T3 Radio Wave Propagation

C. Multipath propagation cancels or reinforces signals

At VHF, a signal often reaches the antenna by several paths at once: directly, and by reflection from buildings, terrain and vehicles. Moving the antenna a few feet changes the path lengths, so the waves reinforce at one spot and cancel at another; at 2 meters a wavelength is only about 2 m, so these peaks and nulls sit close together.

• A. Water vapor concentration does not change over a few feet; humidity matters over long paths, mostly at microwave frequencies.
• B. VHF signals are rarely propagated by the ionosphere, and moving an antenna a few feet barely changes the length of a long ionospheric path.
• D. Doppler shift requires motion during the transmission and produces tiny frequency changes; it does not explain strength differences between fixed antenna positions.

B. Absorbs signals, leading to poor reception of weak signals

Leaves and branches contain water, which absorbs energy at UHF and microwave frequencies. A path through trees weakens signals, so weak signals may become unreadable, and the loss can change as foliage grows or gets wet.

• A. Knife-edge diffraction is the bending of signals over a sharp edge such as a ridge or rooftop; vegetation does not cause it, and it does not distort voice peaks.
• C. Vegetation is passive and cannot amplify a signal; it absorbs and scatters energy.
• D. Vegetation does affect UHF and microwave signals, often noticeably, by absorbing them.

C. Horizontal

Weak-signal CW and SSB operators on VHF and UHF use horizontal polarization by convention, typically with horizontal Yagi antennas. Because everyone uses the same polarization, stations avoid the large loss that mismatched (cross) polarization causes over long paths.

• A. Circular polarization is common for satellite work, where a signal's orientation changes, not for terrestrial long-distance CW and SSB.
• B. Left-hand circular polarization is also used mainly for satellite and space communication, not terrestrial weak-signal work.
• D. Vertical polarization is the standard for FM, repeater and mobile operation, not for long-distance CW and SSB.

B. Received signal strength is reduced

Cross-polarization means one station's antenna is vertical and the other's horizontal. On a direct line-of-sight path, a receiving antenna responds poorly to a wave whose electric field is at right angles to it, so received signal strength drops, often sharply.

• A. Sideband inversion comes from frequency mixing inside a radio, not from antenna polarization.
• C. Echoes come from signals arriving over paths with different delays, not from polarization mismatch.
• D. Polarization mismatch on a line-of-sight path causes a significant loss, so something does happen.

B. Try to find a path that reflects signals to the repeater

VHF and UHF signals reflect from buildings, hills and other large surfaces. When the direct path is blocked, you can aim a directional antenna at a reflecting surface and look for a path that bounces your signal to the repeater.

• A. Changing polarization does not get around an obstruction, and switching to horizontal creates cross-polarization loss with a vertically polarized repeater antenna.
• C. Long path refers to HF ionospheric signals that travel the long way around the Earth; it does not apply to reaching a local repeater.
• D. A higher SWR means a poorer match and less power delivered to the antenna; it cannot help a blocked path.

B. Rapid flutter on mobile signals due to multipath propagation

Picket fencing is the rapid flutter heard on a mobile VHF or UHF FM signal as the vehicle moves through the pattern of peaks and nulls that multipath creates. The flutter has a regular, chopped sound, which gives the effect its name.

• A. Taking turns during a net is ordinary net procedure, not picket fencing.
• C. Ground systems for vertical antennas use radials or ground rods; picket fencing is not a type of ground system.
• D. Carriers at fixed intervals from cable TV are cable leakage interference, not picket fencing.

C. Precipitation

Rain, snow and other precipitation absorb and scatter microwave signals, which can noticeably shorten range. The effect grows with frequency and is minor at HF and lower VHF.

• A. High winds may move antennas, but they do not change how microwave signals travel through the air.
• B. Low barometric pressure on its own does not noticeably reduce microwave range; the precipitation that often comes with storms does.
• D. Colder temperatures do not reduce microwave range; temperature inversions can even extend range through ducting.

D. Random combining of signals arriving via different paths

Signals that travel through the ionosphere often arrive over several paths of different lengths, and those paths change constantly. The signals combine at random, sometimes adding and sometimes canceling, which causes irregular fading.

• A. Faraday rotation changes a wave's polarization as it passes through the ionosphere; it does not shift the frequency.
• B. Thunderstorms cause static crashes (QRN), which are noise rather than irregular fading of a signal.
• C. Intermodulation distortion arises in nonlinear receiver or transmitter circuits; it is not a propagation effect.

B. Either vertically or horizontally polarized antennas may be used for transmission or reception

After passing through the ionosphere, a signal's polarization is elliptical and keeps changing, so it has both vertical and horizontal components. A station can therefore use either a vertical or a horizontal antenna on skywave paths, regardless of the polarization used at the other end.

• A. Polarization changes do not make digital modes unusable; digital modes are widely used over ionospheric paths.
• C. FM voice works over ionospheric paths, for example on 10 meters; elliptical polarization does not prevent it.
• D. Matching polarization matters on line-of-sight paths, but because ionospheric signals are elliptically polarized, the two antennas do not need the same polarization.

D. Error rates are likely to increase

With multipath, copies of a data signal arrive at slightly different times and smear into each other, so the receiver misreads symbols. Error rates rise, and faster data rates, with their shorter symbols, are affected more.

• A. Raising the data rate makes symbols shorter, so delayed copies overlap them more and errors get worse.
• B. Slower data rates can help, but there is no rule tying the rate to the number of paths.
• C. FM does not cure multipath; delayed copies still interfere with an FM data signal.

C. The ionosphere

The ionosphere, a region of the upper atmosphere ionized by solar radiation, bends HF signals back toward Earth. This refraction, usually described as reflection, makes long-distance HF skip propagation possible.

• A. The stratosphere lies below the ionosphere and does not return HF signals to Earth.
• B. The troposphere is the bottom layer of the atmosphere, where weather occurs; it affects VHF and UHF (for example, through ducting) but does not return HF signals.
• D. “Electrosphere” is not the name of a propagation region; HF signals are returned by the ionosphere.

B. Little effect

Rain and fog absorb radio energy noticeably only at microwave frequencies, where wavelengths are short. At 10 and 6 meters the wavelengths are far longer than raindrops, so fog or rain has little effect.

• A. Absorption by rain and fog is significant at microwave frequencies, not at 10 or 6 meters.
• C. Fog and rain do not noticeably deflect 10 and 6 meter signals.
• D. Precipitation does not increase range on these bands.

D. They are at right angles

In an electromagnetic wave, the electric and magnetic fields are at right angles to each other, and both are at right angles to the direction the wave travels. The two fields travel together at the same speed.

• A. The electric and magnetic fields are parts of the same wave and travel together at the same speed.
• B. The fields are perpendicular to each other, not parallel.
• C. The fields do not revolve in opposite directions; even in a circularly polarized wave they rotate together and stay perpendicular.

A. The orientation of the electric field

Polarization is defined by the orientation of the electric field. A wave whose electric field is vertical is vertically polarized, which is what a vertical antenna radiates.

• B. The magnetic field is perpendicular to the electric field; polarization is named for the electric field, not the magnetic field.
• C. The energy ratio between the fields does not define polarization; polarization describes the orientation of the electric field.
• D. Velocity divided by wavelength gives frequency, not polarization.

C. Electric and magnetic fields

A radio wave consists of an electric field and a magnetic field that oscillate together, at right angles to each other, as the wave travels. Together they carry the wave's energy through space.

• A. Impedance and reactance describe circuits and components, not the parts of a radio wave.
• B. Voltage and current exist in wires and feed lines; in space the wave is made of electric and magnetic fields.
• D. Radio waves are non-ionizing radiation; ionizing versus non-ionizing is a classification of radiation, not two components of one wave.

A. Speed of light

Radio waves are electromagnetic waves, like light, so in free space they travel at the speed of light, about 300,000,000 meters per second.

• B. Sound is a pressure wave that needs a medium and travels hundreds of thousands of times slower; radio waves need no medium.
• C. Waves in feed lines travel slower than light by a fraction called the velocity factor; free space has no such reduction.
• D. Radio waves move at the speed of light; any small multiple of the speed of sound is far too slow.

B. Wavelength gets shorter as frequency increases

Wavelength and frequency are inversely related: wavelength equals the speed of light divided by frequency. As frequency increases, each cycle occupies less distance, so the 2 meter band (about 146 MHz) has a shorter wavelength than the 10 meter band (about 28 MHz).

• A. This reverses the relationship; a higher frequency means a shorter wavelength.
• C. Wavelength changes with frequency; it is the speed that stays constant in free space.
• D. Path length has no effect on a wave's wavelength or frequency.

D. Wavelength in meters equals 300 divided by frequency in megahertz

Wavelength equals the speed of light divided by frequency. With the speed of light rounded to 300,000,000 m/s and frequency in megahertz, this becomes wavelength in meters = 300 / frequency in MHz. For example, 300 / 146 MHz = 2.05 m, which is why that band is called 2 meters.

• A. Multiplying frequency by 300 makes the result grow as frequency rises, but wavelength must get shorter as frequency rises.
• B. Dividing frequency in hertz by 300 also makes wavelength grow with frequency, the reverse of the true relationship.
• C. Frequency in megahertz divided by 300 inverts the formula; for 146 MHz it gives about 0.49, not the 2 m wavelength.

A. The approximate wavelength in meters

Amateur bands are commonly named by their approximate wavelength: 146 MHz is the 2 meter band, 440 MHz is the 70 centimeter band and 14 MHz is the 20 meter band. The wavelength comes from 300 / frequency in MHz (300 / 14 MHz is about 21 m).

• B. Letter designators such as L-band or X-band are used for some microwave and radar ranges, but amateur bands are identified by wavelength.
• C. Amateur bands are generally not divided into numbered channels the way citizens band is; operators use frequencies within their privileges.
• D. Only the approximate wavelength is the standard way to name amateur bands, so not every choice is correct.

B. 30 MHz to 300 MHz

VHF (very high frequency) covers 30 MHz to 300 MHz, which corresponds to wavelengths from 10 meters down to 1 meter. The 6 meter, 2 meter and 1.25 meter amateur bands are in the VHF range.

• A. 30 kHz to 300 kHz is LF (low frequency), a thousand times lower than VHF.
• C. 300 kHz to 3000 kHz is MF (medium frequency), which includes the AM broadcast band and the 160 meter band.
• D. 300 MHz to 3000 MHz is UHF, the range above VHF.

D. 300 to 3000 MHz

UHF (ultra high frequency) covers 300 MHz to 3000 MHz (3 GHz), wavelengths from 1 meter down to 10 centimeters. The 70 centimeter (420 to 450 MHz) and 23 centimeter (1240 to 1300 MHz) amateur bands are UHF.

• A. 30 to 300 kHz is LF (low frequency), far below UHF.
• B. 30 to 300 MHz is VHF, the range below UHF.
• C. 300 to 3000 kHz is MF (medium frequency); the numbers match UHF, but the unit is kilohertz rather than megahertz.

C. 3 to 30 MHz

HF (high frequency) covers 3 MHz to 30 MHz, wavelengths from 100 meters down to 10 meters. The 80, 40, 20, 15 and 10 meter amateur bands are HF.

• A. 300 to 3000 MHz is UHF.
• B. 30 to 300 MHz is VHF, the range above HF.
• D. 300 to 3000 kHz is MF (medium frequency), the range below HF.

B. 300,000,000 meters per second

Radio waves in free space travel at the speed of light, 299,792,458 meters per second, which rounds to 300,000,000 meters per second. That rounded value is the source of the 300 in the formula wavelength = 300 / frequency in MHz.

• A. 150,000,000 meters per second is half the speed of light.
• C. The speed of light is about 670 million miles per hour (about 186,000 miles per second); 300,000,000 miles per hour mixes up the units.
• D. 150,000,000 miles per hour is far below the speed of light, which is about 670 million miles per hour.

D. All radio frequencies travel at the same velocity

All electromagnetic waves travel at the same speed in free space, the speed of light, whatever their frequency. Frequency changes the wavelength, not the speed.

• A. Microwaves travel at the speed of light, the same as every other radio frequency in free space.
• B. UHF signals travel at the same speed as all other radio waves in free space.
• C. VHF signals travel at the same speed as all other radio waves in free space.

C. UHF signals are usually not propagated by the ionosphere

The ionosphere usually does not bend UHF signals back toward Earth; they pass through it into space. Without skywave, UHF simplex signals travel mainly by line of sight, so they are rarely heard beyond the radio horizon.

• A. UHF signals are not inherently weak; their range is limited by line-of-sight propagation, not by power.
• B. No FCC rule limits how far a UHF signal may travel.
• D. D region absorption mainly affects the lower HF frequencies during daylight; UHF signals pass through the ionosphere.

C. Long-distance ionospheric propagation is far more common on HF

HF signals are regularly refracted back to Earth by the ionosphere, allowing skip contacts thousands of miles away. On VHF and higher, ionospheric propagation is only occasional, so most contacts there are line of sight.

• A. HF wavelengths are longer, so HF antennas are generally larger, not smaller.
• B. The HF bands are narrow compared with VHF and higher bands, so they accommodate less bandwidth, not more.
• D. HF has more atmospheric static, from lightning and other natural sources, than VHF and higher frequencies.

B. They are distorted, with a characteristic raspy sound

During an aurora, VHF stations aim their antennas toward the auroral zone and bounce signals off the moving, ionized curtain. That motion spreads the signal in frequency, so it arrives distorted with a characteristic raspy or buzzing sound; CW is often easier to copy than voice.

• A. Auroral propagation covers regional distances, not 10,000 miles or more.
• C. Auroras follow geomagnetic storms and can occur in any season and at various times of day, not only on winter nights.
• D. Signals arrive from the direction of the aurora, so stations in the US aim north, not west.

B. Sporadic E

Sporadic E occurs when dense patches of ionization form unpredictably in the E region of the ionosphere. These patches can refract 10, 6 and sometimes 2 meter signals back to Earth, producing strong signals from beyond the radio horizon, especially in late spring and early summer.

• A. Backscatter produces weak signals scattered back from distant areas, such as between stations inside the skip zone; it is not the source of strong VHF openings.
• C. D region absorption weakens signals, mainly at lower HF frequencies, rather than providing long-distance propagation.
• D. Gray-line propagation enhances signals along the day/night boundary, mainly on HF, not on 6 and 2 meters.

A. Knife-edge diffraction

Knife-edge diffraction bends radio waves around a sharp edge, such as a ridge or the top of a building. Some signal reaches the shadow area behind the obstruction, allowing communication without a clear line of sight.

• B. Faraday rotation changes a wave's polarization as it passes through the ionosphere; it does not carry signals past obstructions.
• C. Quantum tunneling is an effect at atomic scales, not a radio propagation mechanism.
• D. Doppler shift is a frequency change caused by relative motion, not a way around obstructions.

A. Tropospheric ducting

Tropospheric ducting occurs when a temperature inversion traps VHF and UHF signals in a layer of the lower atmosphere and guides them far beyond the horizon. It regularly supports contacts of around 300 miles, and sometimes much farther.

• B. The D region absorbs rather than refracts signals, and it does not return VHF or UHF signals.
• C. F2 region refraction carries HF (and occasionally 6 meter) signals, but it does not regularly support VHF contacts above 6 meters or any UHF contacts.
• D. Faraday rotation changes polarization; it is not a propagation mode that extends range.

B. 6 meters

Meteors entering the atmosphere leave short-lived trails of ionized gas that reflect radio signals. These trails reflect lower VHF frequencies more strongly and for longer than higher ones, so 6 meters suits meteor scatter better than the higher bands.

• A. At 33 centimeters the trails reflect far too little energy for practical meteor scatter.
• C. 2 meters is used for meteor scatter, but its reflections are weaker and briefer than on 6 meters.
• D. At 70 centimeters, meteor trails rarely reflect usable signals.

D. Temperature inversions in the atmosphere

A temperature inversion places a layer of warm air above cooler air, the reverse of the usual pattern. The sharp change in the air's refractive index at the boundary bends VHF and UHF signals back down, forming a duct that carries them far beyond the horizon.

• A. Lightning produces static crashes, not ducting.
• B. Sunspots and solar flares affect the ionosphere, while ducting happens in the troposphere, the layer of the atmosphere nearest the ground.
• C. Storm updrafts do not create ducts; ducting needs a stable layer of warm air over cool air.

A. From dawn to shortly after sunset during periods of high sunspot activity

10 meter F region propagation needs strong ionization, which comes from the sun. It occurs mostly in daylight, from dawn to shortly after sunset, and during periods of high sunspot activity, when the ionosphere can refract these higher HF frequencies.

• B. After sunset the F region's ionization declines, so 10 meters usually closes at night even when sunspot activity is high.
• C. During low sunspot activity the ionosphere often cannot support 10 meter F region propagation, even in daylight.
• D. At night during low sunspot activity, ionization is far too weak for 10 meter F region propagation.

A. 6 and 10 meters

During the peak of the sunspot cycle, the F region becomes ionized enough to refract signals on the 10 meter band and, at times, 6 meters, allowing worldwide contacts. Higher bands are beyond what the F region can return.

• B. 23 centimeters (1240 to 1300 MHz) is far too high in frequency for the F region to refract.
• C. 70 centimeters and 1.25 meters are also too high for F region propagation, even at the peak of the cycle.
• D. Only 6 and 10 meters can use F region propagation, so not every choice is correct.

C. The atmosphere refracts radio waves slightly

The atmosphere's density, and with it its refractive index, decreases with height. This bends VHF and UHF waves slightly back toward Earth, so they follow the curve of the Earth a little and reach somewhat beyond the visual horizon.

• A. Radio waves do not travel faster than light; they travel at the speed of light in free space and slightly slower in air.
• B. Dust particles are not what sets the radio horizon; the extra distance comes from refraction.
• D. Dust does not block radio waves in any meaningful way, and blocking would shorten the radio horizon rather than extend it.