DavidKaye said:
What you mean, David, as you well know (but many readers don't know), is that the height above sea lavel of the base of an AM radiator doesn't make much difference. As you know very well, the electrical height of the tower (from the base to the top) has a big influence on the skywave.
Although among non-sectionalized radiators, 5/8-wave towers have the highest groundwave radiation efficiency (an inverse-distance field of ~440 mV/m/kW @ 1 km), such radiators have a vertical radiation pattern with a high-angle lobe, which can cause interference between the reflected skywave and the goundwave during critical hours and at night. If the station is a Class C AM, which is subject to a high level of co-channel interference, this "self interference" is usually not very important. That's why a 5/8-wave tower works so well for KVTO. Class A AMs, generally do not use such tall towers, however, because the self interference would be troublesome to them. Although half wave is the tallest non-sectionalized tower that does not produce a high-angle lobe, heights of 190 to 200 degrees are the most popular among Class A AMs--apparently at those heights, the high-angle lobe is not big enough to be really troublesome and the small added cost of the slightly taller tower is deemed worthwhile.