This is my model (so far) for the shortened 20m vertical I’ve been designing, as visualized by 4Nec2.
The radiating element is 12 feet tall with a 4.2-uH loading coil in the middle, and a 2-ft-long capacity hat at the top. The ground radials are all 0.1-wavelength (6’8″) in length. 4Nec2 actually allows you to click and drag the geometry visualization with the mouse to rotate and manipulate the view interactively. It’s a nice way of checking the model.
The graph above shows the SWR of the antenna model over the 20m band, as computed by 4Nec2. It’s pretty flat. This is pretty consistent with other verticals of this design I’ve used. The flatness of the SWR curve is likely due partly to the losses inherent in verticals of this type.
This plot shows the far-field radiation pattern of the antenna in the vertical plane as computed by 4Nec2. There are no surprises here–the antenna radiates most of its energy in a direction 30 degrees above the horizon. The field strength is 3db below that of an isotropic radiator. There’s no arguing that this is a tremendously-efficient antenna design, but it’ll radiate, and it’ll hear. Incidentally, there is essentially no difference between this plot and the same plot for a full-size quarter-wave vertical–the differences in field strength are negligible.
Lest there be any misunderstanding, I make no claims that this antenna would be a superior performer. This design is the result of a number of size and deployment limitations resulting from where I live and what I have to work with. I think that the results of the 4Nec2 calculations are consistent with what one would expect for a design like this.