> From: jljarvis
> Sent: Friday, March 08, 2002 8:04 AM
> To: towertalk@contesting.com
> Subject: [Towertalk] grounding an elevated vertical
> 120 buried radials are the broadcast norm. But 16 radials
> will get you to 90% efficiency.
More like 80% efficiency (1 dB loss)
> And 3 or 4 elevated radials will do the same.
Not according to professional field strength measurements.
> Empirical data presented to the FCC showed that 4 radials, elevated
> @ 10' were the equivalent of 120 buried radials, in terms of field
> strength. If I recall the experiment, though, there were REMAINS
> of those 120 radials still in the earth, and the ground conductivity
> was exceptionally good.
The key here is the existence of the original radials. They make
a major difference in ground conductivity and thus lower field loss
at the base of the antenna
> The reason to get as high as possible is to minimize the capacitative
> coupling with earth, in the near-field. This will reduce losses.
Right effect, wrong reason. The elevated radials generate a strong
field which cancels at a distance. However the fields do not cancel
directly under the antenna (radials) thus the field is attenuated
by the lossy earth unless steps are taken to reduce the ground losses.
> A 40m vertical, with the base 1/2 wave in the air...and radials
> sloping downward @ 45 degrees or so is a killer.
Work by N7CL shows that ground coupling from radials is very small
at a radial height of 1/2 wave and almost nil by the time one
reaches 4/3 wave high.
Since field strength from a single antenna element (radial) is
inversely proportional to the square of the distance anything one
can do to raise the height of elevated radials will improve
antenna efficiency.
73,
... Joe, K4IK
|