Topband: vertical take off angle

[Eric Gustafson]

Hi Rob,

I just ran your inverted "L" through EZNEC V2 (NEC2 engine).
Here is what I got using "good" ground constants (5 mS, e=13),
and 64 foot radials of #12 wire on the earth's surface.

Radials	     Gain	  TOA
	     (dBi)	(Degrees)
-------	     -----	---------

   0	    -1.53	  23.4

   4	     1.63	  23.8

  10	     1.85	  23.8

  20	     2.04	  23.8

  25	     2.10	  23.8

  30	     2.14	  23.8

  40	     2.20	  23.8

  50	     2.23	  23.8

  60	     2.25	  23.8

 120	     2.29	  23.8



Here is what I got using "poor" ground constants (2 mS, e=13),
and 64 foot radials of #12 wire on the earth's surface.


Radials	     Gain	  TOA
	     (dBi)	(Degrees)
-------	     -----	---------

   0	    -2.62	  26.2

   4	     0.75	  26.4

  10	     1.12	  27.4

  20	     1.43	  27.4

  25	     1.51	  27.4

  30	     1.57	  27.4

  40	     1.65	  27.4

  50	     1.69	  27.4

  60	     1.72	  27.4

 120	     1.77	  27.4


Clearly, these 1/8 wavelength radials are too short to have any
meaningful effect on takeoff angle regardless of the screen
density.  Diminishing returns set in early (somewhere between 20
and 30 radials).


Here is what I got over "poor" ground using the FCC 0.4
wavelength radials at various screen densities.
 
Radials	     Gain	  TOA
	     (dBi)	(Degrees)
-------	     -----	---------

   0	    -2.62	  26.2

   4	     0.81	  26.6

   8	     1.16	  26.4

  16	     1.66	  26.0

  32	     2.29	  24.6

  64	     2.96	  23.4

 128	     3.47	  23.0

 256	     3.72	  23.0

 512	     3.82	  23.0	    


It is obvious from this where the 120 radial requirement came
from at this radial length.  But if you really want to bring the
TOA down with a ground screen under a vertical this tall, be
prepared to go to extremes in both radial length and screen
density.  Here is what happens over "poor" ground with 1.5
wavelength radials at various screen densities.


Radials	     Gain	  TOA
	     (dBi)	(Degrees)
-------	     -----	---------

   0	    -2.62	  26.2

   4	     0.81	  26.6

   8	     1.16	  26.4

  16	     1.66	  26.0

  32	     2.29	  24.6

  64	     2.97	  22.4

 128	     3.57	  19.2

 256	     4.02	  15.6

 512	     4.33	  12.4

1024	     4.53	   9.6

2048	     4.65	   7.4

4096	     4.71	   6.6

8092	     4.73	   6.6


Just out of curiosity, which software package were you using?
What were your conductor diameters?  What assumptions did you
have for ground constants?

73, Eric  N7CL


>From: "Rob" <w4rjk@home.com>
>Date: Mon, 24 Jan 2000 18:38:22 -0500
>
>Due to some recent postings on Inv L's I ran some things through
>a antenna program.  An inv L Apex 100ft top wire 35 feet, says a
>maximum of 14 degrees with 60 radials minimum size 65 ft.  I would
>be interested if others could run the same data and let me know
>what you find out?
>
>I also ran the following:
>
>10 ground radials 22 deg
>
>20 gr  20deg
>
>25 gr 18
>
>30 gr 18
>
>40 gr 16
>
>50 gr 16
>
>60 gr 14 deg
>
>I know there is also the other side of the pattern, but just
>using this info I am interested in your data and the program you
>used to obtain it.  Please reply direct to me.
>
>Thanks
>
>Rob
>W4RJK


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