of material entering the earth's atmosphere although there are ran- Long distance (greater than 1000km) ground wave transmission for dom and predictable variations. Meteors that leave usable trails have diameters between 0.2mm and 2mm, and the trails which last communications and radio navigation. Propagation unaffected by for around half a second to several seconds occur at a height of about ionospheric conditions. High power and large antennas required. 120km. They may extend up to 50km although 15km is a typical Very limited spectrum availability. value. The number of particles entering the earth's atmosphere is inversely proportion to size, SO larger meteors occur too infre- 27.10.2 M.f.: 300kHz to 3MHz quently to be of use whilst the numerous very small particles do not generate sufficient ionisation. The variation in the number of usable Medium to short distance (up to 1000km) ground wave transmission ionised trails occurs on a daily basis with more occurring during for sound broadcasting and mobile communications. Propagation daylight as the earth's rotations tends to sweep up more meteors little affected by ionospheric conditions, but night time interference than at night. There are also predictable showers occurring throug- can be a problem. hout the year. Meteor scatter communications can be established for paths from 27.10.3 H.f.: 3MHz to 30MHz 200km to 2000km, but information has to be transmitted in bursts when a link is established SO the medium cannot be used for services World wide communications using comparatively low power such as speech. Radio waves are reflected from the trails by differ- (1kw), but heavily dependent on ionospheric conditions. Limited ent mechanisms depending upon the density of ionisation; however channel availability. Used for sound broadcasting, point to point and frequencies in the 30MHz to 100MHz range are most effective with mobile maritime and aeronautical communications. 50MHz being the optimum frequency. Horizontal polarisation is generally preferred. When a link is 