Hi Wes,
Thanks for posting up the Severns material. I had started looking for that,
but without memory of exact words (which makes searching difficult), and
without absolute certainty that Rudy was who I heard it from. You likely
saved me an entire working day. If you're ever in the area, that's worth
free pizza and beer and a no-cost friendly guest bedroom accommodation
instead of a motel.
Rudy's contention:
"Confirmation of the NEC predictions was very satisfying but that work must
not be taken uncritically! My articles on that work failed to emphasize how
prone to
asymmetric radial currents and degraded performance the 4-radial elevated
system is. You cannot just throw up any four radials and get the expected
results."
... is oddly separately confirmed by research done by a group of hams in
the North Carolina Triangle area back in 2009. The task was to get an edge
on the highly erratic success/failure of BOG's. We were examining velocity
factor variation in an insulated dipole on ground (DOG) at various points
in our region. The DOG was 151 ft or 47m, picked for round numbers in
length and a probable resonance in or near 160 meters accounting for VF.
Measured resonance of a fixed length gave us velocity factor (VF) and other
data. VF's based on those measurements ranged from 45% to 85%. That huge
variability certainly explained the BOG experience, and suggested a
solution, but that's another subject.
To underscore Rudy's assertion that local in-ground and other influence can
screw up four raised 1/4 wave radials, I offer the DOG VF's, highly
variable in different spots across the region, and in cases highly variable
on the same property. The latter included a few where rotating the DOG 90
degrees about its center produced different VF's, varying 20% in one case.
This was accompanied by the owner's assertion that there was nothing buried
out there he knew about (pipes, electrical or phone feed, etc) and there
had never been a septic field. One feature of this area is veins of sand,
like fingers, near the surface bounded by hard-pan clay. One of these, with
its water content, would have a considerably different effect if a wire ran
across it, than if the wire ran parallel. And it would detune a BOG
specifically tuned to the ground underneath, if the water table dropped and
the sand finger drained out.
So to a lesser degree, but still with effect, similarly oriented 4 x 1/4
wave elevated radials (over that exact spot exposed by the DOG) will have
dissimilar resonances, and the old practice of resonating a PAIR or radials
still would not deal if the variable situation was all under just one of
the radials. The radials would have to be resonated individually with a
single-ended test, largely unfamiliar and requiring a made up testing
adapter and a graphing analyst that can display proper sign of reactance.
Even after that, there still could be differences in ground induction which
could not be adjusted out, causing differing R across the radials even
after X was adjusted to zero.
There is a certain amount of net RF field at ground that is minimized by a
"pure" 4 x 1/4 system. However, if the radials vary in X and/or R, then the
individual field phases at ground shift, and the net RF field under the
radials increases, increasing ground losses. The resulting reactance at
feed can introduce losses on common ferrite blocking devices. Blocking
device and common mode losses can be reduced or eliminated by an Isolation
Transformer, but the ground losses remain.
This is NOT to say don't do it, but if all the potential loss factors for
various counterpoises are considered on a particular ham's property, then
an informed choice can at least optimize to the degree possible. But by all
means put something up and get on the air.
To the degree I can tell, ON4UN's 4 x 1/8 wave elevated radials will always
beat out 4 x 1/4 elevated. At the same height and over the same center, the
4 x 1/8 must have only 1/5 the counterpoise ground power loss in watts of 4
x 1/4. Just be sure to use an isolation transformer with the 4 x 1/8 so
you can use that coil to center the SWR (what everyone does anyway) without
the penalty of common mode and blocking device loss. Connect the radial
center point to the "FCP" connection on an isolation transformer.
The 1/8 wave is not a resonance point or magic length like 1/4 wave. 1/8
wave-ish will always have some degree of reactance. One can make them
uniformly a little shorter to deal with mounting points and keep all the
radial ends in the clear. You just need a little more coil to resonate the
whole thing.
To agree with Wes, gull wings do have an immutable specific loss issue,
which ought to discourage their use. The loss requires technical
explanation, comparative modeling and illustrations to clarify the issue.
The short answer is don't use gull wings, and the long answer I will have
to post on k2av.com in a new section, when I can get to it.
73, Guy K2AV
On Thu, Jul 25, 2019 at 5:10 PM Wes <wes_n7ws@triconet.org> wrote:
> I guess I should have added "useful to me." At my location, I wouldn't
> dream of
> having elevated radials less than 6 feet above ground. Running them
> through the
> cactus and bushes could be just as difficult as on the ground, especially
> when
> they need to be considerably longer. There also needs to be a common-mode
> choke
> and lightning protection is more difficult and there are some mechanical
> issues. I'm going to liberally quote from another of Rudy's papers:
> https://rudys.typepad.com/files/qex-mar-apr-2012-1.pdf
>
> "Among amateurs there's been a long running discussion regarding the
> effectiveness of a vertical with an elevated ground system compared to one
> using
> a large number of radials either buried or lying on the ground surface.
> NEC
> modeling has indicated that an antenna with four elevated λ/4-radials
> would be
> as efficient as one with 60 or more λ/4 ground based radials. Over the
> years
> there have been a number of attempts to confirm or refute the NEC
> prediction
> experimentally with mixed results. These conflicting results prompted me
> to
> conduct a series of experiments directly comparing verticals with the two
> types
> of ground systems. The results of my experiments were reported in a series
> of
> QEX[1-7] and QST[8] articles (.pdf files of these articles are posted at:
> www.antennasbyn6lf.com ). From these experiments I concluded that at
> least under
> ideal conditions four elevated λ/4 radials could be equivalent to a large
> number
> of radials on the ground.
>
> Confirmation of the NEC predictions was very satisfying but that work must
> not
> be taken uncritically! My articles on that work failed to emphasize how
> prone to
> asymmetric radial currents and degraded performance the 4-radial elevated
> system
> is. You cannot just throw up any four radials and get the expected
> results. I'm
> by no means the first to point out that the performance of a vertical with
> only
> a few radials is sensitive to even modest asymmetries in the radial
> fan[10,11,
> 12], the presence of nearby conductors or even variations in the soil
> under the
> fan[9]. These can cause significant changes in the resonant frequency, the
> feedpoint impedance, the radiation pattern and radiation efficiency. These
> problems have been pointed out before but as far as I can tell no detailed
> follow-up has been published. Besides the practical problem of
> construction
> asymmetries, at many QTH's it's simply not possible to build an ideal
> elevated
> system even if you wanted to. There may not be enough space or there may
> be
> obstacles preventing the placement of radials in some areas or other
> limitations. I think it's very possible that some of the conflicting
> results
> from earlier experiments may well have been due to pattern distortion and
> increased ground loss that the simple 4-wire elevated system is
> susceptible to.
>
> As the sensitivity of the 4-radial system and it's consequences sunk into
> my
> consciousness I began to strongly recommend that people use at least 10-12
> or
> more radials in elevated systems."
>
> Based on the foregoing, I would be really reluctant to use a couple of
> gull wing
> radials and call it good.
>
> Wes N7WS
>
> On 7/25/2019 12:37 PM, Richard (Rick) Karlquist wrote:
> >
> >
> > On 7/25/2019 12:01 PM, Wes wrote:
> >
> >> As to gull wing radials, Rudy Severns has looked at these too:
> >>
> https://www.antennasbyn6lf.com/files/antenna_ground_system_experiment_5.pdf
> >> Rudy's work is a treasure trove and I think I have everything he's
> written in
> >> a folder on my hard drive. Believe me, considering how difficult it is
> to
> >> lay radials in my cactus patch, if I thought gull wings would be useful
> I
> >> would have used them. I don't.
> >>
> >
> > Rudy shows gull wing down only 0.65 dB and comments that "radically
> > changing the radial geometry does not seem to have a major impact".
> > So I don't understand your remark that gull wings are not useful.
> > The reduction in drive impedance can be mitigated by increasing the
> > height by about 3 feet. I don't even have cactus but would never again
> > run radials on the ground now that Rudy has educated us.
> >
> >
> > Rick N6RK
> > _________________
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> Reflector
>
>
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