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MODULE A- INTRODUCTION TO RADAR Figure-3.2: The relation between frequency and wavelength A radar operates in microwave region of EM spectrum and it emits the energy in the form of EM wave into the atmosphere through an antenna. While only a fragment of the energy returns, it provides a great deal of information. The entire process of energy propagating through space, striking objects, and returning occurs at the speed of light. Targets are struck by electromagnetic energy and the return signals from these targets are called radar echoes. The table below shows the electromagnetic spectrum and the location of radar frequencies in that spectrum. Figure-3.3 TURKEY RADAR TRAINING 1.0 / ALANYA 2005 27
MODULE A- INTRODUCTION TO RADAR The radar bands located in the spectrum have been designated by certain letters as follows: Band Designation Nominal Frequency Nominal Wavelength HF 3-30 MHz 100-10 m VHF 30-300 MHz 10-1 m UHF 300-1000 MHz 1-0.3 m L 1-2 GHz 30-15 cm S 2-4 GHz 15-8 cm C 4-8 GHz 8-4 cm X 8-12 GHz 4-2.5 cm Ku 12-18 GHz 2.5-1.7 cm K 18-27 GHz 1.7-1.2 cm Ka 27-40 GHz 1.2-0.75 cm V 40-75 GHz 0.75-0.40 cm W 75-110 GHz 0.40-0.27 cm mm 110-300 GHz 0.27-0.1 cm Specific frequencies within above ranges have been assigned for radars by International Telecommunication Union (ITU). The radio frequency bands used by weather radars are located around 2.8 GHz (S-Band), 5.6 GHz (C-Band), 9.4 GHz (X-Band) and 35.6 GHz (Ka-Band). 3. 2. Electromagnetic waves Electromagnetic or radio waves consist of electric (E) and magnetic (H) force fields, which are perpendicular to each other and to the direction of propagation of the wave front, propagate through space at the speed of light and interact with matter along their paths. These waves have sinusoidal spatial and temporal variations. The distance or time between successive wave peaks (or other reference points) of the electric (magnetic) force defines the wavelength λ or wave period T. These two important electromagnetic field parameters are related to the speed of light c. The wave period T is the reciprocal of the frequency f. Frequency refers to the number of completed wave cycles per second. Radar frequency is expressed in units of Hertz (Hz). Higher frequency transmitters produce shorter wavelengths and vice versa. Wave amplitude is simply 28 TURKEY RADAR TRAINING 1.0 / ALANYA 2005
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