Joe N3HEE wrote:
I moved my 160 inverted L to a tall tree in my backyard to get more
vertical
height. The vertical leg is now about 65 feet and the rest (65feet) is
horizontal. .... Any feedback would be great. Thanks -Joe N3HEE
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A NEC4.2 model roughly approximating this system was made using a set of 40
symmetrically-buried radials each 50 feet long, and connected at their
common point by a 40-ft straight wire lying on the earth to an 8-ft buried
ground rod 40 feet away. The L consisted of a vertical and a horizontal
conductor of 65 feet each. The vertical axes of the vertical conductor of
the L and of the offset 8-ft ground rod were aligned. A second 8-ft ground
rod was located at the common-point of the radial field.
The radiation resistance of the L on 1.9 MHz is 21 ohms. System results for
1.9 MHz and earth conductivity of 5 mS/m, d.c. 13 ...
Using the offset radial system: Feedpoint Z = 128 -j 3 ohms, peak gain = -
5.8 dBi at 64 degrees elevation, 2:1 SWR BW = 270 kHz, system radiation
efficiency = 16.4%
With the vertical conductor of the L centered over the common point of the
radials: Feedpoint Z = 29 +j 4 ohms, peak gain = 0.5 dBi at 64 degrees
elevation, 2:1 SWR BW = 90 kHz, system radiation efficiency = 72.4%
The efficiency numbers above are based on a lossless match to the
transmission line connected at the feedpoint. The NEC gain analyses were
based on the far field.
For this comparison study, NEC4.2 shows a system loss of more than 6 dB when
the L uses the offset radial field, however the offset system has the better
SWR bandwidth.
R. Fry
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