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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