Doug Smith W9WI wrote:

47CFR73.189 establishes minimum tower
heights for AM stations, depending on class and frequency. For example,
a Class C station above 1200KHz must use a tower at least 45m tall,
while a Class A station on 650 must use one at least 165m in height.

Under certain circumstances these requirements can be waived if the
station can meet field-strength requirements with a shorter tower.
--

The question was addressing tower height limits which I interpreted as
being maximums rather than minimums. Thanks for your clarification.

There is a company, Valcom, in Guelph, Ontario Canada that is
advertising in the US radio magazines that they are selling MW band
antennas as short as 15 meters. These look like linearly loaded
fibreglass poles with large capacity hats at the top. I wonder if any
of these have been licensed in the USA. Would not a waiver be granted
for such an antenna if the field strength requirements could be met?
It seems to me one could supply the specified field strength with any
reasonably short antenna as long as one was willing to crank up the
transmitter power output to counter the loss in efficiency. I think
that would be permissible under FCC rules but I am not sure.

The Valcom web site does not specifically call out AM broadcast
applications except for TIS stations, probably because the antennas
are only rated to 2 kW over most of the AM band.

www.valcom-guelph.com

Joe Buch