Multipath distortion is caused by radio waves bouncing off of different objects (mountains, large buildings, etc) and then all arriving at the same location but having traveled a different distance. Because of this they arrive at slightly different times (having traveled at different, or multiple, paths) and can interfere with each other.
When you have a solid signal at first and then move and suddenly don't have a strong signal you probably just moved to a location where the multipath distortion was greater. Move back to where you were when you had a strong signal or just try moving a few feet, closer to a window, etc. It usually isn't too hard to locate the "sweet spot".
Note that multipath and other propagation can be affected by the temperature and weather, so you may find that your signal strength changes throughout the course of a day.
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As a general rule of thumb, the shorter the wavelength the more easily the radio waves are able to penetrate the structure of buildings; UHF is higher frequency than VHF, which means that the wavelength on UHF is shorter than VHF. Thus, UHF penetrates through buildings better than VHF -- usually =]
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To remember this, just think that long distance implies over the horizon. You can send the signals in any polarization you care to, but if you want to make contact with other HAMs in other states or countries trying to contact you, with low power, you should use the same polarization they are using to get the strongest signal. It is merely an agreed-on convention or method.
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You may have played with polarized glasses sometime and found that if you hold two pair in line with each other and then rotate one 90 degrees that they darken or block light from coming through. Some 3-D movies have you wear polarized glasses where one eye is vertically polarized and the other eye is horizontally polarized so that each eye can receive a different image projected from the two projectors.
With radio we can have vertically or horizontally polarized antennas. The receiver must have the same polarization in order to pick up the maximum amount of the signal transmitted. If they are not matched they may only detect a small portion and be significantly weaker than it should be. Note the "line of sight" clarification in the question. Once a signal bounces or reflects off of something it may alter its polarity. If you are having a hard time hearing a transmission move positions and tilt the antenna around to maximize reception.
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The great thing about a directional (or "beam") antenna is the ability to tightly focus your transmission in a particular direction. If you don't have a direct path to the receiving station (a repeater, in the case of this particular question), you can point your antenna at something that will "bounce" the signal for you. It's a little like playing pool (you can't get the shot you want, so you find another object, at the correct angle, that can 'bounce' the shot in the correct direction).
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The term "Picket fencing" refers to the effect of the signal cutting in and out of good signal quality due to movement; the imagery implied is that the transmitting station is driving by a picket fence and the signal only makes it through the gaps between the slats. Thus, when you either are missing parts of the signal (wholly or partially) due to consistent gaps in the signal it is commonly referred to as "Picket Fencing"
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Radio waves are also known as
Electromagnetic waves because they have both an
electric and a
magnetic field component to them. It's also important to note that radio waves can be found on the electromagnetic spectrum.
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Fading due to Random combining of signals arriving via different paths is called multipath fading. It is the only option that makes sense in this question.
It is actually possible for reception to be too good. Sometimes when reception is good a signal will arrive at your receiver after reflecting off of different obstacles, such as mountains, buildings, and so on. Each time this happens, it creates a separate "path", and each path is a different distance (or length), resulting in signals that are out of phase with each other.
When these signals all arrive, with some taking longer than others, they can combine in such a way that it can cause the total signal to fade such as when the signal along one path is \(180^\circ\) (or close to it) out of phase with another causing the waves to cancel each other out.
When the signals are less out of phase with each other they can also cause a type of distortion called multipath distortion.
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Skip signals or skywave propagation can be used to communicate beyond the horizon, at intercontinental distances. It is mostly used in the shortwave frequency bands.
Elliptically polarized signals have a vertical and horizontal component, and thus that component can be received by either a horizontally or vertically polarized antenna.
If a signal were strictly horizontal, then receiving it on a vertical antenna would result in significant loss. The same when receiving a vertically polarized signal on a horizontal antenna.
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When the same signal propagates over multiple paths the different paths will generally be a slightly different distance and different angles. As a result, the signal arrives at the destination from multiple directions at multiple times. Since radio waves travel at the speed of light, this difference in time will be very minor, but even a slight difference is enough to cause the recombination of those somewhat out of phase with each other signals at the end to create a type of distortion called multipath distortion. When the signal is a data signal, this distortion causes information loss leading to higher error rates.
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The ionosphere is the upper layer of the atmosphere which becomes ionized when exposed to radiation from the sun. When it is ionized it effectively reflects radio waves back towards the ground which allow the signals to travel farther than they normally could.
To better understand this, think of a room separated into cubicles: you can't see your coworkers in their cubicle because there is a wall in the way. However, if you put a mirror on the ceiling then you would be able to see all of your coworkers by looking at the mirror on the ceiling. The Ionosphere reflects radio waves in more or less the same way.
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