The layers of the atmosphere are in alphabetical order; the D, E, and F layers are considered the ionosphere and the D layer (at a height of 30 to 60 miles) is the region closest to the surface of the earth.

The ionosphere is the portion of the atmosphere responsible for Skywave Propagation (bouncing signals "off of the sky" to come back down long distances away), which makes it of particular interest to Amateur Radio operators.

For more info see Wikipedia: D region

To remember this you may want to remember D for Down.

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Each HF frequency is affected differently by the ionosphere at different times of day, season and angle it strikes the ionosphere. The maximum frequency that will be refracted back to earth, and not just continue into space, at a given time is called the "critical frequency".

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(C). The F2 region appears when the highest layer of the ionosphere (the F region) becomes strongly ionized. As the ionization increases, the F layer splits into the lower F1 region and the higher F2 region. Because it is the highest ionospheric region, long distances may be reached in one bend or "hop" of the signal (up to about 2,500 miles!).

This is why the F2 region is mainly responsible for the longest distance radio wave propagation.

For more info see Wikipedia: F2 region

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Silly hint: Your health is critical with high blood pressure.

In radio wave propagation, the term critical angle refers to the highest takeoff angle that will return a radio wave to the Earth under specific ionospheric conditions. If the angle of the signal leaving your antenna is too low, it may not reach the ionosphere, or if parallel to the earth, not even make it around the curvature of the earth and be lost. If the angle is too great the signal may go straight through the atmosphere and be lost into space. The critical angle, like the critical frequency levels of MUF and LUF are dependent on the conditions of the ionosphere, as the height of layers change with ionization levels, and the angle needed to reach those levels will change with those factors.

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HF frequencies such as the 40, 60, 80 and 160 meter bands are harder to use for long distance communication during the day. The same factors that make the upper E and F layers great for the higher VHF frequencies make the lower D layer more "absorbant" of HF wave signals. The lower HF signals are attenuated or absorbed and so do not pass through for bending at higher ionospheric levels. Ionization is higher during daylight hours, and so the D layer absorbs these frequencies much more during daylight hours. This makes the band much more useful for nighttime use than during the day.

Remember "D" for Daylight, Dull, Diminished. For more info: see Wikipedia

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HF scatter signals often sound distorted because the energy is scattered into the skip zone through several different radio wave paths. When this happens, the original signal is Split or Scattered and will be de-fragmented and may sound distorted at the receiver.

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HF scatter signals often sound distorted because the energy is scattered into the skip zone through several different radio wave paths.

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Note:
A signal which is properly adjusted for angle and frequency is like tossing a clump of clay into the air: it makes a nice arc and comes back to earth at one spot, landing with a nice "thunk".

A signal which is scattered into the skip zone is like tossing up that same lump of clay into the air, but this time it hits the leaves of an overhanging tree which breaks up the clump. Some parts gets stuck in the tree, and the parts that do come down land in a wider area, making lots of smaller "scattered" sounds.

Silly, but easy, tip: the word "scatter" is used in the answer.

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An HF signal in the skip zone gets broken up by the ionospheric conditions, where only a small part of the signal energy is scattered into the skip zone. The signal that does get back to the receiving station arrives in "little bits" at slightly different angles, and so produces a weak and wavering sound.

Hint: 'skip zone' in both question and answer.

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With radio signals you have ground wave, meaning your signal goes between point a-b directly. Then you have sky waves, where your signal hits the ionosphere and bounces back down to earth possibly hundreds of miles away. What happens if you have a station you want to talk to in between the reach of your ground wave and sky wave? Those are the stations in your "skip zone." The best way to reach them is by scatter, meaning your signal will scatter off the ionosphere and possibly hit your station in the skip zone. NVIS or Near Vertical Incidence Skywave is scatter propogation.

Silly Hint: Imagine yourself "scattering" papers about trying to figure out why the signal is heard in the skip zone.

The shortest answer is the correct answer.

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Near Vertical Incidence Sky-wave (NVIS) propagation as the name indicates uses signals which are sent out at very high angles, yet below the critical angle, or these signals would go right out into space! Because these signals take a shorter path through the ionospheric layers, they often have less absorption, attenuation or distortion of the signal. This method can be great for high quality short distance HF propagation using these high elevation angles, especially where there are interferences in the way of direct ground-wave propagation.

Hint: Near vertical implies high elevation angle.

For more info see Wikipedia: Near Vertical Incidence Sky-wave (NVIS)

Oversimplification: basically, instead of an antenna that radiates parallel to the ground, like a mag mount on your car or a Yagi pointed across town, you tip it over 90 degrees so you're sending RF at the sky (Yagi pointed at zenith in "Christmas Tree Mode"). These are also known as "cloud warmers."

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The daylight ionization that makes the upper ionospheric regions great for VHF sky-wave propagation works against HF frequencies below 10 MHz. During the daytime the D layer absorbs these signals. Think of the D layer as a "Daylight Dud". This layer becomes much more usable after dark, when ionization of the band decreases its "absorbent" character.

For more info see Wikipedia: D region

Hint: During Daylight D region

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