The three main modes of propagation of electromagnetic waves are:
(a) ground (or surface) wave
In ground-wave propagation, the radiated wave follows the surface of the earth. It is the major mode of propagation for frequencies up to about 2MHz. Attenuation of the ground wave increases very rapidly above 2MHz and it may extend for only a few kilometres at frequencies of the order of 15- 20MHz. At very low frequencies the attenuation decreases to such an extent that reliable world-wide communication is possible at all times. The ground wave is not so affected by atmospheric effects or time of day as other modes, particularly at frequencies below about 500kHz.
(b) ionospheric wave (sky wave)
Ionospheric propagation is the 'refraction' (ie bending), and hence reflection, of radio waves back to earth by layers of ionised gases. It is the normal mode of propagation over the frequency range of about 1MHz to 30MHz.
(c) tropospheric wave.
This is the major mode of propagation over long distances (ie beyond the line-of-sight range) at frequencies above about 50MHz.
The ionosphere represents less than 0.1% of the total mass of the Earth's atmosphere. Even though it is such a small part, it is extremely important, especially to the radio ham! The upper atmosphere is ionized by solar radiation. That means the Sun's energy is so strong at this level, that it breaks apart molecules. So there ends up being electrons floating around and molecules which have lost or gained electrons. When the Sun is active, more and more ionization happens. Different regions of the ionosphere make long distance radio communication possible by reflecting the radio waves back to Earth. It is also home to auroras.
The Ionosphere is broken down into the D, E and F regions. The breakdown is based on what wavelength of solar radiation is absorbed in that region most frequently.
The D region is the lowest in altitude, though it absorbs the most energetic radiation, hard x-rays. The D region doesn't have a definite starting and stopping point, but includes the ionization that occurs below about 90km.
The E region peaks at about 105km. It absorbs soft x-rays.
The F region starts around 105km and has a maximum around 600km. It is the highest of all of the regions. Extreme ultra-violet radiation (EUV) is absorbed there.
On a more practical note, the D and E regions reflect radio waves back to Earth. Radio waves with shorter lengths are reflected by the F region.
The highest frequency at which a signal aimed straight up at the ionosphere is reflected back to earth is called the maximum useable frequency (MUF) or critical frequency. Frequencies above this will pass through the ionosphere, except at shallow angles where the wavefront is bent back down towards the earth. The peak value of the MUF generally occurs between 1000 and 1600 hours. generally peak values are much higher at sunspot maximum than at the sunspot minimum. Peak values are much higher in the winter than in the summer. There is a much larger variation in the MUF over the day in the winter than in the summer. Around the sunspot maximum, the MUF may exceed 50MHz for short periods, but at the minimum it rarely exceeds 25MHz.
Sporadic E or Es is an unusual form of radio propagation utilizing characteristics of the earth's ionosphere. Whereas most forms of skywave propagation use the normal and cyclic ionization properties of the ionosphere's F region to refract (or "bounce") radio signals back toward the earth's surface, sporadic E propagation bounces signals off of smaller "clouds" of unusually ionized atmospheric gas in the lower E region (located at altitudes of approx. 90 to 160 km). This occasionally allows for long-distance communication at VHF and UHF frequencies not usually well-suited to such communication.
The variability in distance depends on a number of factors, including cloud height and density. Maximum Usable Frequency (MUF) also varies widely, but most commonly falls in the 27–110 MHz range, which includes the FM broadcast band (87.5–108 MHz), and the amateur radio 10 and 6 metre bands. As its name suggests, sporadic E can happen at almost any time, but it does display seasonal patterns. Sporadic E activity peaks predictably in the summertime in both hemispheres. In my experience sporadic E has been most noticeable in mid-to-late June, trailing off though July and into August.
The speed of a radio wave in the atmosphere is determined by the dielectric property of the air. This property depends on the pressure, temperature and humidity of the air. In general as we move upwards through the atmosphere the pressure decreases and temperature falls. This means that the dielectric property changes with height and allows a slight increase in the speed of a radio wave as we move upwards through the atmosphere. This in turn means that if a radio wave moves away from the earth at an angle less than 90 degrees, then the upper part of the wave travels faster than the lower part. Therefore even under normal conditions this can in effect bend, or refract, the wave back down to earth. The normal rate of change of dielectric constant with height refracts the wave so that it follows a curved path of about 1.3 times the radius of the earth. Therefore, we typically can receive signals which are 1.3 times further than we can see by line of sight.
Tropospheric ducting occurs when we get a sharp rate of change in the dielectric constant as we move upwards through the atmosphere. This occurs when we get a rapid increase of temperature and arapid decrease in humidity (dew-point) with height.
Under these conditions we now have the radio wave bent back towards the earth. However, the radio wave can then reflect back of the earth and become refracted again to return earthwards once more. This can sometimes occur a number of times with little attenuation but some fading. The result can be long distance reception of radio waves that would normally have been far beyond the radio horizon. This is not to be confused with skip these signals are not bouncing of the ionosphere but rather traveling in ducts several hundred feet above the ground.
Typical conditions required for a good duct to occur are:
The depth of the duct required for varying wave-lengths is:
Typical meteorological conditions which can be favourable for ducting are:
To decide whether there may be potential for ducting then first consult the Met Office forecast or the data on this site. If they are showing hints of high pressure building or a weak ridge crossing the area then there could well be potential.
Lat : 52,52 N
Long : 1,08 E
Altitude : 175 ft/53 mtr ASL
IARU Locator : JOØ2mg
ITU Zone : 27 (Region 1)
CQ Zone : 14
WAB Square : TM16
WAB Book : 11912
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WAB Book: 11912
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