bottom-mounted loading coil. • Motor runs tap up & down coil to adjust for different bands. • High Sierra (160m to 6m). • Little Tarheel II (80m to 6m). • Yaesu ATAS-120 (40m to 70cm). • Good compromise between performance & convenience.

Dipoles, Ground-Planes, & Random Wires • Random Wires • Random length of wire put up any way you can. • Feedpoint impedance unpredictable. • Radiation pattern unpredictable. • Connected directly to transmitter or antenna tuner. • Station equipment part of antenna system. • RF burns from contacting equipment possible. • Can yield good results on any band where impedance match can be achieved.

Dipoles, Ground-Planes, & Random Wires • Effects of Height Above Ground • Feedpoint impedance affected by height above ground. • Below 1/2λ above ground, impedance steadily decreases as height decreases. • Above 1/2λ above ground, impedance repeatedly increases then decreases as height increases. • Impedance variations decrease in amplitude until settling on free-space value (72Ω) several wavelengths above the ground.

Dipoles, Ground-Planes, & Random Wires • Effects of Height Above Ground • Radiation pattern affected by height above ground. • Signals reflects off ground and combines with direct signal to change pattern. • Below 1/2λ above ground, dipole is nearly omni-directional with maximum radiation straight up. • Near Vertical Incidence Skywave (NVIS). • Useful on 80m & 40m for communicating with close-in stations.

Dipoles, Ground-Planes, & Random Wires • Effects of Polarization • Radiation pattern affected by height above ground. • Reflection of signal off of the ground results in signal loss. • Loss is less if wave is horizontally polarized. • Vertical antennas have lower angle of radiation than horizontal antennas mounted near the ground. • Vertical antennas preferred for DX on lower frequency bands.

G4E01 -- What is a "capacitance hat", when referring to a mobile antenna? A. A device to increase the power handling capacity of a mobile whip antenna B. A device that allows automatic band-changing for a mobile antenna C. A device to electrically lengthen a physically short antenna D. A device that allows remote tuning of a mobile antenna

G4E02 -- What is the purpose of a "corona ball" on a HF mobile antenna? A. To narrow the operating bandwidth of the antenna B. To increase the "Q" of the antenna C. To reduce the chance of damage if the antenna should strike an object D. To reduce high voltage discharge from the tip of the antenna

G4E06 -- What is one disadvantage of using a shortened mobile antenna as opposed to a full size antenna? A. Short antennas are more likely to cause distortion of transmitted signals B. Short antennas can only receive vertically polarized signals C. Operating bandwidth may be very limited D. Harmonic radiation may increase

G9B01 -- What is one disadvantage of a directly fed random-wire antenna? A. It must be longer than 1 wavelength B. You may experience RF burns when touching metal objects in your station C. It produces only vertically polarized radiation D. It is not effective on the higher HF bands

G9B02 -- What is an advantage of downward sloping radials on a quarter wave ground-plane antenna? A. They lower the radiation angle B. They bring the feed-point impedance closer to 300 ohms C. They increase the radiation angle D. They bring the feed-point impedance closer to 50 ohms

G9B03 -- What happens to the feed-point impedance of a ground-plane antenna when its radials are changed from horizontal to downward-sloping? A. It decreases B. It increases C. It stays the same D. It reaches a maximum at an angle of 45 degrees

G9B04 -- What is the low angle azimuthal radiation pattern of an ideal half-wavelength dipole antenna installed 1/2 wavelength high and parallel to the Earth? A. It is a figure-eight at right angles to the antenna B. It is a figure-eight off both ends of the antenna C. It is a circle (equal radiation in all directions) D. It has a pair of lobes on one side of the antenna and a single lobe on the other side

G9B05 -- How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna? A. If the antenna is too high, the pattern becomes unpredictable B. Antenna height has no effect on the pattern C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional D. If the antenna is less than 1/2 wavelength high, radiation off the ends of the wire is eliminated

G9B06 -- Where should the radial wires of a ground-mounted vertical antenna system be placed? A. As high as possible above the ground B. Parallel to the antenna element C. On the surface or buried a few inches below the ground D. At the top of the antenna

G9B07 -- How does the feed-point impedance of a 1/2 wave dipole antenna change as the antenna is lowered from 1/4 wave above ground? A. It steadily increases B. It steadily decreases C. It peaks at about 1/8 wavelength above ground D. It is unaffected by the height above ground

G9B08 -- How does the feed-point impedance of a 1/2 wave dipole change as the feed-point location is moved from the center toward the ends? A. It steadily increases B. It steadily decreases C. It peaks at about 1/8 wavelength from the end D. It is unaffected by the location of the feed point

G9B09 -- Which of the following is an advantage of a horizontally polarized as compared to vertically polarized HF antenna? A. Lower ground reflection losses B. Lower feed-point impedance C. Shorter Radials D. Lower radiation

General License Class