[Milsurplus] NDB's and LOM's

[CL in NC]

Locator Outer Marker was a low freq transmitter with omnidirectional antenna, and normally a 2 letter ID,  colocated with an outer marker which ran on 75Mhz, radiating straight up.  Not too many left now as outer markers are and have been decommisioned due to the advent of GPS fixes.  Non directional beacons were and still are owned by airports and the feds, they have 3 letter ID's some times 4.  Their grandaddies were the A/N directional beacon sites, and many NDB's were once 5 tower arrays radiating a directional signal with the letter A or N received depending on what quadrant you were in.  All current NDB's have to be maintained to federal standards whether private owned or owned by the FAA, and are usually inspected by an FAA rep once a year for power, freq, mod percent and mod tone freq.  The antenna system has its power set by a flight inspection aircraft that orbits the station and reads RF level, and instructs to raise or lower the power as needed, normally only done once unless the antenna changes, but, they are checked whenever ILS or VOR systems are being checked and they are in the area.  Usually, the indication of power that has to be maintained is antenna current, not watts.  The watt indication if the transmitter has a wattmeter is relative to the antenna current.  At one time, there was a required check to measure antenna base resistance on a regular basis.  When I did it, the FAA had an ancient piece of General Radio test equipment in a wooden box with black bakelite panel, but it still gave a accurate feed point z for the various antennas, which were towers with capacity hats, or vertical wires with capacity hats, some only 50ft tall.  There are commercial NDB's that have 2 identical transmitters in one cabinet, one radiates all the time, the second one is keyed and is 1020 or 400HZ off freq and gives a 1020 or 400 hz beat note for the ID.  Many municipal airports are keeping their NDB's up and running as it is the only navaid they have.  The reason the aviation continues to use AM and no other mode, except SSB for over ocean work, is that it is a simple system, low parts count, thus a longer mean time between failures.  The did not want FM due to the capture effect of the stronger signal.  Of course today, all of it being computer based, AM receivers are not the simple things they used to be.  AM gave aircraft and controller a positive indication of interference due to heterodyne, and they could ask for repeats, but todays modern gear, it is not usual to hear two aircraft talking at once with no beat note.  I am not a all the eggs in one basket fellow, eliminating all the ground based NDB/LOM may not be the smartest thing, and the FAA has rethought the total elimination of VOR's, deciding to keep Gateway VOR's and certain others in remote locations on the air as a back up to GPS...just in case.

Charlie, W4MEC in NC