4 - 13 VHF and UHF communications into the Earth’s surface. This can be overcome by increasing antenna height, power output, and radio frequency. Since radio frequencies are pre designated and power output is limited by the capabilities of the radio set, antenna length and height are the two variables that can be manipulated to increase radio communication range. Using the following formulas it will be possible to plan for the use of field expedient antennas, determine the best location to gain/ maintain communication, and plan for communication windows as necessary. a. To calculate the physical length of an antenna in feet, use the following equation. It will give you the antenna length in feet for a 1/ 4 wavelength of the frequency. To determine the antenna length in feet for a full wavelength antenna, multiply the antenna length by 4: X = 234/ Freq (X = the length of the antenna in feet; Freq = the radio frequency used) EXAMPLE 234/ 38.950 = 6.01 feet (Quarter Wavelength Antenna) 6.01 feet x 2 = 12.02 feet (Half Wavelength Antenna) 6.01 feet x 4 = 24.04 feet (Full Wavelength Antenna) b. Curvature of the earth allows for a person 5 foot 7 inches looking across a flat surface to see a distance of about 4.7 km (Figure 4-6), anything beyond this distance is below the horizon and essentially dead space. To overcome this, the person must move to a higher elevation to see beyond 4.7 km. LOS communication is subject to this same principle. Use the following formula to calculate the required antenna height for a given distance (keep in mind that if you are in low ground such as a valley, draw, depression, the height of the antenna will be greater; when on high ground the antenna height may be shorter). Use the following formula to compute height of antenna to compensate for curvature of the earth: X = 234/ Freq (X = the length of the antenna in feet; Freq = the radio frequency used Distance in km (Dkm) from receiving station = square root of √ (12.7 x Am) where Am is the antenna height in meters. EXAMPLE Known height: Dkm = √(12.7 x Am), Dkm = √(12.7 x 1.7m) or Unknown height: Am= 0.07874 x (Dkm)², Am= 0.07874 x (4.7km)² Figure 4-6. CURVATURE OF THE EARTH