For convenience they have been divided into two groups:

MICROWAVES

RADIO WAVES

Microwaves

The range of wavelengths emitted by a generator depends on its temperature. The radiation is emitted in a band of wavelengths around a peak value which moves to shorter wavelengths depending on the reciprocal of the Kelvin temperature. For example, around room temperature, 300K, the peak wavelength is about 10-5m and is, therefore, in the infrared region. At a higher temperature, say 6,000K, the peak is about 5 x 10-7 m which is in the visible region. At much lower temperatures, say 3K, the peak wavelength is in the microwave region.

Uses of microwaves

Microwaves have become well known as a method of rapidly heating food. The waves are absorbed quickly by water molecules in the food and this results in the rapid generation of heat throughout the food giving uniform cooking. A magnetron is used to generate waves having a wavelength of 12cm at a frequency of 2500 MHz. The food rotates on a flat turntable inside a small oven. The oven has a window fitted with a wire mesh to reflect the microwaves back inside onto the food.

The programme shows how the Search for Extra-terrestrial Intelligence (SETI) uses microwave detection to monitor signals arriving on the Earth from space. Radio astronomy developed rapidly as satellites and space probes were launched in order to increase our knowledge and understanding of the universe.

Worldwide communications have seen an unprecedented development since the launch of satellites which can receive and then transmit TV signals "carried" by microwaves. There is now a wide range of satellites in geostationary orbits around the Earth which have diverse uses including weather pattern observation, surveying and mapping the Earth, military uses and navigation for land vehicles, ships and aircraft. This latter system is extremely accurate and is superseding more conventional methods based on time and distance measurements.

The search for evidence of life in space requires a system of interstellar communication. Such a system would require energy radiated which has a number of specific characteristics. It would have to travel with the speed of light, in straight lines and have a low level of natural occurrence so that a received signal would be immediately noticeable. It would be absorbed neither by the sparse gas and dust which exists between stars, nor by our atmosphere of mainly nitrogen and oxygen. This would mean that the signals could be received by the large dishes of our radio telescopes on earth.

Radio waves

TYPICAL WAVELENGTHS - Between 10-1km and 1 km

SOURCES - Cathode ray tubes, Oscillating electrical circuits

Background on radio waves

The generation of the first radio waves is attributed to Hertz in 1887. He used an induction coil to make a spark and detected the resultant waves with a spark gap receiver. Marconi followed this by using the spark method to send signals in Morse code. It was soon realised that voice transmissions were not possible without a continuous carrier wave.

Early broadcasts used frequencies in the range 100kHz to 1MHz using amplitude modulation of long wave (LW) and medium wave carriers. But speech transmission requires 3kHz and 15kHz.

It was Marconi who first suggested sending out waves which would be reflected off objects back to their source and hence allow the calculation of the range and bearing of the object - radar! He realised that this would work in all bad weather including fog. Unfortunately this idea was not developed until thirty years later as war in Europe became increasingly probable. In the event, radar was a major factor in the Battle of Britain which helped the RAF to detect the Luftwaffe.

The next development was the invention of the cavity magnetron, a sophisticated vacuum valve controlled by a combination of electric and magnetic fields. This enabled the generator of 'radar' waves to be carried by aircraft seeking enemy aircraft at night. Waves with a wavelength of 10 cm were used since they were not significantly reflected from the ground. These 'ground' reflections would swamp the returning signal from enemy aircraft.

Uses of radio waves

Ultra High Frequency (UHF) waves are used to transmit television programmes: Very High Frequency (VHF) waves are used to transmit local radio, police and emergency service messages. Medium and long waves are used to transmit over long distances including aircraft to air-traffic controllers. They can get around the curvature of the Earth and over obstacles because of their long wavelengths which range from 1 metre to 1 kilometre. Diffraction of these waves enables them to 'bend around' and over obstacles.

In the earliest days of aviation the only communication between aircraft and the ground was by lamp signals. Later, some aircraft were able to carry radio sets but the messages could only be transmitted in Morse code. This process was slow and cumbersome but an improvement on signalling using lamps.

Next came the radio telephone (RT) - an enormous leap forward which allowed pilots to speak to air-traffic controllers. This improved air safety since advice and assistance could be given much more quickly. Soon they were using VHF because the high quality of transmission and reception were essential as the volume of air traffic increased rapidly.

Radio waves are now used to navigate aircraft and shopping all around the Earth. They also enable pilots to land their aircraft in all weather conditions. The pilots have a radio aid which enables them to locate the centre, and to control the descent to land on the near end of the runway. This aid is called the Instrument Landing System (ILS). It has been improved and refined over the years so that it can be coupled to the automatic pilot control in the aircraft so that it can fly all the way to the landing regardless of the visibility and cloud base. Not all airports, nor all aircraft have this equipment, but this will happen eventually.

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