Antennas Length Transceivers 11-38 Figure 11-85.  The basic stages used in a receiver to produce an  output from a radio wave.  1. Length 2. Polarization 3. Directivity A local oscillator in the receiver produces a frequency that is  different than the radio frequency of the carrier wave. These  two frequencies are mixed in the mixer. Four frequencies  result from this mixing. They are the radio frequency, the  local oscillator frequency, and the sum and difference of  these two frequencies. The sum and difference frequencies  contain the information signal. The frequency that is the difference between the local  oscillator frequency and the radio frequency carrier wave  frequency is used during the remaining processing. In VHF  aircraft communication radios, this frequency is 10.8 MHz.  Called the intermediate frequency, it is amplified before it is  sent to the detector. The detector, or demodulator, is where  the information signal is separated from the carrier wave  portion of the signal. In AM, since both sidebands contain  the useful information, the signal is rectified leaving just one  sideband with a weak version of the original transmitter input  signal. In FM receivers, the varying frequency is changed to a  varying amplitude signal at this point. Finally, amplification  occurs for the output device.  [Figure 11-85] functions of the device. So is the antenna. This saves space  and the number of components used. Transceivers are half  duplex systems where communication can occur in both  directions but only one party can speak while the other  must listen. VHF aircraft communication radios are usually  transceivers.  [Figure 11-86]  As stated, antennas are conductors that are used to transmit  and receive radio frequency waves. Although the airframe  technician has limited duties in relation to maintaining and  repairing avionics, it is the responsibility of the technician to  install, inspect, repair, and maintain aircraft radio antennas. Three characteristics are of major concern when considering  antennas:  The exact shape and material from which an antenna is made  can alter its transmitting and receiving characteristics. Also  note that some non-metallic aircraft have antennas imbedded  into the composite material as it is built up. Over the years, with the development of transistors, micro- transistors, and integrated circuits, radio transmitters and  receivers have become smaller. Electronic bays were  established on older aircraft as remote locations to mount  radio devices simply because they would not fit in the flight  deck. Today, many avionics devices are small enough to be  mounted in the instrument panel, which is customary on most  light aircraft. Because of the number of communication and  navigation aids, as well as the need to present an uncluttered  interface to the pilot, most complicated aircraft retain an area  away from the flight deck for the mounting of avionics. The  control heads of these units remain on the flight deck.  When an AC signal is applied to an antenna, it has a  certain frequency. There is a corresponding wavelength for  that frequency. An antenna that is half the length of this  wavelength is resonant. During each phase of the applied  AC, all voltage and current values experience the full range  of their variability. As a result, an antenna that is half the  wavelength of the corresponding AC frequency is able to  allow full voltage and full current flow for the positive phase  of the AC signal in one direction. The negative phase of  the full AC sign wave is accommodated by the voltage and  current simply changing direction in the conductor. Thus, the  applied AC frequency flows through its entire wavelength,  first in one direction and then in the other. This produces the  strongest signal to be radiated by the transmitting antenna. It  also facilitates capture of the wave and maximum induced  voltage in the receiving antenna.  [Figure 11-87] A transceiver is a communication radio that transmits  and receives. The same frequency is used for both. When  transmitting, the receiver does not function. The push to  talk (PTT) switch blocks the receiving circuitry and allows  the transmitter circuitry to be active. In a transceiver, some  of the circuitry is shared by the transmitting and receiving  Most radios, especially communication radios, use the same  antenna for transmitting and receiving. Multichannel radios  could use a different length antenna for each frequency,  however, this is impractical. Acceptable performance  can exist from a single antenna half the wavelength of a  median frequency. This antenna can be made effectively  shorter by placing a properly rated capacitor in series with  the transmission line from the transmitter or receiver. This  electrically shortens the resonant circuit of which the antenna  RF amplifier Local oscillator Mixer IF amplifier Detector/ Demodulator AF amplifier