Timz From United States of America, joined Sep 1999, 6903 posts, RR: 7
Reply 3, posted (13 years 9 months 4 weeks 1 day 2 hours ago) and read 1026 times:
I'm no expert, but I think an NDB is about the simplest possible radio navaid. It just transmits a signal, and it's up to you in the aircraft to have direction-finding equipment that tells you where the NDB is, relative to the aircraft, not relative to north. In other words, the pilot can read an instrument that tells him the NDB is straight ahead of him, or 90 degrees to his left or right, or whatever, but if he wants to know whether he is south or east or north of the NDB he has to look at his compass.
Airplay From , joined Dec 1969, posts, RR:
Reply 7, posted (13 years 9 months 4 weeks 23 hours ago) and read 1006 times:
An NDB is a non-directional radio beacon. The beacons broadcast in AM in the 200KHz to 400Khz. ADF receivers are designed to receive from 200 KHz to 1700 KHz so they can make use of existing commercial AM radio stations.
The ADF receiver uses 2 antennas to find the direction of the beacon, a loop antenna, and a sense antenna. Some older ADFs use a separate "long wire" for the sense antenna, but most modern ADFs use a "combined" loop/sense antenna.
The loop antenna consists of 2 loops of wire which are placed 90 degrees apart. The broadcasted radio signal is induced into each winding.
When the aircraft is flying directly to the station, the amplitude will be maximum in one winding and minimum in the other. When the aircraft is flying abeam the station, the ampiltudes in the windings are reversed. Any intermediate bearing will induce amplitudes dependant on the relative bearing to the NDB.
The amplitudes in the windings are identical for reciprical bearings: the amplitude is maximum in one winding when going directly to or directly from the station. This is where the sense antenna comes in. The phase of the radio signal in the sense antenna is used as a "decision maker" and allows the ADF to determine which is the appropriate reading.
Once this is all figured out, the ADF supplies the pilot with the relative bearing to the station, not the heading. The pilot must determine the heading to fly to get to the station. Sometimes the ADF indicator has a moving compass card incorporated in the display which is interfaced to the aircraft compass system. This type of display is called a "radio magnetic indicator" or RMI.
Many RMIs have 2 needles to accomodate multiple ADFs. Others have swithing available to be able to display "automatic VOR" which is merely an indication of your VOR radial.
Many middle marker beacon transmitters have been replace with an NDB. This allows them to perform a dual purpose as a non-precision approach nav aid, or as a middle marker for precision (ILS) approaches. Overflying an NDB is indicated by the needled turning 180 degrees around.
N766AS From , joined Dec 1969, posts, RR:
Reply 8, posted (13 years 9 months 4 weeks 2 hours ago) and read 978 times:
>NDB's are often placed a couple miles from a runway to aid the pilot in flying a percise approach
You're right... I live right near one that is coupled with the OM for SEA 34 (ODD). When it is pared with an outer marker, the NDB is referred to as a LOM (locator outer marker).
As for other uses- it is used quite a lot more than you may think in commerical aviation. At many smaller, regional airports, NDBs are part of departure or arrival procedures, using NDB bearings and such as navaids. Oh, and the infamous NDB approaches...