HamSandwich

Extra lettuce, Extra class.

Study › Amateur Extra › E3A

E3A

RADIO WAVE PROPAGATION

Electromagnetic Waves and Specialized Propagation: Earth-Moon-Earth (EME) communications; meteor scatter; microwave tropospheric and scatter propagation; auroral propagation; daily variation of ionospheric propagation; circular polarization

Drill results are kept in this browser. Log in to keep them on your account and get a study plan.

E3A011 of 14

What is the approximate maximum separation measured along the surface of the Earth between two stations communicating by EME?

Why EME works like a passive repeater: your signal must travel to the Moon and back, so both stations need the Moon above their horizons at the same time. Two stations can each have the Moon just on their horizon when they are on nearly opposite sides of the Earth, which is about half the Earth's circumference, roughly 12,000 miles. Mutual visibility of the Moon, not distance or Moon phase, is the real limit.
Watch out The perigee and apogee answers suggest the Moon's distance sets the range, but the Moon's distance only affects path loss and echo delay, not how far apart two stations on Earth can be.
EME range = wherever both stations can see the Moon, about 12,000 miles, half way around the Earth.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A022 of 14

What characterizes libration fading of an EME signal?

Why The Moon is a rough, irregular reflector, and its libration (the slight wobbling of its face as seen from Earth) constantly changes the relative path lengths from the thousands of scattering points on its surface. Those reflections add and cancel at random, producing rapid multipath interference on the received signal. The result is the fluttery, irregular fading EME operators hear, sometimes fast enough to make CW sound rough.
Watch out The choice about the echo returning several hertz lower in frequency describes Doppler shift from the Earth-Moon relative motion, a separate EME effect; a slow pitch change would also be Doppler, not libration.
Libration = the Moon wobbles, so its rough face scatters many echoes: flutter. Doppler = frequency shift, a different problem.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A033 of 14

When scheduling EME contacts, which of these conditions will generally result in the least path loss?

Why EME path loss follows the inverse square law on both the up and down legs, so total loss scales with the fourth power of distance. Perigee is the closest point of the Moon's elliptical orbit, roughly 356,000 km versus about 407,000 km at apogee, which is worth about 2 dB of extra signal. Operators also favor times when the Moon is high and away from the noisy galactic plane, but path loss itself depends only on range.
Watch out A full Moon is about illumination by the Sun, not distance, and it has nothing to do with radio reflection; in fact a full Moon near the Sun-opposite point can add solar or sky noise concerns but does not change path loss. MUF is an ionospheric parameter and is irrelevant to a signal that must pass through the ionosphere to the Moon.
PERIgee = PROXIMITY. Closest Moon, strongest echo, about 2 dB better than apogee.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A044 of 14

In what direction does an electromagnetic wave travel?

Why An electromagnetic wave is transverse: the electric field and the magnetic field are at right angles to each other, and the direction of propagation is perpendicular to both of them. This is described by the Poynting vector, the cross product of E and H, which points along the direction energy flows. So if the E field is vertical and the H field is horizontal, the wave moves straight out at 90 degrees to both.
Watch out The idea that the wave travels parallel to the fields describes a longitudinal wave like sound, not an EM wave; radio waves have no field component along the direction of travel.
Think of your right hand: E, H and direction of travel are three mutually perpendicular axes, like the corner of a box.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A055 of 14

How are the component fields of an electromagnetic wave oriented?

Why In a radio wave traveling through free space, the electric field and the magnetic field are perpendicular to each other, and both are perpendicular to the direction the wave is traveling. That is why it is called a transverse electromagnetic (TEM) wave. Each field regenerates the other as the wave propagates, and their mutual right-angle geometry is what sets the direction of travel (the Poynting vector, E cross H).
Watch out The choice about being 90 degrees out of phase confuses space with time: in a plane wave in free space the E and H fields peak at the same instant, in phase, while it is their orientation in space that is 90 degrees apart.
E and H: right angles in space, in step in time.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A066 of 14

What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?

Why The maximum usable frequency depends on ionization in the F region, which is produced by solar radiation. As the path goes into darkness the ionization decays, the critical frequency drops and the MUF for that path falls, so the band you were using may now pass through the ionosphere instead of refracting back. Moving down to a lower HF band puts your signal back below the MUF and keeps the path open, which is why 20 meters often closes at night while 40 and 80 meters open up.
Watch out Going to a higher band is exactly the wrong direction, since the frequency is already approaching the MUF and a higher one will punch through the layer. Raising the takeoff angle shortens the hop distance, which hurts rather than helps a long-distance path.
Sun goes down, frequency goes down. Less ionization means a lower MUF.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A077 of 14

Atmospheric ducts capable of propagating microwave signals often form over what geographic feature?

Why Tropospheric ducts form when a temperature inversion puts warm, dry air above cool, moist air, creating a sharp drop in refractive index that traps VHF/UHF/microwave signals in a waveguide-like layer. That layering happens most reliably over large bodies of water, where cool water chills the surface air and keeps it humid while warmer air sits above. Marine ducts over the ocean, the Gulf of Mexico, or the Great Lakes routinely support contacts of hundreds of miles on bands from 144 MHz up through the microwave bands.
Watch out Clouds themselves, whether stratocumulus or nimbus, are not the mechanism; a duct is about the refractive index gradient of the air, and stratocumulus decks are simply a common visible sign of an inversion rather than the duct itself. Mountain ranges support knife-edge diffraction and rain scatter, not ducting.
Ducting loves water: cool moist air under warm dry air over the sea makes a natural waveguide.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A088 of 14

When a meteor strikes the Earth's atmosphere, a linear ionized region is formed at what region of the ionosphere?

Why Meteors burn up and ionize the air at roughly 85 to 120 km altitude, which is exactly the height of the E region. The friction of the meteoroid leaves a dense, cylindrical (linear) column of ionization along its path that can refract or reflect VHF signals for a fraction of a second up to a few seconds. That is why meteor scatter contacts on 6 meters and 2 meters behave like brief, intense sporadic-E style openings out to about 1200 miles.
Watch out The F1 and F2 regions sit much higher, around 150 to 400 km, well above where meteors ablate, and the D region (50 to 90 km) is the absorbing layer that mainly attenuates lower HF by day rather than forming meteor trails.
Meteor trails burn up around 100 km, the same height as sporadic E. Meteors = E region.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A099 of 14

Which of the following frequency ranges is most suited for meteor-scatter communications?

Why Meteor trails leave a brief column of ionization that behaves like a short-lived ionized reflector, and its electron density is only high enough to reflect signals up to roughly 150 MHz. The sweet spot runs from about 28 MHz through 148 MHz, which covers 10, 6 and 2 meters, with 6 meters generally considered the best meteor scatter band. Below that range normal ionospheric propagation and higher noise levels mask the short pings; above it the trails are too weakly ionized to return a usable signal.
Watch out The 220 to 450 MHz choice is tempting because it is still VHF/UHF, but the ionization density in a meteor trail cannot support reflection that high, so returns are extremely rare and weak.
Meteor scatter lives on 10, 6 and 2 meters: think 28 to 148 MHz, with 6 meters the classic ping band.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A1010 of 14

What determines the speed of electromagnetic waves through a medium?

Why In free space electromagnetic waves travel at c, about 3 x 10^8 m/s, but in any material they slow to v = c/n where n is the index of refraction. That index comes from the medium's permittivity and permeability (n = sqrt of the relative values multiplied together), so the electrical properties of the material set the propagation speed. This is the same quantity that governs velocity factor in feed lines and bending of waves in the ionosphere.
Watch out Resistance and reactance describe circuit behavior, not wave speed in a medium; birefringence is a material having two different indices for different polarizations, and evanescence describes a non-propagating field that decays with distance.
v = c/n: the index of refraction is literally the ratio telling how much a medium slows a wave.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A1111 of 14

What is a typical range for tropospheric duct propagation of microwave signals?

Why Tropospheric ducting happens when a temperature inversion creates a layer of warm, dry air over cooler, moist air, forming a waveguide-like duct that traps VHF/UHF/microwave signals and keeps them from spreading upward. Signals can travel far beyond the normal radio horizon with low loss, and the commonly cited working range is on the order of 100 to 300 miles. Exceptional ducts over water can stretch farther, but the pool's expected answer is the few-hundred-mile figure.
Watch out The 10 to 50 mile choice is just ordinary line-of-sight VHF/UHF range with no duct at all, and the 1,200 and 2,500 mile figures are more typical of ionospheric F-layer HF hops, which microwaves do not use.
Ducting = a few hundred miles: remember 100-300. Anything over 1,000 miles is ionospheric, not tropospheric.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A1212 of 14

What is most likely to result in auroral propagation?

Why Auroral propagation happens when charged particles from the Sun, funneled by Earth's magnetic field into the polar regions, ionize patches of the E region at high latitudes. That requires a disturbed magnetic field, which is exactly what a severe geomagnetic storm (high K index, following a coronal mass ejection or fast solar wind stream) produces. Stations aim their antennas north at the auroral curtain and use it as a reflector, mostly on 6 meters and 2 meters, with signals arriving badly distorted and buzzy so CW is favored over SSB.
Watch out Quiet geomagnetic conditions are the opposite of what is needed and generally give the best HF conditions instead; meteor showers support meteor scatter, a completely separate mode using ionized trails from burning meteors.
Aurora needs a storm: high K index, antennas north, buzzy CW. Quiet geomagnetic field means no aurora.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A1313 of 14

Which of these emission modes is best for auroral propagation?

Why Auroral propagation reflects signals off the rapidly moving, turbulent curtains of ionized particles in the auroral zone, which smears the signal with severe Doppler spreading and a characteristic raspy buzz. CW survives this because the information is carried only by the presence or absence of a tone, so even a hissy, distorted note is still copyable. Voice and digital modes need the fine frequency and phase structure that auroral reflection destroys.
Watch out SSB is the usual DX voice mode and is sometimes used on aurora at VHF, but the distortion makes speech muffled and hard to understand, so it is second best at most; RTTY and other tone-decoding modes fail outright when the tones are smeared.
Aurora = buzzy, warbly signals. Only a mode that just needs on or off survives: CW.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E3A1414 of 14

What are circularly polarized electromagnetic waves?

Why Polarization describes the orientation of the electric field (and the magnetic field perpendicular to it). In a circularly polarized wave, the E and H field vectors rotate steadily about the direction of travel, completing one full turn each RF cycle, so the tip of the field vector traces a helix through space. This is produced by combining two linearly polarized components of equal amplitude that are 90 degrees out of phase, as with crossed dipoles fed in quadrature or a helical antenna. Circular polarization comes in right-hand and left-hand senses and is popular for satellite and EME work because it tolerates the unknown or shifting orientation of the far end, including Faraday rotation.
Watch out The idea of a field "bent into a circular shape" misreads the name: nothing about the wave is curved, it is the direction the field vector points that rotates. An ordinary loop antenna radiates linear polarization, not circular.
Circular = rotating, not curved. Two linear waves 90 degrees apart in phase make the field spin like a corkscrew.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
Baofeng UV-5R Dual Band Two Way RadioSponsored · View on Amazon →Baofeng BF-F8HP Pro Dual Band HandheldSponsored · View on Amazon →
← E2E All groups E3B →