33 30 24 2I I5 I2 6 3 NAV HDG GS GS 188 ° G HDG 183 CDI “TO” indicator CDI lateral deviation index Azimuth scale Actual heading of aircraft Omnibearing selector Figure 11-102.  A mechanical HSI (left) and an electronic HSI (right) both display VOR information.  should indicate 0° showing FROM on the indicator or 180°  when showing TO. If an RMI is used as the indicator, the test  heading should always indicate 180°. Some repair stations  can also generate signals to test VOR receivers although not  on 108.0 MHz. Contact the repair station for the transmission  frequency and for their assistance in checking the VOR  system. A logbook entry is required. be determined by monitoring the strength of the signal  received. This was possible because a radio wave striking a  loop antenna broadside induces a null signal. When striking  it in the plane of the loop, a much stronger signal is induced.  The NDB signals were modulated with unique Morse code  pulses that enabled the pilot to identify the beacon to which  he or she was navigating.  NOTE: Some airborne testing using VOTs is possible by  the pilot. An error of ±4° should not be exceeded when testing a VOR  system with a VOT. An error in excess of this prevents the  use of the aircraft for IFR fight until repairs are made. Aircraft  having dual VOR systems where only the antenna is shared  may be tested by comparing the output of each system to  the other. Tune the VOR receivers to the local ground VOR  station. A bearing indication difference of no more than ±4°  is permissible. With RDF systems, a large rigid loop antenna was installed  inside the fuselage of the aircraft. The broadside of the  antenna was perpendicular to the aircraft’s longitudinal  axis. The pilot listened for variations in signal strength of  the LF broadcast and maneuvered the aircraft so a gradually  increasing null signal was maintained. This took them to  the transmitting antenna. When over flown, the null signal  gradually faded as the aircraft became farther from the station.  The increasing or decreasing strength of the null signal was  the only way to determine if the aircraft was flying to or from  the NDB. A deviation left or right from the course caused the  signal strength to sharply increase due to the loop antenna’s  receiving properties.  The ADF improved on this concept. The broadcast frequency  range was expanded to include MF up to about 1800 kHz.  The heading of the aircraft no longer needed to be changed  to locate the broadcast transmission antenna. In early model  ADFs, a rotatable antenna was used instead. The antenna  rotated to seek the position in which the signal was null.  An automatic direction finder (ADF) operates off of a  ground signal transmitted from a NDB. Early radio direction  finders (RDF) used the same principle. A vertically polarized  antenna was used to transmit LF frequency radio waves in  the 190 kHz to 535 kHz range. A receiver on the aircraft  was tuned to the transmission frequency of the NDB. Using  a loop antenna, the direction to (or from) the antenna could  11-46 Automatic Direction Finder (ADF)