Laurence Glavin said:
I recall that a few years ago, on "Let's Talk About Radio", there was a discussion about a totally new AM tower design (in use in Egypt, if not elsewhere) that was unobtrusive enough not to raise the ire of the NIMBYS, and close enough to the performance of classic AM tower designs to be used as a replacement.
I have never seen any credible reports that these unconventional designs actually work. Generally, the proponents make extravagant claims and claim to have measurements to support those claims. But there seems to be a significant lack of supporting measurements from credible observers who have no dog in the fight.
The unconventional AM radiators that the FCC has approved are the Valcom Fireglass whip (useful at 1180 kHz and above at relatively low power levels) but not approved for use as elements of directional arrays; the KinStar, a massively top-loaded design from KinTronics, which depends on some magical coupling networks devised by the highly regarded consulting form of duTreil, Lunden, and Rackley to enable antennas that would normally have very low driving-point impedance to be driven by transmitters that are designed to work only with loads that exhibit much higher impedance; the Paran antenna, which, instead of one tall tower, uses four short towers that support a large top load--Parans work all the way down to the bottom of the AM dial and several of the very few that have been built operate at moderately high powers, but like the Valcom whip, Parans are not FCC approved for use in DAs and the towers are not simple--I believe they incorporate concentric sections that are insulated from one-another.
The FCC also allows a variety of sectionalized AM antennas, but most of those are taller than conventional vertical radiators. There are a couple of examples of sectionalized AM antennas that claim electrical heights that are about twice their physical height (see KSAZ). I have been unable to find a description that I can comprehend of why these work--and, indeed, they don't seem work as advertised; the radiation efficiency does not match the claimed electrical length.
I suspect that the focus of any session on AM antennas at this year's just-ended Radio Show was the recently approved Method of Moments computer-modeling-based AM directional antenna performance-verification technique, which several consulting engineering forms recently persuaded the FCC to accept. MoM promises to be a great boon to the firms that agitated for it, but, like any technique that is based on computer models, its accuracy depends upon the accuracy of the models that are used in the simulation. I'm not so sure that accurate models are always available. I believe that three MoM-based DAs are already in operation in this market--the WXKS/WRCA/WUNR triplex at 750 Sawmill Brook Parkway in Newton.
