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Below are some numbers and comments on these topics.
NEC was used to model the three antennas below, and determine their performance. Here are the results for the groundwave field at one mile for ground-mounted antennas with 25 ohm r-f ground loss and the r-f loss of a typical loading coil:
Configuration > NEC Calculated Field for 1 kW Tx Power > Computed Field for 80 mW Tx Output Power
1. 3-meter, no hat > 19.52 mV/m > 17.46 µV/m
2. 3-meter, 4 spoke hat, spokes at 90 degree interval, each spoke 1 m long > 28.61 mV/m > 25.59 µV/m
3. 2-meter, 2-spoke hat (in line), each spoke 1/2-m long > 16.15 mV/m > 14.44 µV/m
Configuration 1 is a classic, ground-mounted 3-meter vertical. The other two are configurations suggested by earlier posters in this thread.
If the spokes lie in the horizontal plane and are arranged symmetrically they will not produce useful radiation. This is because the currents that flow in them always move in opposite directions in each in-line pair, and the radiation from one cancels the radiation from the other. Getting an FCC field inspector that understands this probably is dicey, and the 4-spoke configuration above might be seen as exceeding the 3-meter definition in Part 15. It does give the best performance of the three, though.
Configuration 3 uses a 2 meter vertical section and two 1/2-meter spokes, so the total structure is 3 meters. But it doesn’t perform as well as configuration 1 (no spokes), because even though the average current in the vertical section has increased, it is flowing along a shorter radiator.
For a 3-meter vertical with no hat, using a 6″ OD pipe versus a 1/2″ OD pipe has a negligible affect on the radiation resistance and current distribution of the radiator, so there is no real change in the radiated field. The 6″ OD pipe does have lower reactance, and it changes at a slower rate with frequency — so there is a benefit to it in that its VSWR bandwidth is better, and the station might have better audio bandwidth at receivers. Using the 6″ pipe might save a bit of r-f loss in the loading coil needed to resonate it, also.
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