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E3B

RADIO WAVE PROPAGATION

Transequatorial propagation; long-path propagation; ordinary and extraordinary waves; chordal hop; sporadic-E mechanisms; ground-wave propagation

E3B011 of 13

Where is transequatorial propagation (TEP) most likely to occur?

Why Transequatorial propagation depends on the ionization crests that form on either side of the geomagnetic equator, so it links stations that are roughly equidistant north and south of it. That means the path runs north-south, perpendicular to the magnetic equator, and typical distances are about 2,000 to 3,000 miles total. The signal effectively takes a high chordal hop between the two ionization crests, often supporting VHF contacts well above the normal MUF.

Watch out Stations sitting on the geomagnetic equator itself are not in the right geometry; the enhanced ionization crests lie a bit north and south of it, and the choice with antipodal points describes long-path propagation instead.

TEP = Trans (across) the Equator: north-south path crossing the magnetic equator, about 2,000 to 3,000 miles.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B022 of 13

What is the approximate maximum range for signals using transequatorial propagation?

Why Transequatorial propagation (TEP) works between stations roughly equidistant north and south of the geomagnetic equator, each up to about 2,500 miles from it. Signals refract off dense, bulging ionization in the equatorial anomaly regions, so the total path length tops out near 5,000 miles. TEP often supports VHF contacts (6 m and even 2 m) around dusk when the MUF along that path far exceeds normal predictions.

Watch out The choice saying 2,500 miles is the distance from the geomagnetic equator to each station, not the end-to-end path; doubling it gives the actual maximum range.

2,500 miles each side of the magnetic equator, so 5,000 miles total.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B033 of 13

At what time of day is transequatorial propagation most likely to occur?

Why Transequatorial propagation depends on the equatorial ionization anomaly, two dense crests of F-region ionization that form on either side of the magnetic equator as daytime ionization is lifted and redistributed by the equatorial fountain effect. Those crests take hours to build, so they reach maximum density late in the day, and signals ricochet between them in the afternoon and early evening. TE paths run roughly north-south across the magnetic equator and often support frequencies well above the normal MUF, sometimes into the VHF range.

Watch out Morning or noon looks appealing because that is when solar ionization is strongest overhead, but the anomaly crests need time to develop after that ionization peaks, so the opening lags the sun by several hours.

TE needs the ionosphere to charge up all day: think 'after dinner, not after breakfast.'

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B044 of 13

What are "extraordinary" and "ordinary" waves?

Why When a radio wave enters the ionized plasma of the ionosphere, the Earth's magnetic field makes the medium birefringent: the wave splits into two components, called the ordinary (o) and extraordinary (x) waves. Each is elliptically polarized and rotates in the opposite sense, and each sees a different refractive index, so they refract, absorb, and travel slightly differently and arrive independently. Their recombination at the receiver with varying phase is one cause of polarization fading. This effect is called magnetoionic splitting.

Watch out The name tempts people into thinking "extraordinary" means an unusual or rare long-skip path, but the term is purely a plasma physics label for one of the two magnetoionic modes, not a description of distance.

Ordinary/extraordinary = one wave split in two by Earth's magnetic field, both elliptically polarized.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B055 of 13

Which of the following paths is most likely to support long-distance propagation on 160 meters?

Why On 160 meters the D layer is the limiting factor: daytime ionization absorbs low-frequency signals before they can reach the F layer and return. Absorption falls roughly as the inverse square of frequency, so at 1.8 MHz even modest D-layer ionization kills the path. Only when the entire path lies in darkness does the D layer recombine enough to let the signal make it to the F layer and back, which is why topband DX is worked in the middle of the night, ideally around gray line.

Watch out A sunlit path is exactly the worst case, since the D layer is fully ionized. High-latitude paths add auroral absorption and flutter, and a north-south path matters for transequatorial propagation on VHF, not for 160 meter absorption.

Topband loves the dark: 160 m DX needs the whole path in night, no D layer, no absorption.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B066 of 13

On which of the following amateur bands is long-path propagation most frequent?

Why Long-path signals take the longer great-circle route around the Earth, often 25,000 miles or more, so the band has to support many low-loss hops (or chordal hops) along a path where much of the ionosphere is in darkness. The 20 meter band is the classic long-path band, especially near sunrise and sunset along the grey line, with 40 meters working at night when absorption drops. These bands sit in the sweet spot: high enough to escape heavy D-layer absorption, low enough that the MUF supports the whole path.

Watch out 160 and 80 meters suffer too much absorption and noise to survive a trip most of the way around the world, and 10 and 6 meters would require a very high MUF sustained over the entire enormous path, which is rare.

Long path lives on 20 (with 40 at night); think grey-line DX on 14 MHz.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B077 of 13

What effect does lowering a signal's transmitted elevation angle have on ionospheric HF skip propagation?

Why Skip distance is set by geometry: the signal leaves the antenna, travels to the ionospheric layer at some height and returns. The shallower (lower) the takeoff angle, the farther downrange the wave travels before it reaches the reflecting layer and comes back, so each hop spans more ground. That is why DXers favor low takeoff angles, with single hops of 2000 miles or more possible off the F2 layer, while high angles give short hops or near-vertical local coverage.

Watch out The choice about MUF going down has it backwards: lowering the elevation angle makes the wave strike the layer more obliquely, which raises the maximum usable frequency for that path. Critical frequency is measured straight up at vertical incidence and does not depend on your antenna's takeoff angle at all.

Low angle, long hop. Flatter launch equals farther DX (and a higher MUF, not a lower one).

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B088 of 13

How does the maximum range of ground-wave propagation change when the signal frequency is increased?

Why Ground-wave (surface-wave) signals travel by hugging the Earth's surface, and the ground acts as a lossy dielectric that absorbs energy from the wave. That absorption grows rapidly as frequency rises, so the useful range shrinks: 160 and 80 meters can give hundreds of miles of ground wave over good soil, while at 10 meters ground wave is only good for a few miles. This is why AM broadcast and LF/MF services rely on ground wave for daytime coverage.

Watch out The idea that range increases with frequency confuses ground wave with skywave or line-of-sight VHF work; higher frequencies do not penetrate or follow the ground better, they are attenuated more. The 8 MHz peak choice is simply invented, there is no such peak.

Lower frequency, longer ground wave. Think of AM broadcast stations at 1 MHz covering whole states in daytime.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B099 of 13

At what time of year is sporadic-E propagation most likely to occur?

Why Sporadic-E clouds of intensely ionized patches in the E layer (about 60-70 miles up) form most often in a broad season centered on the summer solstice, with a weaker secondary peak near the winter solstice. In the northern hemisphere that means a strong May-through-August season for 10, 6 and sometimes 2 meters. The mechanism is tied to wind shear and meteoric ionization rather than to solar flux, so Es appears even at sunspot minimum.

Watch out The equinox answers describe the pattern for F2-layer DX and transequatorial propagation, which favor spring and fall, not Es. The winter-solstice choice names the real but much weaker secondary Es peak.

Sporadic-E = summer sizzle: the big 6 meter season peaks around the June solstice.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B1010 of 13

What is the effect of chordal-hop propagation?

Why In normal multi-hop skip, the wave comes back to Earth between hops and the ground or sea reflection throws away a big chunk of the signal, plus the wave passes through the absorbing D region twice per hop. In chordal hop the signal skips along between ionospheric tilts, refracting from one region to the next without ever touching the ground, so those reflection and extra absorption losses are avoided. The result is unusually strong, low-loss signals over very long paths, which is why transequatorial and long-path openings can sound so good.

Watch out The idea that the MUF is lower is backwards: chordal paths often support higher usable frequencies, and nothing about this mode changes the speed of radio waves or atmospheric noise levels.

Chordal hop = the ionosphere passes the signal along without letting it touch the lossy ground.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B1111 of 13

At what time of day is sporadic-E propagation most likely to occur?

Why Sporadic-E is caused by dense, patchy ionization forming in the E layer, and the E layer is a daytime, sunlight-driven region. Ionization builds after sunrise, so Es openings peak during daylight hours with a broad midday peak and a second peak in the late afternoon or early evening, often lingering past sunset in summer. Overnight the E region largely recombines and disappears, so Es is unlikely in the small hours.

Watch out The overnight choices describe when the E layer has recombined; the F2 layer is what supports long-haul nighttime work on the lower HF bands, not sporadic-E.

E for Earth-lit: the E layer needs sun, so sporadic-E is a sunrise-to-sunset phenomenon, peaking in summer.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B1212 of 13

What is chordal-hop propagation?

Why In ordinary multi-hop propagation the wave bounces between the ionosphere and the ground, and each ground reflection costs several dB of loss. Chordal hop occurs when tilts or gradients in the ionosphere refract the wave back up at a shallow angle so it travels chord-like from one ionospheric refraction to the next without ever touching the ground. Skipping those lossy ground reflections is why chordal-hop paths, common on transequatorial and long-path openings, deliver unusually strong signals over very long distances.

Watch out Propagation across the geomagnetic equator describes transequatorial propagation, which often uses chordal hops but is defined by geography, not by the missing ground reflection; departure from the great circle bearing describes skewed or skew-path propagation.

Chord = a straight line inside the circle: the signal stays up in the ionosphere and never touches the ground.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

E3B1313 of 13

What type of polarization is supported by ground-wave propagation?

Why Ground-wave (surface-wave) propagation depends on the wave clinging to the boundary between air and the conducting earth. A horizontally polarized electric field runs parallel to that conductive surface, so the ground effectively short-circuits it and the wave dies within a short distance. A vertical electric field is perpendicular to the ground and survives, which is why AM broadcast and 160/80 meter surface-wave work uses vertical antennas over good ground or a radial system.

Watch out Horizontal polarization is the usual choice for HF skywave dipoles and beams, but at ground level those signals are absorbed almost immediately, so they contribute nothing to surface-wave range. Circular and elliptical polarization are terms from satellite and VHF work, not surface-wave propagation.

Ground wave wants the E-field standing up: vertical polarization. Lay it flat and the earth shorts it out.

HamSandwich explanation, first draft. The question and answers are the NCVEC text.

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