At 08:28 AM 6/13/2005, Terry Conboy wrote:
>According to modeling in EZNEC, as long as the two radials are symmetrical
>(180 degrees apart, same slope), the azimuth and elevation patterns are
>very uniform, and the horizontal radiation cancels quite well in the far field.
>
>I modeled a 40m ground plane made from 33 foot lengths of #12 copper wire
>with the feed at 50 feet over "average" earth and 45 degree sloping
>radials. The elevation patterns with 2 and 4 radials are nearly
>identical. There is a very slightly oval azimuth pattern (+/- 0.2 dB)
>with 2 radials, with the slight maximum broadside to the radial
>plane. The maximum of the horizontally polarized signals were down 18 dB
>from the main lobe.
>
>The pattern with 4 radials was omni within 0.01 dB, at all the elevation
>angles I checked (15, 30 & 45 degrees).
>
>Certainly, in the real world, other nearby metal conductors (house wiring,
>gutters, etc.) will modify the pattern.
I assume the radials are on the order of 33 ft long too, so sloping down at
45 degrees, they droop about 20 feet. That would make the ends roughly 30
ft off the ground, so the dielectric properties of the earth won't have a
huge effect. However, if you start looking at radials that are, say, 10
feet off the ground and horizontal, small scale variations in the
dielectric properties of the earth under those radials make a big
difference. Changes in the water content will change the dielectric
constant (and conductivity), which will change the effective propagation
velocity along the radial. That will have an effect similar to making the
radial "not straight" and a different length.
Probably not a huge difference in the overall pattern, probably a bigger
effect on feed point impedance, but certainly potentially bigger effects
than the difference between 2 and 4 drooped radials.
>73, Terry N6RY
>
>At 05:52 PM 2005-06-12, Jim Jarvis wrote:
>>If you have a limited number of radials, as in 2,
>>you will definitely have favored directions.
>>
>>In fact, even with 4 or more, you have to consider
>>that the mean vertical angle of the lowest lobe will
>>vary....being lower where the radials are present, and
>>higher in between, as Dale observes, below:
>>
>>>-0-
>>>When I was a kid living in Whittier, CA, I had a roof-mounted 14avq with the
>>>requisite pair of radials cut for each band. The radials were strung out
>>>the length of our house roof -- North-South. I'm sure the years have
>>>obscured/warped my perception of what I may have experienced, but it seems
>>>like I worked a truckload more stations in the pacific NW/VE7/KL7/JA/UA than
>>>to the east and west.
>>>
>>>dale, kg5u
>
>
>
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>any questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|