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Tech Talk: Whatever capacitance any given cap hat adds, is the same no matter where it is placed. However, whether or not a cap hat increases the effective length and/or increases the radiation resistance and/or increases overall losses, depends on where (how high above the coil) the cap hat is placed. For example, when placed too close to the loading coil, the capacitance can have a detrimental affect on the coil's Q, and will indeed produce an increase in the measured input impedance. For example, the left photo* depicts a cap hat incorrectly installed. The input impedance and bandwidth will indeed increase in this example, however the changes are due to increased coil losses, and not by an increase in radiation resistance (Rr) . . .

Cap hats typically increase bandwidth. The amount of the increase is based on a myriad of factors, most of which you cannot measure directly. Here too, you have to be careful about applying a change in input impedance (SWR over any given bandwidth), as a positive indication. In fact, the bandwidth of an antenna with incorrectly mounted cap hat (directly atop the coil), will be greater than a correctly mounted one, and with far less efficiency as well.

Cap Hats

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What da ya know, every once in a while you antenna gunslingers get it right