General · correct answers only, in question-pool order.
Higher sunspot numbers generally mean a greater chance of good HF propagation at the higher frequencies.
A sudden ionospheric disturbance disrupts daytime ionospheric propagation more strongly on lower frequencies than on higher ones.
The increased ultraviolet and X-ray radiation from a solar flare reaches Earth quickly enough to affect radio propagation in about eight minutes.
During periods of low solar activity, 15, 12, and 10 meters are the least reliable choices listed for long-distance communication.
The solar flux index is a measure of solar radio radiation at a wavelength of 10.7 centimeters.
A geomagnetic storm is a temporary disturbance in Earth's geomagnetic field.
The 20-meter band can usually support worldwide daylight propagation at any point in the solar cycle.
A geomagnetic storm can degrade HF propagation at high latitudes.
High geomagnetic activity can create auroras that reflect VHF signals.
The Sun's surface layers rotate around its axis in roughly a 26- to 28-day cycle, producing periodic changes in HF propagation conditions.
A coronal mass ejection can take from about 15 hours to several days to affect radio propagation on Earth.
The K-index describes the short-term stability of Earth's geomagnetic field.
The A-index describes the longer-term stability of Earth's geomagnetic field.
Charged particles reaching Earth from solar coronal holes usually disturb long-distance HF communication.
If a skywave signal reaches you by both short-path and long-path propagation, the different travel times can produce a slightly delayed echo.
The official G3B02 source identifies path distance and location as factors affecting the maximum usable frequency.
The official G3B02 source lists time of day and season among the factors that affect maximum usable frequency.
The official G3B02 source identifies solar radiation and ionospheric disturbances as factors affecting the maximum usable frequency.
For long-distance skip, the least attenuation usually occurs at a frequency just below the maximum usable frequency.
One way to check current propagation on a desired band is to use internet-connected automated receiving stations and see where your transmissions are being received.
Signals above the LUF but below the MUF are refracted by the ionosphere and returned to Earth.
Signals below the lowest usable frequency are usually attenuated too much to reach the destination effectively.
LUF means Lowest Usable Frequency for communication between two specific points.
MUF means Maximum Usable Frequency for communication between two points.
One normal F2-region hop can cover approximately 2,500 miles along Earth's surface.
One normal E-region hop can cover approximately 1,200 miles along Earth's surface.
If the LUF rises above the MUF, there is no frequency window left for ordinary skywave communication over that path.
Lower HF frequencies typically experience high levels of atmospheric noise or static during the summer.
The D region is the ionospheric region closest to Earth's surface.
At a given incidence angle, the critical frequency is the highest frequency that will still be refracted back to Earth.
F2-region skip reaches farther because the F2 region is the highest of the ionospheric regions listed.
The critical angle is the highest takeoff angle that will still return a radio wave to Earth under the specified ionospheric conditions.
During daylight hours, the D region absorbs signals on 40, 60, 80, and 160 meters, making long-distance communication on those bands more difficult.
HF scatter signals often have a fluttering sound.
HF scatter can sound distorted because energy reaches the skip zone over several different scattered paths.
HF scatter signals in the skip zone are usually weak because only a small portion of the transmitted energy is scattered into that area.
Scatter propagation can place signals inside the transmitting station's skip zone.
Near vertical incidence skywave uses high elevation angles for short-distance MF or HF communication.
During daylight, the D region is the most absorbent ionospheric region for signals below 10 MHz.