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Re: Topband: Confusion in ON4UN's Low Band DXing radial length calculati

Subject: Re: Topband: Confusion in ON4UN's Low Band DXing radial length calculations.
From: Jim Brown <>
Date: Thu, 18 Dec 2014 22:25:14 -0800
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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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