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G3C

RADIO WAVE PROPAGATION

- Ionospheric regions; critical angle and frequency; HF scatter; near vertical incidence skywave (NVIS)

G3C011 of 11

Which ionospheric region is closest to the surface of Earth?

Why The ionosphere is layered by altitude, and the letters run in order going up: D is lowest (roughly 50-90 km), then E (about 100 km), then F1 and F2 (150 km and up, merging into a single F region at night). Because the D region sits in the densest part of the upper atmosphere, collisions are frequent there and it acts mainly as an absorber of lower HF signals during daylight rather than a reflector.

Watch out The F2 region is the one most often used for long-distance HF skip, so it is the most familiar name, but it is the highest region, not the closest to Earth.

Alphabet order equals altitude order: D, E, F1, F2 from the ground up. D is 'Down low' and 'Daytime absorber'.

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

G3C022 of 11

What is meant by the term "critical frequency" at a given incidence angle?

Why The ionosphere bends (refracts) signals back toward Earth only up to a limit set by its electron density and the angle at which the wave enters. For a given incidence angle, the highest frequency that still gets bent back instead of passing into space is the critical frequency; above it the signal punches through the layer and is lost. Measured straight up (vertical incidence) this is foF2, and the usable frequency rises as the angle gets lower, which is why long, low-angle paths support higher bands than short ones.

Watch out The choice about the lowest frequency refracted back confuses this with the LUF, and the LUF is set by D-layer absorption, not by failure to refract. Lower frequencies are refracted more easily, not less.

Critical = ceiling. Go above it and your signal goes through the ionosphere, not back down.

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

G3C033 of 11

Why is skip propagation via the F2 region longer than that via the other ionospheric regions?

Why Single-hop distance is set by geometry: the higher the reflecting layer, the shallower the angle at which a ray can still be bent back and the farther it lands. The F2 region sits roughly 250 to 400 km up, well above the E region near 100 km, so one F2 hop can span about 2500 miles while an E-region hop is limited to roughly 1200 miles.

Watch out F2 is indeed the most heavily ionized region, but that high electron density is what raises the MUF, not what stretches the hop; the extra distance comes purely from the layer's altitude.

Highest mirror, longest bounce: F2 at ~250+ km gives ~2500 miles per hop, E at ~100 km only ~1200.

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

G3C044 of 11

What does the term "critical angle" mean, as applied to radio wave propagation?

Why Takeoff angle is measured up from the horizon, and the steeper a wave hits the ionosphere, the harder it is to bend it back down. For a given frequency and level of ionization there is a maximum takeoff angle at which the wave still refracts back to Earth; go any steeper and it punches through into space. That limit is the critical angle, and it is the angular counterpart of the critical frequency, which is the highest frequency returned when the wave is sent straight up.

Watch out The choice naming the lowest takeoff angle inverts the idea: low, shallow angles are the easiest to return and give the longest skip distances, so there is no lower cutoff. Long path and short path azimuths are bearings in the horizontal plane, nothing to do with elevation.

Too steep and you leak through: the critical angle is the HIGHEST angle that still comes back.

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

G3C055 of 11

Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?

Why The D region is the lowest ionospheric layer (about 50-90 km) and it only exists while the sun is up, because it requires constant solar ionization and its dense air lets ions recombine within minutes after sunset. Rather than refracting signals back to Earth, the D region absorbs them, and that absorption gets much stronger as frequency goes down (roughly proportional to 1/f squared). So the low bands, 160, 80, 60 and 40 meters, lose their energy on the way up during the day, and only come alive for DX after dark when the D region disappears.

Watch out The F region is what actually refracts HF signals back to Earth for long-haul work; it does not absorb the low bands, and daytime is when it is strongest, splitting into F1 and F2.

D is for Daytime and Death of the low bands. Lower frequency, more D-region absorption.

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

G3C066 of 11

What is a characteristic of HF scatter?

Why Scatter signals reach the receiver by many small ionospheric or tropospheric scattering paths at once, and those paths vary in length and change moment to moment. The many components add and cancel with slightly different delays and Doppler shifts, so the received signal has a rapid flutter or watery, distorted quality. Scatter signals are also typically weak and the audio is hard to copy.

Watch out The choice about high intelligibility is the opposite of reality, since multipath distortion makes scatter phone signals sound rough and hard to understand, and scatter is not a nighttime-only mode. Very large sudden swings in strength describe other fading mechanisms, not the fast flutter of scatter.

Scatter = many scattered paths = flutter. Weak, watery, hard to copy, not crisp.

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

G3C077 of 11

What makes HF scatter signals often sound distorted?

Why Scatter fills the skip zone with weak energy that arrives after bouncing off many small irregularities in the ionosphere (or off the ground/troposphere), so the receiver hears several copies of the same signal that traveled slightly different path lengths. Those copies arrive a few milliseconds apart and add and cancel randomly, which smears the audio and gives scatter signals their characteristic hollow, fluttery, distorted sound. This is classic multipath distortion, the same effect that makes long-path and chordal-hop signals sound rough.

Watch out Blaming an unstable ionospheric region is tempting because ionization really does fluctuate, but that produces fading in amplitude, not the smeared multipath distortion; ground wave absorption and the presence or absence of the E region have nothing to do with how scattered signals sound.

Scatter = many paths at once. Multiple paths, multiple arrival times, mushy audio.

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

G3C088 of 11

Why are HF scatter signals in the skip zone usually weak?

Why The skip zone is the ring between the end of ground wave coverage and where the first skywave hop lands, so no direct refracted signal reaches it. What you hear there is scatter: small amounts of energy randomly redirected by irregularities in the ionosphere, by the ground at the far end of the hop, or by the troposphere. Because only a tiny fraction of the transmitted power gets deflected back into that zone, the signal is weak, often watery sounding and fluttery.

Watch out The magnetosphere is far above the ionosphere and plays no role in normal HF scatter, and ground wave dies out well before the skip zone, so neither explains what you hear there.

Scatter = crumbs, not the loaf. Only a sliver of energy lands in the skip zone, so it sounds weak and watery.

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

G3C099 of 11

What type of propagation allows signals to be heard in the transmitting station's skip zone?

Why The skip zone is the dead region between the end of ground wave coverage and the point where the first ionospheric hop returns to Earth, so normal refraction puts no signal there. Scatter propagation, where a small part of the signal is scattered in the ionosphere (or off terrain, aircraft, or meteor trails) in directions other than the main refracted path, spills weak energy back into that gap. Scatter signals are typically weak, distorted, and have a watery or fluttery sound because the energy arrives over multiple slightly different paths.

Watch out Short-path is just the shorter of the two great-circle routes to a distant station and still lands beyond the skip zone; chordal hop describes signals refracting between ionospheric layers without touching the ground, which extends long-distance range rather than filling the skip zone.

Skip zone isn't totally dead: weak, watery, fluttery signals sneak in by scatter.

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

G3C1010 of 11

What is near vertical incidence skywave (NVIS) propagation?

Why NVIS works by radiating signals almost straight up at elevation angles near 90 degrees so they reflect from the F (or E) region and rain back down over a circle roughly 0 to 300 miles around the station. It fills in the skip zone that normal low-angle skywave leaves empty, and it needs a frequency below the critical frequency (typically 80 m and 40 m, sometimes 60 m) since only frequencies under f-critical come back when sent straight up. Low horizontal antennas, about a tenth to a quarter wavelength high, naturally produce this high-angle pattern.

Watch out Propagation near the MUF is the opposite situation: the MUF applies to low-angle, long-distance paths, while NVIS deliberately uses frequencies well below the critical frequency so near-vertical signals are refracted rather than punching through.

NVIS = shoot straight up, come straight down. Short range, high angle, low frequency, low antenna.

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

G3C1111 of 11

Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?

Why The D region is the lowest ionized layer, about 60 to 90 km up, where air density and therefore electron-neutral collision rates are highest. Electrons set in motion by a passing wave collide with neutral molecules and lose that energy as heat instead of re-radiating it, so the wave is attenuated rather than refracted. This absorption rises sharply as frequency falls (roughly as 1/f squared), which is why 160, 80 and 40 meters are short-range in daylight but open up at night when the D region quickly disappears.

Watch out The E and F regions are the ones that actually refract HF signals back to Earth; the F2 region is the main daytime long-haul reflector, not an absorber.

D is for Daytime Deadener: lowest region, kills the low bands until sunset.

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

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