The somewhat devastating report from NEC 4.2 offered by Mr Fry is all the
more gloomy if one factors in the now-common acknowledgement that NEC 4.x
underestimates ground losses with less than ideal radial configurations. Or
stated another way, the gloomy NEC 4.2 report is the very best the bad
situation could be, and likely is significantly worse.
The "shape" of radiation patterns of this genre of antenna rarely depend on
radials. The "magnitude" of the pattern depends on the radials. Mr. Fry's
link shows a very typical shape to the pattern, also seen in an L with a
"gold standard" radial system, the latter being much louder at the distant
end.
73, Guy.
On Mon, Nov 24, 2014 at 9:05 AM, Richard Fry <rfry@adams.net> 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
>
>
>
>
>
>
>
>
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
>
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|