RE: NEC-LIST:MF Ground Constants near High Power Transmitters

Gentlemen:

Even if there is a lot of power loss due to I²R in the antenna hardware and
ground system, think of the total area of the ground radials under a
high-efficiency MF antenna - ~100 meter radius gives an area of ~31K m².
Directional arrays have more area, of course, and a typical three element
in-line array has as much as 2 or 3 times that area, depending upon the
array geometry.

The worst design I've seen in years, a really awful 5 kilowatt 3 tower
tight-spaced array with an RSS/RMS ratio of well over 3/1, gives about 70%
efficiency for 1 ohm loss per element, and that's still only 1.5 kW for an
input of 5 kW, and that's 0.025 watts per M², which is very small comfort
for worms. And if it were a megawatt nobody would have built such a bad
design.

A typical high power array (this example is one Ron Rackley and I just
designed for use in the middle east) with 600 kW input has a loss assuming 2
ohms per element of just under 8 kW. Most of this would be loss in the
"earth-return" currents to the ground radials, and it's still only about 0.1
W/m².

I also can point out that at installations in Alaska where equipment
shelters have sunk into the permafrost, the antenna systems haven't!

  ben dawson

Benjamin F. Dawson III, P.E.
Hatfield & Dawson Consulting Engineers
9500 Greenwood Avenue North
Seattle, WA 98103 USA
206 783 9151
206 789 9834 Facsimile
ben-dawson_at_hatdaw.com

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