Topband: Broadband Inverted L

[Yuri Blanarovich]

Hi Richard,

How does the radiation diagram, pattern look with offset radials?
I would suspect some enhancement in the direction of the old radial 
field. (RF mirror)

Do the numbers apply to overall average over 360 deg or is there 
variation in different directions.

Yuri, K3BU.us

 
 
 On Mon, Nov 24, 2014 at 09:05 AM, Richard Fry wrote:
 
 > 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
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