Radio Propagation Technician License Class Session 3 N1AW revised 4/2013

Scatter - you can see the beam of the searchlight, because the air is not completely transparent.

Diffraction: What happens when a wave encounters a barrier?

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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. Note, source is Local copy is at – Refracción.avi (If the movie doesn't play, please pause the Power Point and click on the link for the Ripple tank refraction movie on the session 3 page)

Ground wave – speed of radio wave is slower in earth, low frequencies bend to follow curve of the earth’s surface.

Radiation from the sun affects the upper layers of the Earth's atmosphere by causing ionization. Electrons are knocked loose from atoms, resulting in a plasma, a mix of free electrons and charged atoms (ions). Although the radiation from the sun that we see as light and feel as heat is relatively constant, very short wave radiation (ultraviolet and X-ray) varies from time to time. This is the radiation that causes ionization. This invisible and unfelt radiation is highly correlated with the number of sunspots visible on the sun. The SSN (SunSpot Number) varies over an approximately 11 year cycle – in 2013 we are near the peak of the 24 th cycle (sunspots have been counted since Galileo Invented the telescope).

Solar flares produce X-rays which can disrupt normal radio communication via the ionosphere. Solar flares are more numerous during periods of high sunspot numbers.

How does the ionosphere reflect radio waves? Free electrons and ions in the atmosphere act like little antennas that are activated by energy absorbed from radio waves and then reradiate that energy … but when an electron collides with an ionized atom the charges are neutralized and the antenna action stops. … when collisions occur rapidly the energy is not reradiated as radio waves, it is absorbed as heat.

Where the atmosphere is denser, at lower altitudes, electrons can't go very far before they collide with ions and are neutralized

Where the air is thinner free electrons live longer and can reradiate radio energy.

In addition to sunspot numbers, radio frequency radiation from the sun is also correlated with very short wave ionizing radiation. Radio telescopes record the intensity of solar radio emissions at a wavelength of 10.7 cm, or, in frequency units, 2800 MHz (2.8 GHz). The correlations are strong enough that given a sunspot number (SSN) or a 10.7 cm solar flux value, computer programs can make world wide predictions of ionospheric ionization levels, and from these can calculate the reliability of long distance radio transmissions at any time between any two points on the surface of the Earth. There are several websites where you can see these predictions on a real near-time basis plotted on a map of the world.

Critical F-2 layer frequency – highest frequency at which a radio wave directed vertically is reflected back to the transmitter site.

MUF – Maximum Usable Frequency – defined for a specified distance, in this case for a 3000 Km path. The computer program calculates this world-wide based on sunspot number (SSN)