Topband: Radials help
w2xj at nyc.rr.com
Fri Feb 10 15:06:01 PST 2012
I think this is all being way over thought. It would be good to go back
and study the work of Dr. Brown of RCA in the early 30s. At the time
broadcast stations were using various iterations of wire antennas as we
still use today on our HF bands. Dr Brown developed the theory and the
math underlying what came to be the standard practice for MW
broadcasting world wide. There have been numerous studies that build on
that work including some in QEX that document the effect of varying
numbers and lengths of radials.
In a nutshell virtually no commercial broadcast is reasonant nor is that
even part of the design criteria. A sterotypical idealized antenna would
consist of a quarter wave vertical with 120 ground radials a quarter
wavelength or more. In the real world this can be reduced to a 45 degree
mast and 4 to 8 ground radials with some loss of efficiency but not as
much as one might think. My advice for the top band put up as much tower
as possible with 60 feet being a desireable minimum and about 360 feet
the maximum. Use a buried ground system. In broadcast we usually use #10
bare copper. Put in as many as is pratical and as long as practical. The
key is to have a proper matching network at the antenna. In broadcast we
always have an outdoor ATU at the tower base (or a dog house). For
simplicity an L network will do the job although a T network is better.
Either network can be adjusted to match any frequency in the 160 M
band. There are also more elaborate network design that can flatten out
the reactance slope to create a much broader match without retuning.
On 2/10/12 1:21 PM, Guy Olinger K2AV wrote:
>> 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.
> 73, Guy.
> UR RST IS ... ... ..9 QSB QSB - hw? BK
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