Hi Marconi. I do recall that test model that was constructed after reading the Avanti patent. The patent suggested increasing the angle could increase gain and is why I built it to test their idea.
Back at that time I posed a question on my copy of the patent in the area for column #3 line 66 saying, "SW proved this wrong ?"
Tuning at a distance of 50 miles on HF proved very difficult. My partner and I used a 27.120 RF generator with a very stable 100 watt carrier into an Interceptor 25K for the test. At this distance, the atmospheric conditions do effect signal strength regardless of how stable the test equipment is.
Those fluctuations could easily be a few db within moments and at other times, remain stable for many minuets. That mislead me into thinking there was some improvement when continued testing revealed that the variables on HF at 50 miles were larger than the gains I was measuring with antenna changes.
It was only after more testing on VHF that I found the gain dropped when the antenna was tuned with a larger loop. The larger loop with more angle means the radials must be shorter to maintain 1/4 wavelength resonance. This is the opposite direction than what appears to focus the TOA on the horizon.
When you lengthen the cone, you delay the signal to the top section by a few degrees with respect to that which appears on the outside of the cone. This provides a few degrees of "Electrical Beam Tilt" which places the maximum signal much closer to the horizon than is possible with single section, vertical antennas.
Also. don't forget that both the original Sigma IV, LW-150 and old Vector 4000 used radials that were many inches shorter than 1/4 wavelength and were not taking advantage of phase delay between sections. The FM version of the antenna had been using near 1/4 wavelength radials that are resonant with the loop around the top for many years before the new V4K followed.
In summary, there are 3 main thing to consider with the dimensions of the cone.
1) Its outside surface and ring must be fairly close to a 1/4 wavelength resonance regardless of the size of the ring or length of the radials. Changing one means you must change the other to keep this resonance.
2) Changing the length of the cone also changes the phase angle of the signal driving the top section, useful for beam steering.
3) Changing the length of the cone also changes how much of the out of phase radiation coming from the central element inside the cone, is shielded from contributing to the rest of the pattern. Too short a cone and lots of the out of phase radiation escapes into the far field, lifting the pattern above the horizon. Too long a cone and the delay to the top section becomes too much as you begin to cover over more than the just the first 1/4 wavelength of the inner radiator.
Keep in mind, the velocity factor to the vertical element inside of the cone is different than the exposed, outside surface of the cone. So you can see there are at least 3 or 4 different things going on as we make changes in the cone. Finding the best balance between them all, that puts the most signal on your target areas is what the goal is whenever you're working with beam tilt. Unfortunately with the mounting heights used on HF, what works best for one person is not always the same in the next installation.
When the antenna is less than 1 or 2 wavelengths above ground (as is the typical case on HF) small changes is height make larger changes in the TOA. Once the antenna is many wavelengths above ground (easy to do on VHF), the effects of height on TOA become less. The typical goal here is to maintain a signal parallel to the horizon and is easier to repeat from one installation to the next.
For as simple as the antenna looks, there is certainly more going on than meets the eye at first glance.
