Technician · correct answers only, in question-pool order.
VHF signals can arrive by more than one path after reflecting from nearby objects. Moving only a few feet changes the path lengths, so the signals may add together or cancel.
Leaves, branches, and other vegetation absorb energy increasingly well at UHF and microwave frequencies, weakening signals that pass through them.
Weak-signal CW and SSB work on VHF and UHF conventionally uses horizontal polarization, so stations use matching horizontal antennas to avoid polarization loss.
When one antenna is vertical and the other is horizontal on a line-of-sight path, their polarizations do not match well, so less signal is transferred between them.
Buildings and terrain can reflect VHF and UHF signals. A directional antenna may find an indirect reflected path to a repeater even when the direct path is blocked.
As a vehicle moves through overlapping reflected signals, the paths alternately reinforce and cancel. The rapid rise and fall in signal strength sounds like flutter called picket fencing.
Rain, snow, and other precipitation absorb and scatter microwave energy. That additional loss can reduce communication range.
Ionospheric signals can reach the receiver over several paths. As those paths change, the arriving signals combine differently and produce irregular fading.
Ionospheric propagation can change a signal into elliptical polarization, so the arriving wave is not purely vertical or horizontal. Either antenna orientation can therefore receive some of the signal.
Multiple delayed copies of a digital signal can overlap at the receiver and make symbol decisions harder. The result is an increased data error rate.
The ionosphere contains charged particles that can refract or return HF signals toward Earth, allowing contacts well beyond the normal line-of-sight range.
Fog and ordinary rain cause little attenuation on 10 meters and 6 meters compared with their much larger effects at microwave frequencies.
A radio wave has electric and magnetic fields that are perpendicular to each other and also perpendicular to the direction the wave travels.
Radio-wave polarization is defined by the orientation of its electric field. A vertically polarized wave has a vertical electric field; a horizontally polarized wave has a horizontal one.
A radio wave is electromagnetic energy made of linked electric and magnetic fields that travel together through space.
In free space, radio waves are electromagnetic waves and travel at the same fundamental speed as light.
Frequency and wavelength are inversely related because their product is the wave's propagation speed. Raising frequency therefore shortens wavelength.
Using frequency in megahertz, an easy amateur-radio approximation is wavelength in meters equals 300 divided by frequency in MHz.
Amateur bands are commonly identified both by frequency and by an approximate wavelength, such as the 2-meter, 20-meter, or 70-centimeter bands.
VHF means Very High Frequency and spans 30 MHz through 300 MHz.
UHF means Ultra High Frequency and spans 300 MHz through 3000 MHz, or 3 GHz.
HF means High Frequency and spans 3 MHz through 30 MHz.
The speed of electromagnetic waves in free space is approximately 300 million meters per second.
Microwave, UHF, VHF, and all other radio-frequency electromagnetic waves travel at the same speed in free space. Frequency changes wavelength, not free-space propagation speed.
UHF is normally a line-of-sight service because the ionosphere usually does not return UHF signals to Earth. Without an unusual propagation mode, reception generally ends near the radio horizon.
HF signals are commonly refracted by the ionosphere and returned to Earth, making routine long-distance communication possible far beyond the horizon.
Auroral backscatter reflects VHF signals from disturbed regions near the aurora. The rapidly changing path produces a distinctive rough or raspy sound.
Sporadic E forms temporary highly ionized patches in the E region that can support strong, unexpected propagation on 10 and 6 meters and sometimes as high as 2 meters.
Radio waves can bend around a sharp terrain edge or obstruction. This knife-edge diffraction can deliver usable signal into an area that lacks a direct line-of-sight path.
Tropospheric ducting occurs when atmospheric layers refract VHF and UHF signals so they remain trapped near Earth for long distances, sometimes hundreds of miles.
Meteor trails briefly ionize the upper atmosphere and can reflect VHF signals. The 6-meter band is especially well suited to taking advantage of these short-lived paths.
A temperature inversion places warmer air above cooler air and can create a refractive layer in the troposphere. That layer may trap VHF/UHF signals and form a duct.
Ten-meter F-region propagation improves when solar activity is high and the ionosphere is strongly ionized. The best long-distance conditions are generally from daylight hours into shortly after sunset.
Near the peak of the solar cycle, increased ionization can support F-region propagation at unusually high HF and low-VHF frequencies, including 10 meters and sometimes 6 meters.
The lower atmosphere bends radio waves slightly toward Earth. This refraction makes the practical VHF/UHF radio horizon extend somewhat beyond the geometric visual horizon.