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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Think of VHF as very sharp waves that poke right through the atmosphere. Whereas, HF has very dull waves that bounce off the atmosphere. So VHF is mostly line of sight and HF is long distance. In this answer the only true option is long distance communications.
HF antennas are much bigger, tend to have smaller bandwidth, and there is generally a lot more noise / static.
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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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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 poorly understood.
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.
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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.
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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There are several modes that can allow communication that ranges "over-the-horizon" or beyond line-of-sight such as Ducting and Troposheric scatter. The key words to differentiate these two in this question is "regular basis". Ducting requires specific atmospheric conditions such as a temperature inversion. Tropospheric scatter works in the VHF, UHF and microwave frequencies where the signals are bent or reflected back to earth in a somewhat random manner to station a significant distance away on a regular basis.
More information can be found here.
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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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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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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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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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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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