e radio² link operates at 38.71 GHz. What is the equivalent wavelength at this frequency? 38.71 109 x 3 3 1 0 m/sec a :0.00775 m or 7.75 mm. Figure 2.6 is an outline drawing of the radio frequency spectrum from nearly zero Hz to 100 GHz. The drawing shows several frequency bands assigned to specific services. 2.3.3.1 Introduction to Phase. The IEEE defines phase as "a relative measurement that describes the temporal relationship between two signals that have the same fre- quency." We can plot a sine wave (representing a certain frequency) by the method shown in Figure 2.7, where the horizontal lines are continuation of points a, b, c, and SO on, and the vertical lines a', b', c', and SO on, are equally spaced and indicate angular degrees of rotation. The intersection of lines a and a', b and b', and SO on, indicates points on the sine wave curve. To illustrate what is meant by phase relation, we turn to the construction of a sine wave using a circle as shown in Figure 2.7. In the figure the horizontal scale (the abscissa) represents time and the vertical scale (the ordinate) represents instantaneous values of Sound waves travel at 1076 ft/sec (331 m/sec) in air at 0°C and with 1 atmosphere of atmospheric pressure. However, our interest here is in radio waves, not sound waves. This is termed millimeter radio because wavelengths in this region are measured in millimeters (i.e., for frequencies above 30 GHz) rather than in centimeters or meters. SIGNALS CONVEY INTELLIGENCE 100,000,000 Radio Relay 30,000,000 Radio Relay and Satellite Communications Radio Relay and Satellite Communications 1,000,000 Cellular TV Broadcasting 300,000 Domestic Public Land Mobile Short Haul Toll Rural Subscriber Maritime Mobile 100,000 FM Broadcasting Domestic Public Land Mobile 30,000 International Overseas 10,000 Ship Telephone - High Seas Portable Emergency Restoration of Toll Circuits Ship Telephone - Coastal Harbor Radio Broadcasting Marine Loran C Omega Figure 2.6 The radio-frequency spectrum showing some frequency band assignments.