Any time you hear stations directly (not using a repeater system) from far away, you can assume some sort of atmospheric condition is helping the signal to travel. Most long-distance radio waves bounce off the ionosphere. However, signals in the UHF spectrum have such a short wavelength that they don't bounce off the ionosphere at all -- they pass right through it into outer space. (This is why higher frequencies are ideal for communicating with satellites). Thus, if you hear a signal in the UHF band, it's safe to assume the source of that signal is nearby.

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Tags: uhf propagation radio waves ionosphere arrl chapter 4 arrl module 8

Unlike light, radio waves are not reflected by the Ionosphere; they are refracted or bent back toward earth. "Refracted" is the key word in this answer. Sporadic E is just one type of refraction.

Also, the E Layer is one of 4 layers in the Ionosphere Layers: D layer; E layer; Es layer (or sporadic E-layer); F layer

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Tags: ionosphere propagation vhf arrl chapter 4 arrl module 8

There is a lot going on here for a quick explanation but the best way to understand what is happening is to think of throwing a rock in a small pond and then shining a flashlight on the water. The beam is dancing around on the ripples and waves. The radio waves are doing the same thing in the atmosphere that being excited by the energy causing the auroral phenomenon and just about as quickly.

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Tags: vhf propagation ionosphere arrl chapter 4 arrl module 8

Memory Aid: Sporadic = Occasional

Every now and then a type of propagation occurs that carries the RF energy within a particular range of frequencies quite a long distance, refracting it in just the right way over and over. This type of propagation is known as Sporadic E. It occurs when clouds of intensely ionized gas form in the E region of the earth's ionosphere typically between 90 and 120 km in altitude. The mechanisms behind the formation of the ionized gas clouds are beyond the scope of this text.

Backscatter generally scatters a signal back towards its source, which would not result in strong over-the-horizon signals. The operative word in D layer absorption is absorption where the RF signal is attenuated, not refracted, in the ionospheric layer closest to the ground. Gray-line propagation is a reference to a 45 - 60 minute period around twilight when D layer absorption is diminished but some refraction of signals on the 10 and 15-meter bands can occur before the solar ionization in the E and F layers is diminished with nightfall.

For more information see: "Sporadic E, Es Propagation" on Electronics-Notes.com.

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Tags: propagation vhf uhf 2 meter 10 meter 6 meter arrl chapter 4 arrl module 8

In general, radio signals don't penetrate dirt or rock very well at all. So if you're hearing a signal on the other side of a mountain, it's likely due to knife-edge diffraction, a physical phenomenon that occurs when waves hit a sharp edge. The diffracting object or aperture effectively becomes a secondary source of the propagating wave.

Faraday rotation is way too complex to be explained or even included on a Technician Class license exam.

Quantum tunneling has to do with devices like Tunnel Diodes, which aren't discussed in the Technician Question Pool, and certainly have nothing to do with radio waves.

Doppler Shift, although a topic that does appear in the Question Pool, has to do with the source of the signal moving toward or away from you, and has nothing to do with hearing a signal despite obstructions.

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Tags: arrl chapter 4 arrl module 8

There are several modes that can allow communication that ranges "over-the-horizon" or beyond line-of-sight such as Ducting and Tropospheric scatter. The key to differentiate these two in this question is the mentioning of "VHF" frequencies.

Tropospheric scatter is where the signals are bent or reflected back to earth in a somewhat random manner to a station a significant distance away on a regular basis. But it works in the UHF and microwave frequencies, and is best around 2 GHz.

Tropospheric ducting happens when a large mass of cold air is overrun by warm air causing a temperature inversion, it is relatively common during summer and autumn months and can work as low as 40 MHz, and most commonly works above 90 MHz which covers most the VHF bands.

More information can be found here.

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Tags: propagation ionosphere vhf uhf arrl chapter 4 arrl module 8

Meteor scatter communication is done by reflecting radio waves off ionized particles in the ionosphere that were caused by meteors passing through. The 6-meter band is excellent for meteor scatter due to its wavelength, and because it is a quiet band. Wavelengths longer than 6 meters are not effectively reflected by meteor scatter; shorter wavelength bands, such as the 2-meter band, are not as quiet which makes it difficult to hear these weak signals from 500 to 1500 miles away.

Here is a memory aid: The "6" in 6 meters looks like a meteor with a curved tail

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Tags: 6 meter propagation arrl chapter 4 arrl module 8

To remember this answer, think of heating ducts in a building that carry different-temperature air long distances from a central unit, just like the temperature inversions that cause tropospheric ducting.

Tropospheric ducting is an atmospheric effect caused by a differential temperature layer that causes reflection or refraction of radio wave. These reflective layers can form a radio wave "duct", much like the ducts that are used to duct warm or cool air through our homes. These ducts are often caused by thermal inversions and other weather phenomena. Radio wave propagation can extend from 300 to 500 miles, sometimes as far as 1000 miles, through tropospheric ducting.

The troposphere is the lowest level of the atmosphere and is where temperature inversions occur; understanding this relationship will help you choose the correct answer.

Further information can be found at http://en.wikipedia.org/wiki/Tropospheric_propagation#Tropospheric_ducting

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Tags: propagation ionosphere arrl chapter 4 arrl module 8

Remember that 10 meters follows the sun and thus is best in daylight hours.

The 10 meter band is best during daylight hours due to the nature of this wavelength and how it refracts through or reflects off of the F2 layer of the ionosphere.

During periods of increased sunspot activity, band openings may begin well before sunrise and continue into the night.

In areas near the equator, 10 meters is effective even during periods of low solar activity. This is demonstrated by good propagation between areas in Africa to the Caribbean.

More information is found here.

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Tags: 10 meter propagation arrl chapter 4 arrl module 8

When we talk about radio wave propagation we often say that it is "line of sight". This may cause you to think that the "radio horizon" is the same as the "horizon" that you can see with your eyes. But, this is not always the case depending of the frequency band and atmospheric conditions that may make radio waves go beyond the horizon that we can see.

Thus the "radio horizon" is the distance the radio signal between two points is blocked by the curvature of the Earth.

Key to remember this question and answer pair is that horizon relates to the curvature of the Earth and that radio waves are blocked differently than light (or eye-sight).

Learn more here.

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Tags: propagation arrl chapter 4 arrl module 8

When we talk about radio wave propagation we often say that it is "line of sight". This may cause you to think that radio signals will only travel to where you can see with your eyes. But, this is not always the case depending on the frequency band and atmospheric conditions that may make radio waves go beyond the horizon that we can see. VHF and UHF can bend somewhat around the curvature of the Earth and thus travel further than we can see. This assumes that there are not other significant obstacles that may block the signal such as buildings, trees and hills. This is also called the "radio horizon", or the distance where the radio signal between two points is blocked by the curvature of the Earth.

The key to remembering this question and answer pair is that VHF and UHF signals can bend (or "curve") around Earth.

Learn more at http://en.wikipedia.org/wiki/Radio_horizon.

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Tags: vhf uhf propagation arrl chapter 4 arrl module 8

Since they're talking about sunspot cycle, they're talking about ionospheric refraction. But 23 centimeters and 70 centimeters have wavelengths that are too short to be reflected or refracted by the ionosphere - they pass right through without enough bending to make it back to earth.

Six and ten meters are refracted somewhat, but only when we have high sunspot activity is there enough ultraviolet radiation to bend a signal all the way back down to the earth - without the high ultraviolet, the signal bends, but not enough to get it back to earth.

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