2 11-39 Antennas are polarized. They radiate and receive in certain  patterns and directions. The electric field cause by the voltage  in the conductor is parallel to the polarization of an antenna.  It is caused by the voltage difference between each end of the  antenna. The electromagnetic field component of the radio  wave is at 90° to the polarization. It is caused by changing  current flow in the antenna. These fields were illustrated in  Figure 11-76  and  11-77 . As radio waves radiate out from  VHF radio frequencies used by aircraft communication radios  are 118–136.975 MHz. The corresponding half wavelengths  of these frequencies are 3.96 – 3.44 feet (47.5–41.2 inches).  Therefore, VHF antennas are relatively long. Antennas  one-quarter of the wavelength of the transmitted frequency  are often used. This is possible because when mounted on  a metal fuselage, a ground plane is formed and the fuselage  acts as the missing one-quarter length of the half wavelength  antenna. This is further discussed in the following antenna  types section. The formula is derived from the speed of propagation of  radio waves, which is approximately 300 million meters per  second. It takes into account the dielectric effect of the air at  the end of an antenna that effectively shortens the length of  the conductor required. Many radios use a tuning circuit to adjust the effective  length of the antenna to match the wavelength of the  desired frequency. It contains a variable capacitor and an  inductor connected in parallel in a circuit. Newer radios use  a more efficient tuning circuit. It uses switches to combine  frequencies from crystal controlled circuits to create a  resonant frequency that matches the desired frequency. Either  way, the physical antenna length is a compromise when using  a multichannel communication or navigation device that must  be electronically tuned for the best performance.  A formula can be used to find the ideal length of a half  wavelength antenna required for a particular frequency as  follows:  is a part. An antenna may be electrically lengthened by  adding an inductor in the circuit. Adjusting antenna length  in this fashion allows the use of a single antenna for multiple  frequencies in a narrow frequency range. Antenna Length (feet) = 468  F MHz Figure 11-87.  An antenna equal to the full length of the applied AC  frequency wavelength would have the negative cycle current flow  along the antenna as shown by the dotted line. An antenna that is  ½ wavelength allows current to reverse its direction in the antenna  during the negative cycle. This results in low current at the ends of  the ½ wavelength antenna and high current in the center. As energy  radiates into space, the field is strongest 90° to the antenna where  the current flow is strongest.  Figure 11-86.  VHF aircraft communication transceivers. Polarization, Directivity, and Field Pattern