Multipath propagation In radio communication

Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. When the same signal is received over more than one path, it can create interference and phase shifting of the signal. Destructive interference causes fading; this may cause a radio signal to become too weak in certain areas to be received adequately. For this reason, this effect is also known as multipath interference or multipath distortion.

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Just like with light or heat, vegetation tends to absorb UHF and microwave signals. This is particularly noticeable at higher frequencies such as wireless networking where vegetation can significantly block the signal if you try to do a long range point-to-point link through a bunch of trees.

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To remember this, just think that long distance implies over the horizon (horizon-tal).

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.

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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" comes from the visual image of watching a light move on the other side of a picket fence (a fence with slats which are separated by a space); you would see the light appear to flicker as it is periodically hidden by a slat and then visible again. Imagine the same effect but with sound / signal -- that's "picket fencing".

Thus if someone is mobile and their signal flutters in and out it is often called "picket fencing", though the cause is actually due to the signal taking multiple paths and combining in different ways as they move rather than an actual "picket fence" type impediment between them and the receiving antenna.

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The main thing to remember is that the higher the frequency, the more RF energy is absorbed (and converted into heat) by water and solids.

Since microwaves are Extremely High Frequency or greater, the weather condition that would decrease range at microwave frequencies would be precipitation. None of the other options have any significant effect.

The easiest way to remember this is your microwave oven. The microwave oven functions as an oven because the oven microwave frequency used is optimized for being absorbed by water. Because most food contains a large amount of water, this energy absorption readily heats the food in the oven. Microwaves getting absorbed by water and converted into heat is great for heating food, but not for transmitting a radio signal.

If your microwave oven were a longwave or shortwave oven instead, it wouldn't be much of an oven because your leftovers wouldn't be getting too terribly warm at the same power.

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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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The main thing to remember is that the higher the frequency, the more RF energy is absorbed (and converted into heat) by water and solids.

For 10m and 6m wavelengths, the frequency isn't high enough to be affected by fog and light rain so fog and light rain will have little effect on those bands.

The easiest way to remember this is your microwave oven. The microwave oven is a microwave oven because the oven microwave frequency used is optimized for getting absorbed by and thus heating the water in your food.

If your microwave oven were a 10m or 6m wave oven instead, it wouldn't be much of an oven because your leftovers wouldn't be getting very warm at the same power usage. 10m and 6m are rather far from "micro" when it comes to wavelength.

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