Topband: Shortend Vertical model questions

[Tom Rauch]

> Wow!!!! What a group of knowledgeable folks. I thank each and everyone
> of you for your input. I certainly have learned alot about verticals.
> It's back to the keyboard for more design work. I've got 6.5 acres
> here to play with so I think I can come up with a large wire top-hat
> for loading. I had also considered using a large wire cage structure
> for the vertical element which would also help in broadening out the
> response, and several folks suggested that.

Hi Tony,

Most of the bandwidth in a loaded system (like a short vertical) 
comes from keeping reactance low, so system Q is low. 
Remember in a series resonant circuit, less reactance means 
lower Q. (Low Q does NOT mean efficiency is low.)   

In an antenna element, Q is largely a function of the boundary 
where the electric field terminates (at the open antenna end).

Current distribution over the radiating area (or ampere-feet of 
radiator area), determines IRE-standard loop radiation resistance.

Coil location and element thickness is secondary to other 
concerns when the antenna has a large hat.  

With that in mind, I modelled a 30-foot vertical using a 26-foot long 
four-wire hat made of small wire.

With a #16 wire vertical element, 2:1 SWR bandwidth was 24kHz

With a 6-inch diameter vertical element, 2:1 VSWR bandwidth was 
26 kHz

That's a quite a change in diameter, yet bandwidth only changed 
the slightest amount...less than you would likely ever notice.

If you use a hat, use a large enough element to support the system 
mechanically. Don't worry about the common myth that making the 
vertical element thicker somehow makes a noticeable difference in 
bandwidth in hat-loaded elements. As a matter of fact, a thicker 
vertical section will actually reduce the radiation resistance in 
some cases, and reduce system efficiency!

Use a big wire hat, put the coil where convenient (at ground level), 
use a reasonable size coil with a modest Q (no need for big copper 
tubing, number ten or twelve airwound wire is fine), and a GOOD 
ground system with at least two dozen long radials (don't believe 
claims a few elevated radials, or worse yet a few short elevated 
radials, will make a good ground system) and you'll be within a dB 
of anything else you might possibly do.

You can put the coil directly from the antenna base to the ground 
system, tap up on the coil with the center conductor, and use that  
single coil for matching AND loading. That will save components, 
make adjustment **very** easy, still maintain about the same 
system efficiency, and will even increase bandwidth!!

With a somewhat thin vertical wire and the hat mentioned above, 
84% of the current remains at the top when the coil is mounted at 
the earth level. The change in radiation resistance is only 15% from 
having the coil inconveniently located at the top, and the loss 
change is somewhat offset by a reduction in required coil 
reactance. With a larger hat, the change is even less!  

While every system can be designed to have problems if we aren't 
careful, with a reasonably large wire hat (my model used four 26-
foot #16 wires on a 30-foot vertical) the system is NOT critical, and 
that includes loading coil location, coil design,  and element 
thickness.
73, Tom W8JI
W8JI@contesting.com