At 08:32 PM 2/25/02 -0500, Tom Rauch wrote:
>When I use a single ground of 3/4 inch copper pipe, a 4-5 foot pipe
>has somewhere around 100 ohms of resistance. I had something
>over that value near Atlanta, and less than that value at other
>locations, but that is a good average.
Tom's reply got me to wondering what I could measure for ground resistance.
Using an old Simpson 260 VOM, I measured the loop resistance at the
termination end of one of my test shorty's. The termination ground rod is a
piece of brass rod 1/4 inch in diameter and in the ground about 2 feet. The
feed point ground is a piece of 1/2 inch square steel tubing in the ground
about 30 inches. (These are not permanent rods!) The distance between them
is 270 feet.
Across the 270 ohm termination resistor, there is 0.2 VDC with about 0.05
VAC modulating it. With the termination removed, I measured 65 ohms for one
polarity and 130 ohms for the opposite. The average would be about 100 ohms
and this number includes the combined resistance of two grounds, the
transformer, and at least 20 ohms for the galvanized 18 gauge wire. Looks
like about 40 to 45 ohms for each ground rod.
Surge resistance / SWR measurements are complicated by the enormous amount
of AM BC energy in the antenna. My best guess is the surge impedance is 290
ohms. I would sure like to know if there is a better way to make these
measurements in the presence of all that RF.
By the way, this beverage was a little weaker than the flag on PW0T last
night, but the jammer on frequency was almost totally nulled out on the
beverage. It's handy to have many RX antennas to chose from!
73,
Larry - W7IUV
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