What is the law for lpam antenna and using a capitance hat on the antenna? Would you have to shorten the antenna to comply or not
use one?
To be clear, there is no law concerning the use of a capacitance hat for "Part 15" station antennas. However, if the total length of the conductor for antenna, transmitter, feedline and ground cannot exceed 3 meters. Most antenna designers for Part 15 AM broadcasting agree, the use of a top hat does not significantly improve the efficiency of a very, very short and narrow pass band system.
Very short AM broadcast radiators in the neighborhood of 100 inches or so, exhibit a lot of capacitive reactance. So, to get the antenna system to resonate, adding an inductive loading coil at the base of the radiator has been the best solution. The loading coil also fits into the allowable 3 meter length.
The very significant drawbacks to this type of system is the ohmic loss in the coil wire and the very narrow pass band of the antenna.
A capacitance hat could broadand the system somewhat, however,
its size and the structure needed to mechanically stabilize it would be difficult at best.
Marshall Johnson, Sr.
Rhema Radio - The Word In Worship
http://www.rhemaradio.org
AFAIK, if you use a capacitance hat, the antenna length is measured from the antenna base, up to the hat, and then out to the end of one of the hat radials (the longest if all are not equal length). If the far ends of the hat radials are connected through a "hoop" wire, that hoop does not count, as it adds nothing electrically because the radials are also joined at the center.