Technician License Class Session 2 Radio Propagation Technician License Class Session 2 Al Woodhull, N1AW, January 19, 2012

Last week I showed you some line-of-sight coverage maps Last week I showed you some line-of-sight coverage maps. In our area the topography is a major factor in coverage, but in a totally flat area the curvature of the earth seems to determine how far apart two stations can be and still communicate.

Well, there are ways to communicate beyond the distance indicated in the previous slide. Three mechanisms exist: scatter, refraction, and diffraction.

Radio waves can be scattered like light, by all sorts of things. Scatter - you can see the beam of the searchlight, because the air is not Completely opaque.

Diffraction: What happens when a wave encounters a barrier? Diffraction is a little harder to understand. Diffraction: What happens when a wave encounters a barrier?

What Huygens theorized about light can be applied to any kind of wave that spreads through space.

Y

y If some part of the wave front is blocked, the end of the wavefront is now curved.

Refraction occurs when a wave goes from one region to another where the speed of propagation is different – this can cause the direction of the wave to change …. Let’s see a movie. I couldn’t get a movie into Powerpoint. (Show Refraction - Refracción.mp4 here). Ripple tanks allow one to do experiments with water waves travelling in 2 dimensions. The speed of the wave varies with the depth of the tank, the tank is shallower to the left of the diagonal division. Source of the movie: http://www.youtube.com/watch?v=stdi6XJX6gU

A low frequency radio wave penetrates somewhat into the ground, but its speed is slower there. This causes the wavefront to tilt, bending it to follow the curve of the earth. Ground wave – speed of radio wave is slower in earth, low frequencies bend to follow curve of the earth’s surface.

Really spectacular radio propagation comes about through refraction in the atmosphere. All living things on the earth live in a layer that, compared to the size of the earth is like the shell of an egg or the outer layer of an onion. I’m not sure of the exact number, but something like 90% of the atmosphere’s gases are in the troposphere, the lowest layer.

Within the troposphere changes in air density can cause refraction of radio waves just the way difference in depth can refract water waves in a ripple tank. So the radio horizon is somehat further away than the visual horizon because of tropospheric bending.

Weather conditions that result in layers of air with different temperatures can cause refraction that keeps radio waves going much further than usual, as if in a duct.

So far we have talked about refraction within the troposphere So far we have talked about refraction within the troposphere. Even more dramatic refraction can take place in higher levels, due to the effects of radiation from the sun. As it turns out, the radiation that causes these effects varies with the number of sunspots.

Solar flares can also cause very large changes in the amount of radiation that affects layers of the atmosphere that in turn affect radio propagation.

Various kinds of radiation from the sun – X-rays, different wavelengths of ultraviolet light – penetrate to different distances through the atmosphere. There is also some variation in the gaseous composition of the atmosphere with height. This leads to layers containing ionized gases at different heights.

At lower levels where the atmosphere is denser, free electrons can only travel short distances before they recombine with ions, so the ionization dissipates quickly after the sun sets. (This diagram does not show that the E layer also becomes considerably less strongly ionized at night). The F1 and F2 layers combine (I haven’t seen an Explanation of why this happens) at night. The air is very thin at the height of the F layers, free electrons have long lives, and thus the ability to reflect (actually refract) radio waves can persist long after dark. However, the ionization levels do decrease after dark, so the highest frequencies that might be usable in the daytime are not reflected at night.

The virtual height of an ionospheric layer is greater than the actual height, because the path of a radio wave is bent gradually, not reflected as if from a mirror.

At low levels molecules and ions are numerous and a free electron doesn’t go very far or stay free very long.

At higher levels molecules and ions are fewer and farther apart and a free electron may go farther and stay free for a longer time.

Self-explanatory.

Also mention sporadic E – like clouds of unusually strong ionization in the E layer. Discovered by hams, still not entirely understood, but a source of great delight for hams because of the unusual opportunity for long distance communication at frequencies where radio waves are not usually returned to earth by refraction.

Self-explanatory.

This is also why many small local a. m This is also why many small local a.m. broadcast radio stations are required to reduce power or even shut down at night, to prevent interference with other stations on the same frequency.

This is getting pretty complicated, perhaps more than you need to know for the Technician license test.

The critical frequency is the highest frequency at which a radio wave directed vertically will be reflected back to the point where it originated. It can be predicted on a worldwide basis, and this map is available online, updated every five minutes.

This is a real-time map similar to the one for critical frequency, but showing the maximum usable frequencies for h.f. communication between two stations 3000 Km apart.