Topband: Measuring field strength (for the purpose of radialevaluation)

[Rick Karlquist]

Telegrapher9 at aol.com wrote:
> Rick and Tom (W8JI),
>
>> 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.
>>
>     Dave WX7G
>

When I first built my 230 ft diameter ground screen (3 ft by 3 ft
wire grid, all junctions soldered) I put a 40 meter vertical at
the center and drove it against the center junction of the grid
and measured 70 ohms drive impedance with a bridge.  Most verticals
have far less than the 5 miles of wire I had, yet they are usually
below 70 ohms.  I later determined that this configuration had equal
field strength to a system with 120 radials that had 36 ohms drive
impedance.

I think the reason for these "strange" results is that current
distribution of the antenna can change due to radials.  You cannot
partition the antenna neatly into a radiating portion and a counter
poise, and even if you could, there is no guarantee that the place
you choose for a feedpoint is right on this partition.  I believe
that the grid behaved as a "bottom hat" and the impedance bridge
was between the hat and the vertical portion.  The actual "ground"
was out at the periphery of the grid.  Thus the impedance bridge
was not really at the true "bottom" of the vertical and hence not
surprisingly read a higher than expected impedance.  Whether or
not this heuristic explanation is correct, the results are what
they are.

Rick N6RK