[BC] Ground systems

[Phil Alexander]

On 12 Feb 2006 at 11:18, Barry Mishkind wrote:

> At 08:20 AM 2/12/2006, RSTYPE at aol.com wrote
> >Based on Dr. Brown's work in the 30's, the ideal ground system would 
> >be 113 radials
> >(which has been increased to 120 for the convenience of a nice round number)
> >located on the surface of the earth.
> 
>          According to Brown himself, the 113 number came
>          from him running out of wire for the grounds at 113.
> 
>          Various early applications also specified 113, following Brown,
>          and it appears the FCC did round up for convenience.
> 
>          There are some graphs/charts around showing the
>          point of diminishing returns in adding more radials.
>          I can't put my finger on it right now.

Barry,

I'm not sure it was *just* a matter of convenience. There is evidence
I've seen that indicates diminishing gains up to ~ 180 radials in
longer radial systems although I can't put my finger on it at the
moment. Consider the rule on this topic.

73.189(b)(4)
	At the present development of the art, it is considered that where a
	vertical radiator is employed with its base on the ground, the ground 
	system should consist of buried radial wires at least one-fourth wave 
	length long. There should be as many of these radials evenly spaced 
	as practicable and in no event less than 90. (120 radials of 0.35 to 
	0.4 of a wave length in length and spaced 3° is considered an 
	excellent ground system and in case of high base voltage, a base 
	screen of suitable dimensions should be employed.)

Also see: <http://www.fcc.gov/mb/audio/amgnd.html>
That may be two of the charts you are thinking of although these show
deductions for systems less than prescribed by 73.189(b)(4).

Elsewhere I've read information suggesting that the improvement from radial
length beyond 90 deg. is not realized in systems of less than 120 radials
and tends to approach maximum at about 180 radials. IOW there is an 
indication that the fractional lambda cord length between radial ends 
may have some importance. This tends to indicate, to me at least, that 
the optimum system would be 180 radials about 140 deg. in length, having 
50 ft interspersed radials or ~ 50' x 50' ground screen if the tower has 
a base resistance > 50 ohms.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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