Thursday, August 27, 2009

Propagation - some basics

Propagation will account for four questions in Section A. There are some basics which need to be known, starting with the ionospheric layers. The ionosphere is a region of the atmosphere between 100km and 400km above the surface of the Earth. It is above the troposphere, which is where all the clouds are located! The ionosphere is affected by radiation from the Sun, in the form of the Solar Wind. Air molecules are ionised, particularly by ultra violet radiation from the Sun. Solar flares can cause Aurora Borealis (Northern Lights) which can help with propagation of radio signals. Anyway, I'm distracting myself here. Back to the layers. You have to know these for the exam.

On right is a diagram of the ionosphere layers from the IRTS study CD.

What's important to note, apart from the heights they occupy, is that the D layer disappears at night, and the two F layers, F1 and F2, merge at around 250km above the earth. The F layer is at its weakest just prior to dawn.

The ionised regions of the atmosphere will reflect radio waves by refraction (gradual bending) within the layer.

The ionosphere is essential for long distance short wave radio communications.

As seen above, the layers will vary in height and density at different times of the day/night and different times of the year. One thing you MUST DEFINITELY KNOW about propagation is the length of the so-called "Sunspot Cycle". This is an 11-year cycle during which the number of sunspots on the surface of the Sun goes from maximum to minimum. Right now, we're in an extended minimum, with 0 sunspots. Things should pick up soon though and in a couple of years' time we should be experiencing great DX conditions if the solar activity increases.

Tropospheric propagation is for higher frequencies, usually above about 30-40Mhz.

Sporadic E propagation is more likely in the Summer, and affects the 2-metre band (144 - 146Mhz).

Ground Wave propagation: during the day, ground wave propagation is usable on frequencies up to approximately 2Mhz.

Some questions which have come up on QADV and elsewhere which need to be absorbed are as follows:

What is the mode of propagation called 'ducting' caused by:

a Refraction in the troposphere
b Variations in the earth's magnetic field
c Solar flares
d Absorption in the D region of the ionosphere.

The correct answer is a Refraction in the troposphere

The biggest daily variation in the maximum usable frequency occurs in:

a Spring
b Summer
c Autumn
d Winter

The correct answer is d Winter.

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