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Re: Topband: Using 4 - 6 elevated radials in lieu of 120 buried wires

To: topband@contesting.com
Subject: Re: Topband: Using 4 - 6 elevated radials in lieu of 120 buried wires
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 4 Jan 2024 23:59:32 -0800
List-post: <mailto:topband@contesting.com>
On 1/4/2024 9:53 PM, Robin wrote:
Milt Jensen, N5IA (SK) constructed his original (circa 1990s) 160M station TX antenna based on a similar design I encouraged him use.  He built a 180 ft tower with an insulator at 50 ft,  Four elevated quarter wave radials, each  made of a box of four pieces of # 12.  This was on his "city" lot where buried radials were impossible

It worked very well,  We did not have the tools to make real field strength measurements to compare to a model, but on air performance was excellent

Some thoughts about that particular installation and why it worked well, based on my study of Rudy Severns' excellent work on the topic.

The earth is a big resistor, and thus a lossy place for return current to flow. Radials are intended to prevent the antenna's field, and its return current, from seeing the earth.

The closer they are to the earth, the greater the resistance that couples to them. The more there are of them, the more evenly current divides between them, and since P = I squared R, where R is the loss coupled from the earth, when the radials are close to the earth, the more radials we have, the less total power will be dissipated in their combination. THAT'S why more on-ground radials are important. That is, because power is related to I squared, but I divides by the number of radials, the total power lost is inversely proportional to the number of equal radials.

Current in radials close to the ground also is affected by variations in the conductivity under them, which caused Rudy to recommend as many elevated radials as practical. Also to keep the currents more equal, he recommended making them slightly shorter than resonant. And he observed that in radials longer than a quarter wave, current increases as we move away from the feedpoint, reaching a maxima a quarter wave from the end.

But because Milt's radials were so high, there would have been very little coupling to the earth, thus none of Rudy's concerns matter. AND the higher feedpoint which is the current maxima on a quarter-wave antenna, (and thus the higher max current point), it improves the vertical pattern.

BTW -- none of this thinking is new to me, it's what I learned from studying Rudy's work -- except that the light bulb that lit up for me was that power division concept as WHY "more is better" for on-ground radials.

Another thing I learned from N6BT, who we all know has worked extensively with verticals, is that to be reasonably efficient on topband they must be at least 20 ft above the ground.

73, Jim K9YC

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