Richard (Rick) Karlquist (by way of Bill Tippett
<btippett@alum.mit.edu>) wrote:
>So I think the answer to this "paradox" is that you
>need very close radial spacing close to the antenna
>where the current density is highest. Farther out it
>is OK to have greater spacing.
>
>Rick N6RK
>
>_______________________________________________
>
>
Rick,
That has been the belief in the broadcast industry for as long as I can
remember. Every presentation at NAB and IEEE I have ever listened to or
read seem to support the thesis that a 50x50 or 100X100 piece of
expanded copper mesh under a 90 degree tower AM broadcast tower
establishes the near field FSM normallty made at one mile from the
tower. Adding 30 or 60 radials, so I have read, will make little or no
significant difference to this reading. But as the man said topbanders
seldom use FSM readings of ground wave levels as a determination of what
works best for sky wave propagation requirements. So IMHO if you have
limited options for a location a large screen of expanded copper mesh
(I know its expensive and chicken wire will work in the short term) you
will not be disappointed with the screen as a viable substitute for
limited space or other long radial restrictions.
Herb, KV4FZ
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