Hello All,
I've been working with the "ENDFEED" antenna analysis program written by Reg Edwards G4FGQ (it and his other programs can be downloaded from here: www.btinternet.com/~g4fgq.regp/#S102 ), and the results suggest that the "Inverted Coathanger" wire antenna used by some aviation NDBs (Non-Directional Beacons) is effectively (electrically) half the height of an un-hatted vertical monopole antenna (or a "Tee" antenna) of the same physical height due to the "coathanger's" sloping wire sides. However, it may improve AM audio.
I think this upside-down wire triangle antenna functions as if it were one half of a fan dipole (like a UHF television "bow tie" antenna, only much larger and inductively-loaded at the feedpoint). The wide dipole has a more broadbanded response and will pass a wider bandwidth signal. The NDB MOG - 382 in Montague, California (see: www.auroralchorus.com/ndb/ndbgllry.htm , www.auroralchorus.com/ndb/382mog.jpg , and www.auroralchorus.com/ndb/382mog_2.jpg ) had an "Inverted Coathanger" antenna and ran TWEB (transcribed voice weather advisories), and the "Inverted Coathanger" antenna was likely chosen to improve the audio quality of its AM signal.
I'm going to give this antenna configuration a try with my Part 15 C-QUAM AM Stereo transmitter because its wider bandwidth will help the audio quality even if it's a somewhat less efficient radiator. Also, it's very easy to build--a single length of wire can be run through two elevated supports and then be twisted together below the lowest "coathanger" corner (with the bare ends soldered together) to form the short vertical downlead. Mounted on an indoor wall, its height will be less than 3 meters (2.4 meters [8'] at most).
-- Black Shire
I've been working with the "ENDFEED" antenna analysis program written by Reg Edwards G4FGQ (it and his other programs can be downloaded from here: www.btinternet.com/~g4fgq.regp/#S102 ), and the results suggest that the "Inverted Coathanger" wire antenna used by some aviation NDBs (Non-Directional Beacons) is effectively (electrically) half the height of an un-hatted vertical monopole antenna (or a "Tee" antenna) of the same physical height due to the "coathanger's" sloping wire sides. However, it may improve AM audio.
I think this upside-down wire triangle antenna functions as if it were one half of a fan dipole (like a UHF television "bow tie" antenna, only much larger and inductively-loaded at the feedpoint). The wide dipole has a more broadbanded response and will pass a wider bandwidth signal. The NDB MOG - 382 in Montague, California (see: www.auroralchorus.com/ndb/ndbgllry.htm , www.auroralchorus.com/ndb/382mog.jpg , and www.auroralchorus.com/ndb/382mog_2.jpg ) had an "Inverted Coathanger" antenna and ran TWEB (transcribed voice weather advisories), and the "Inverted Coathanger" antenna was likely chosen to improve the audio quality of its AM signal.
I'm going to give this antenna configuration a try with my Part 15 C-QUAM AM Stereo transmitter because its wider bandwidth will help the audio quality even if it's a somewhat less efficient radiator. Also, it's very easy to build--a single length of wire can be run through two elevated supports and then be twisted together below the lowest "coathanger" corner (with the bare ends soldered together) to form the short vertical downlead. Mounted on an indoor wall, its height will be less than 3 meters (2.4 meters [8'] at most).
-- Black Shire