Topband: Elevated Radials
Guy Olinger K2AV
olinger at bellsouth.net
Tue Mar 5 16:28:08 EST 2013
Pretty picture for perfect ground, but no dice in real life.
The very same NEC 4.2 with raised quarter wave radials over routine real
life ground made of dirt will show field intensities in the ground. Since
there is a field in the ground, and the ground is conductive with a
resistance there will be current and I squared R losses. The fields
immediately underneath the radials will be different than in the areas that
bisect the radials. This difference increases with poorer earth.
This loss MUST be subtracted from the power fed to the antenna.
All one has to do to examine this is to run near field tables with negative
elevations. Squaring the field values and summing them up is a way to
evaluate and understand changes in ground loss with changes in radials.
On Tue, Mar 5, 2013 at 5:56 AM, Richard Fry <rfry at adams.net> wrote:
> Developing this topic a little more, the link below shows a NEC4.2 model of
> a 1/4-wave, vertical monopole driven against a set of 4 x 1/4-wave
> horizontal wires used as an elevated counterpoise. The base of the
> and the elevation of the radial wires are set to 4.9 meters, as in the
> Culpepper system I posted earlier in this thread (Sat, 2 March 2013 at
> 04.33:34 -0600). The relative amplitudes and phases are shown for each
> conductor. Frequency is 1490 kHz. The system was modeled over perfect
> This system produces an inverse distance groundwave field of ~313 mV/m at
> 1 km for 1 kW of applied power. This is the maximum theoretical field
> possible for those conditions for a perfect, series-fed, 1/4-wave monopole
> base-driven against a perfect ground plane.
> The azimuth radiation pattern is perfectly circular. This is as expected,
> because the only conductor producing useful far-field radiation is the
> vertical monopole, itself. There is no physical reason why that radiation
> should be other than omnidirectional, at all elevation angles.
> But while the performance shown by this NEC model is identical to that of a
> perfect monopole with its base attached to a perfect, flat ground plane of
> infinite extent, none of the r-f current flowing on the vertical section
> needed to flow through the earth to reach the monopole. In fact, the NEC
> model HAS no structural connection to the earth !
> No earth currents will flow along the vertical monopole in such elevated
> systems even when they are installed just above a non-perfect ground plane,
> such as the earth.
> This illustrates how such an elevated system using only 4 x 1/4-wave
> horizontal wires as a counterpoise can equal the performance of a
> conventional 1/4-wave monopole using as many as 120 x 1/2-wave radials
> buried in real earth.
> Topband Reflector
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