The signals produced by a VOR transmitter propagate 360°  from the unit and are used by aircraft to navigate to and from  the station with the help of an onboard VOR receiver and  display instruments. A pilot is not required to fly a pattern to  intersect the signal from a VOR station since it propagates  out in every direction. The radio waves are received as long  as the aircraft is in range of the ground unit and regardless  of the aircraft’s direction of travel.  [Figure 11-97] A VOR transmitter produces two signals that a receiver  on board an aircraft uses to locate itself in relation to the  ground station. One signal is a reference signal. The second  is produced by electronically rotating a variable signal. The  variable signal is in phase with the reference signal when at  magnetic north but becomes increasingly out of phase as it  is rotated to 180°. As it continues to rotate to 360° (0°), the  signals become increasingly in phase until they are in phase  again at magnetic north. The receiver in the aircraft deciphers  the phase difference and determines the aircraft’s position  in degrees from the VOR ground based unit.  [Figure 11-98] Most aircraft carry a dual VOR receiver. Sometimes, the  VOR receivers are part of the same avionics unit as the VHF  communication transceiver(s). These are known as NAV/ COM radios. Internal components are shared since frequency  bands for each are adjacent.  [Figure 11-99] Large aircraft may have two dual receivers and even dual  antennas. Normally, one receiver is selected for use and the  second is tuned to the frequency of the next VOR station to  be encountered en route. A means for switching between  NAV 1 and NAV 2 is provided as is a switch for selecting the  active or standby frequency.  [Figure 11-100]  VOR receivers  are also found coupled with instrument landing system (ILS)  receivers and glideslope receivers. A VOR receiver interprets the bearing in degrees to (or from)  the VOR station where the signals are generated. It also  produces DC voltage to drive the display of the deviation  from the desired course centerline to (or from) the selected  station. Additionally, the receiver decides whether or not  the aircraft is flying toward the VOR or away from it. These  items can be displayed a number of different ways on various  instruments. Older aircraft are often equipped with a VOR  11-44 Figure 11-99.  A NAV/COM receiver typically found in light aircraft.  Figure 11-97.  A VOR transmitter produces signals for 360° radials  that an airborne receiver uses to indicate the aircraft’s location in  relation to the VOR station regardless of the aircraft’s direction of  flight. The aircraft shown is on the 315° radial even though it does  not have a heading of 315°.  Figure 11-98.  The phase relationship of the two broadcast VOR  signals.  0° Magnetic radial 90° Magnetic radial 180° Magnetic radial 270° Magnetic radial Fixed signal Rotating signal 0° Magnetic north 010 350 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190 020 030 040 050 060 070 080 090 100 110 120 130 140 150 160 170 180 315 135 COMM KY196A TSD PULL 25K PULL TEST OFF STBY CHAN CHAN