Topband: Measuring field strength (for the purpose ofradialevaluation)

[Joe Subich, W4TV]

Dave, 

> So, I stand corrected on being too loose with the method. I 
> should have pointed out the limitation of the "decreasing input 
> impedance method." And that limitation is that the radials must 
> be symmetrical and all of the same length.

The "problem" with simulation is the assumption that ground (earth) 
is homogeneous and uniform.  With "real" ground this is certainly 
not the case - probably even with (relatively) large open areas 
that were formerly tilled fields.  In the typical urban/suburban 
site there are many things in the earth that will effect the 
soil characteristics - particularly on 160 meters where the skin 
depth extends many feet into the earth.   

> Now it sounds like even this is not the case with actual 
> measurements. Have you measured increasing input impedance 
> for additional radials in a symmetrical arrangement? If so, I 
> would like to understand this.  Understanding and getting sims 
> to match measurements (and vise versa) is what I do.

I do not think that even NEC-4 can be trusted (Christman 
notwithstanding) to be more than a rough estimate with real 
earth. My own yard goes from extremely poor soil (Florida 
sand dunes) to reasonably good areas of extensive treatment 
for my wife's flower gardens to worse than poor (places in 
which original construction debris was buried).  All of that 
area would be encompassed by a radial field of 60 x 100' 
radials.  

73, 

   ... Joe, W4TV