Topband: Confusion in ON4UN's Low Band DXing radial length calculations.

[Jim Brown]

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

http://k9yc.com/160MPacificon.pdf

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