Total posts : 45366
IIRC, and I’ve looked at several different history threads, the FCC considers that the 3m antenna length does not include the wire in a base loading coil, but concerning a hat, it does include the length of a top hat radial, IOW, the length of the vertical plus the radius of a flat top hat.
Even though the function is capacitive, it does have the potential to increase radiation by building more output power, i.e., it helps prevent losses as the signal goes up the vertical. Of course, it’s also different than ground radials.
I would think that in a normal land-based environment, if it weren’t for the restriction mentioned above, the combination of ground radials and capacitive hat could do two things for a given signal strength:
1) Decrease the number and length of ground radials.
2) Decrease the height of the vertical antenna component.
That is, if you have a height restriction, such as you have in close proximity to airport landing patterns, along with limited property boundaries at the base of the antenna, then you could still have similar signal strength to a system with more/longer ground radials and increased height with no hat.
The best example for lower power stations may be the old-style LF NDB systems at airports. Because of the nature of our local airport, they recently replaced one here (“FHR” @ 284kHz) and I became interested in the old one, so I checked it all out. It is 50 ft. high triangular tower with 12 buried ground radials 30ft. each, and 6 capacitive hat radials about 5 ft each. The TX puts out 25 watts, and an ADF in an average aircraft can pick it up reliably from at least 30 miles out.