Topband: Broadband Inverted L

[Richard Fry]

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
__________

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