First - Happy New Year to everyone! I'm hoping to engage the brilliant minds of this august group... Yesterday, I visited a local NDB transmitter site as an excuse to get outdoors on the last day of the year, and it got me thinking about capacitance top hats. I understand that the purpose of top loaded capacitance is to get more RF current moving up the vertical element, which is typically accorded all--or the lion's share--of being the RF radiating element. I also understand that, in contrast to the vertical element, there's very little current moving in the horizontal arms of the top hat (not zero, however, since the alternating RF voltages in the top hat arms have to get changed along their entire lengths somehow, and that's going to require at least some current, no matter how small). Finally, I understand that the alternating RF electric field generated in the horizontal top hat arms works capacitively against ground, essentially completing an RF return circuit back to the transmitter via ground and providing a "reason" that RF wants to flow up the vertical element in the first place. Got all that. Check.
But here's my question: exactly how much do the top hat arms ALSO radiate some RF that works additively with the bulk of the RF radiated from the vertical element? Is it actually zero? Substantial? Trivial? In between? Looking at the top hat array of wires at the NDB I visited yesterday (4 x 40' wire arms at 40' above ground), it's hard for me to believe the answer is ZERO RF radiation from the top hat arms. I'd love to hear some opinions on this. Also, how much capacitance (nF/uF/mF?) could one actually expect from 4 x 40' wire arms at 40' above ground?
Thanks everyone for any thoughts. Again, Happy New Year, and hope it's a great one for everybody. Looking forward to another year of reading this great forum.
Bruce WA1HGJ