Topband: Radials help
Guy Olinger K2AV
olinger at bellsouth.net
Fri Feb 10 10:21:02 PST 2012
> According to either the ARRL "ANTENNA HANDBOOK", or ON4UN's "LOW-BAND DX
> HANDBOOK", the velocity factor of insulated wire placed atop the ground is
Unfortunately, and inconveniently, not to cast aspersions on anyone, BUT
actual measurements in the Raleigh area showed that velocity factor of a
wire laying on the ground and used for 160 meters was nothing like a
uniform 50%. Measurements were taken with a 151' (46m) dipole on ground,
with an analyzer to find primary resonance (and therefore velocity factor
with a formula) and RF resistance at zero reactance, half of this being the
effective series resistance of an electrical 1/4 wave radial identically
installed in that spot.
The measured velocity factor ranged from 45% to 80%, with the effective
series resistance ranging from 30 to over a hundred ohms. There were wild
variations both in velocity factor and effective series resistance on the
same property, often had just by reorienting the DOG 90 degrees.
These measurements were extremely dependent on the actual height above the
dirt. Notching the wire down just into the dirt (not waiting for the grass
to gradually bury it) produced the most repeatable measurements. If the
end was not specifically insulated (like dipping in liquid tape, etc), the
ends had to be bent up to get a measurement, because the ends are voltage
nodes, even at the tiny antenna analyzer output voltages. Having an
uninsulated end down just down in the damp botched the measurement.
If any of you think an insulated radial field can just plopped down based
on a formula on just any plot of land and be efficient, think again. All
that is necessary to be abysmally INefficient is for the construction
ground fill underneath your sod to be variable in composition, or contain
metallic pipes or buried wires or a septic system. In this case your
radials are no longer ELECTRICALLY dense and uniform, current distribution
becomes wacky, effectively removing radials from the system, and the radial
system has become an unbalanced ground heater, and quite inferior to an
elevated counterpoise. (Sound familiar?)
Have a read on W7ADC's (the excellent Mr. Archibald Doty) work in NCJ on
radials. 1983 and 2011. Note the variability in the SAME dense radial
field, and his conclusions. Largely ignored, and price paid for ignoring.
Measurements made out in the convenient middle of a plowed and disc'd Iowa
cornfield are idyllic because the dirt is uniform and wonderfully
conductive, and UNAVAILABLE to the tortured topbanders trying to get
anything to work on the only and the MF-dreadful plot of land they own.
The starting presumption on a given plot of land should be that
on/in-ground radials will NOT work well unless PROVEN otherwise. The odds
are simply dreadful against it. I doubt they are even as good as one in a
hundred that they could beat a well-designed elevated counterpoise.
If one is stuck with in/on ground, then DEAL with the variability and
INDIVIDUALLY PRUNE radials, notched into the dirt to their final resting
place, to equal effective series resistance to FORCE the UNIFORM in "dense
and uniform". And if you're not willing to bother with dense, you're going
to need an amp to compete with barefoot and QRP using efficient antennas.
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