Amateur Extra · correct answers only, in question-pool order.
For EME, two Earth stations can communicate as long as both can see the Moon. Their separation along Earth's surface can therefore approach about 12,000 miles.
Libration fading in EME is caused by the changing geometry of reflections from the Moon's irregular surface, producing rapid multipath changes and a fluttery, irregular signal.
EME path loss is lowest when the Moon is at perigee because that is when it is closest to Earth.
An electromagnetic wave travels in a direction perpendicular to both its electric field and its magnetic field.
The electric and magnetic fields of an electromagnetic wave are oriented at right angles to each other.
When darkness lowers the MUF for a path, moving to a lower HF band keeps the operating frequency below the new MUF so ionospheric propagation can continue.
Tropospheric ducts over large bodies of water can form where strong temperature and humidity gradients bend microwave signals and trap them in a refractive layer.
Meteor trails form ionized columns primarily in the E region, where VHF signals can be briefly scattered back to Earth.
Meteor scatter is most useful from roughly 28 to 148 MHz: low enough for efficient trail reflection and high enough to avoid ordinary HF ionospheric propagation dominating.
The propagation speed of an electromagnetic wave in a medium is determined by the medium's index of refraction.
Microwave tropospheric ducting commonly supports paths on the order of 100 to 300 miles, substantially beyond the normal line-of-sight horizon.
Severe geomagnetic storms produce strong auroral activity and ionization in polar regions, creating conditions for auroral radio propagation.
Auroral propagation severely distorts voice signals but CW can remain recognizable despite the characteristic rough, fluttery tone.
In circular polarization, the electric and magnetic field vectors rotate as the wave travels.
Transequatorial propagation is most likely on paths roughly perpendicular to the geomagnetic equator, with stations separated by about 2,000 to 3,000 miles.
Transequatorial propagation can support signal paths of roughly 5,000 miles under favorable conditions.
Transequatorial propagation is most common in the afternoon and early evening, when equatorial ionization patterns favor the path.
The ionosphere can split a wave into ordinary and extraordinary components that propagate independently with different elliptical polarizations.
On 160 meters, ionospheric and atmospheric absorption are lowest along paths in darkness, making all-dark paths the best choice for long-distance propagation.
Long-path propagation is encountered most often on 40 and 20 meters, where ionospheric conditions commonly support the very long route around Earth.
Lowering the takeoff angle increases the horizontal distance traveled before the signal returns to Earth, so each ionospheric hop covers more ground.
Ground-wave propagation becomes less effective as frequency increases, so its maximum useful range decreases with increasing frequency.
Sporadic-E propagation is most common around the solstices, especially during the summer solstice season.
Chordal-hop propagation avoids one or more lossy ground reflections by refracting successively through the ionosphere, so the signal can suffer less loss than ordinary multi-hop propagation.
Sporadic-E is primarily a daytime phenomenon in this pool, with the greatest likelihood between sunrise and sunset.
Chordal-hop propagation consists of successive ionospheric refractions without an intervening reflection from Earth's surface.
Ground-wave propagation depends on the electric field interacting with Earth's surface, so vertical polarization is supported while horizontal polarization is heavily attenuated.
Short-term HF radio blackouts are caused by solar flares, whose X-ray and ultraviolet radiation rapidly increase D-region ionization and absorption on the sunlit side of Earth.
The A-index and K-index measure geomagnetic disturbance. Rising values indicate that Earth's geomagnetic field is becoming more disturbed.
When geomagnetic indices are elevated, paths through the auroral oval experience especially strong absorption and disruption.
Bz is the north-south component of the interplanetary magnetic field carried by the solar wind.
A southward Bz opposes Earth's magnetic field and allows stronger magnetic reconnection, increasing the chance of disturbed geomagnetic conditions.
Atmospheric refraction bends VHF and UHF signals slightly toward Earth, so the radio horizon is about 15 percent farther than the purely geometric horizon.
Solar flares are classified A, B, C, M, and X in increasing X-ray intensity. Class X is the strongest of those listed.
The NOAA geomagnetic-storm scale runs from G1 through G5, with G5 designating an extreme geomagnetic storm.
Amateur propagation-reporting networks commonly collect reception reports from digital modes and CW to show where signals are being heard.
The 304A parameter measures solar ultraviolet emission near 304 angstroms and is correlated with the conventional solar flux index.
VOACAP is a propagation-prediction program used to model expected HF communication paths and reliability.
A sudden broadband rise in HF background noise can indicate that a coronal mass ejection has impacted Earth or that a solar flare has occurred.