Topband: Hindering factors in the science of back yard 160m vertical ins
rfry at adams.net
Sun Nov 6 02:41:34 PST 2011
Guy Olinger wrote:
>Hams have suffered in the confined circumstances by attempting various
>minimalist extrapolations of commercial radial methodology. The rub is
>that commercial grade research was never done on non-dense,
That isn't exactly correct, Guy.
The 1937 real-world experiments of Brown, Lewis & Epstein of RCA Labs
includes measured data for sets of of radials varying in number from 2 to
113, and in length from 0.137 to 0.412 lambda. The paper at
http://i62.photobucket.com/albums/h85/rfry-100/BLandERadials.gif shows the
results for the longer two sets.
>Bell Labs and all the radio pioneer giants have already been there,
>but not for the non-dense non-uniform radial-disadvantaged crowd,
>and not much for skywave.
The results of the BL&E experiments show that for monopole heights ranging
from about 45º to > 90º using 113 or more symmetrical, buried radials each
at least 0.412 lambda physical length in free space, the groundwave field
measured 3/10 mile from the monopole will be within several percent of its
theoretical maximum value for the same applied power to a perfect monopole
driven against a perfect ground plane.
In essence this means that the real conductivity of the earth in which that
number/length of radials is buried has a very little affect on the radiation
"launched" by that monopole antenna system.
These tests were done at 3 MHz at a site in NJ where earth conductivity was
not better than 4 mS/m, so they certainly are applicable for 160-m monopole
systems used by hams.
A good means to test the performance of a monopole antenna system is to
measure the groundwave field it produces within 1/2 mile or so, and compare
it to the theroretical value it would have if that power was radiated by
that system with zero loss in its connection to r-f ground (as per BL&E).
Doing this accurately takes an expensive, calibrated F.I. meter which few
hams have, but maybe they could borrow/rent one for a few hours from a local
AM broadcast station. The results of the test would show whether they could
benefit from adding more/longer wires to a given set of less than 120 x
1/4-wave buried radials.
Rather than be overly concerned about the conductivity of the earth within
1/2-lambda of a monopole, one could install a "broadcast" type system of 120
x 1/4-wave buried radials, and be done with it. Such systems using monopoles
having physical heights of 45º to > 90º will radiate about 95% of the
applied power, regardless of soil conductivity within the area of the
Once radiation is launched from a vertical monopole* it is subject to
whatever real earth conductivity exists beyond the radius of the buried
radial system. That will affect both the groundwave and the skywave.
But there isn't much that be done about this, except to move the monopole
system to a geographical region having better conductivity.
* The shape of the elevation pattern from a given monopole is the same no
matter what the application. All unloaded monopole heights up to 5/8 lambda
radiate (launch) maximum field in the horizontal plane. Hams may be more
interested in skywave propagation paths than some Class C AM broadcast
stations, but that does not mean that a monopole has zero field in the
horizontal plane and at small elevation angles above it, as sometimes is
concluded by looking at "far-field" plots produced by NEC software, and
found in antenna textbooks.
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