On Sun, Mar 8, 2015 at 5:57 PM, Eduardo Araujo via Topband <
topband@contesting.com> wrote:
>
> When I read that they have measured the current in one radial and
> multiplied it by the number of radials, I wonder why even though I used
> simple instruments, got some different result......my method, my instrument
> or both perhaps??
>
The Brown, Lewis and Epstein study (BL&E) was most likely done over very
uniform, and over pretty good dirt for RF. I would trust your measurements
as far more typical of ham circumstances.
A few years back a group of hams in the Raleigh NC area made many
measurements of resonant frequency and R at that frequency and +/- 50 kHz
placing a 151 foot dipole on ground (DOG) as if they were constructing a
BOG. The point was to try to understand some things that were happening
with BOG deployments. The 151 feet was chosen to give resultant resonances
near/in the 160 meter band, given the kinds of velocity factors we were
already measuring in preliminary stages. It also had an even equivalent in
meters (46).
The answer was that those figures were wildly variable, even in different
spots in the same back yard. Sometimes there were large variations between
one placement and another with same center and at right angles.
Should note that it is not possible to model radial-by-radial strangeness
in any NEC variant as the ground approximation schemes DEPEND on a
monolithic uniformity around the compass. Some sites, due to the large
essentially flat acreage, DO fit that requirement. Particularly commercial
AM broadcast, and their propensity to site at low, large, flat sites.
To defend BL&E a wee bit, it contains some number of photographs of the
site (figures 19-24) and equipment. The site depicted in the photos is very
flat farmland. Figures 20 and 21 show the site to be flat at least to a
distance of a mile by visual cues. Most people would call the site "black"
dirt. Figures 22-24 have a built in calibrating grey scale indicator from
clearly discernible black meter cases and white meter faces typical for the
time.
The photographs depict dark dirt, very flat dirt, presumably deposited over
a very wide area by ancient alluvial processes, likely undisturbed at depth
for millennia. Just looking at it, one would expect radials to behave the
same all around, no obvious reason to expect otherwise. We do not know from
the study text if they did prior experimentation that suggested/validated
taking the single radial approach. In terms of time/expense to conduct the
study, taking measurements on all the radials over the test circumstances
included in their testing matrix would be an order of magnitude increase in
expense.
This is in contrast to many of our ham situations with no reason to expect
uniformity of ground composition across a radial field.
>From various clues in my case, including excavations to flatten out US 64
Hwy when they four-laned it some years ago, the ground beneath contains red
or grey clay hard pan, a varying depth layer of carbon/charcoal from an
ancient forest fire, several varying layers of sand in areas draining to
nearby creek beds, a layer of grey clay usually near the top, plus a
varying depth of organic "stuff". Probably the best space-wise site for
traditional radials on my and my neighbor's property combined would have
half the wire above a layer of sandy soil, and the rest over grey clay,
with the sand going underneath the clay layer. That part of the property
was a great site for the necessary septic field, further ruining any chance
for uniformity at RF.
A specific description from you about the ground used for radials would be
very interesting.
73, Guy K2AV
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