On Thu,12/18/2014 1:18 PM, Doug Turnbull wrote:
I appreciate some guidance with this matter. I would like a
radial field which would take me to within 0.5/1 dB of the maximum
achievable for reducing near field losses.
Some of the best work I've seen published on this topic is by Rudy
Severns, N6LF. Look for the 2-part QEX piece he did about 3 years ago.
It's on his website and is well worth studying.
For my part, I modeled a half-wave 160M dipole parallel to the earth
starting at heights of about 5 ft down to a few inches, varied the
length so that the antenna was resonant at each height, computed Vf from
the result, and plotted it. That work is slides 40 and 41 in
Obviously what happens in the real world will depend on soil conditions.
Think about this in the light of Rudy's work -- the standard for
broadcast radials has been a half wave length. Taking Vf into account,
they would be more like 3/4 wavelength or even longer. Rudy observes
that current distribution will depend on the length of the radial, and
the boundary condition is that it must be minimum at the end. He notes
that if a radial is some length between 0.25 and 0.5 wavelength, the
current will peak 0.25 from the far end, and that peak will be greater
than the current at the tower base, and because that current is greater,
the loss will be greater. He observes that loss will be minimized when
the peak current is at the feedpoint. He also observes that loss will be
minimized by making the all the radial currents as nearly equal as
possible (again, because loss is I squared R), and by sharing that
current by more radials (again because loss is I squared R). Rudy
further observes that radial current can be unbalanced by variations in
soil conditions, including factors like variations in skin depth, and by
their electrical length. Like I said, it's REALLY GOOD reading, and it
all makes sense.
And thanks for the QSO on 10M.
73, Jim K9YC
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