Topband: FCP model

[Jim Brown]

On 8/1/2012 9:07 AM, Jim Hoge wrote:
> Team Vertical has employed a gull wing style of raised radials that would eliminate your concerns. I'm not the expert here.... just mentioning another alternative.

That's my implementation, more or less.  My feedpoint is about two feet 
above the ground, the radials rise quickly from that level, pulled up at 
the far end by an anchor in a tree, at a midpoint by a rope in another 
tree, and held up closer to the antenna by sections of 3/4-in PVC 
conduit that are held erect by 4 ft sections of small diameter rebar 
shoved into the earth.  At about ten feet from the feedpoint the radials 
are about 7-8 ft high, by 20 ft they're 10-12 ft.  We all do what we can 
based on our real estate and our resources. I have a LOT of trees and 
the earth is mostly soft.

On 8/1/2012 5:47 AM, Charlie Young wrote:
> My models don't show the significant effect of close proximity to ground on the radial resonant length.

You can get at least a first approximation by modeling a dipole at the 
height where you plan to put radials. I did that this spring in 
preparation for this elevated radial project. I did a series of 
computations, each for a different height, varying the length for each 
height until the "dipole" was resonant.  What we are seeing is the 
change in the velocity of propagation along the antenna caused by 
proximity to the earth.

What I learned is that the velocity factor is on the order of 0.7-0.8 
for radials more or less laying on the ground, increasing quickly with  
even six inches of elevation, and by the time you've reached 4 ft or so 
of height the resonant length is pretty close to what it would be if the 
antenna was high in the air.  What changes with additional height is 
mostly the feedpoint impedance.  I should put those plotted curves on my 
website.

> Consequently, I just build the antennas, adjust the radials to the real world length, and don't worry about what the model says.

Guy has several times recommended tuning the lengths of radials by 
connecting them in pairs, dipole-style, to an MFJ259 or equivalent. .

73, Jim K9YC