To: <towertalk@contesting.com>
> Date: Mon, 11 May 1998 16:01:58 -0700 (PDT)
> From: Ron Youvan <ka4inm@gte.net>
Hi Ron,
> Parasitic elements near a driven element, lowers the feedpoint impedance
> of the driven element, it also compresses the baloon and provides gain.
> INCREASING the feedpoint impedence of a driven element (like it being a
> loop)
> lowers the current (at the same power level, increasing the voltage)
>reduces the IR losses, increasing the `efficency'
The loop simply splits the current into two areas of half the
level, the sum of which is nearly identical to driven element
current in a yagi of the same pattern and spacing. The sum of
currents in each half of the quad driven element (with the same
pattern and spacing) is almost exactly identical to current in the
yagi driven element, both driven elements have the same total
current.
The quad feedpoint connects in series with only one current
area. Since the feedpoint sees half the current, and power is the
same, voltage must be double or Mr. Watt will rise from the grave and
haunt us. That means impedance at the feedpoint is four times higher.
Isn't it amazing. Two very different antennas have the same
integrated total of current when radiating the same power with the
same approximate directivity.
Magic with better efficiency?? Not if you consider this.
If we take the extra wire used in the quad, and instead of throwing
the extra copper away we parallel the wires and make the yagi
element with twice as much surface area by using both wires in
parallel with small spacing. The resistance of the yagi element is
now about half that of the quad. The total loss is now equal to the
quad, and we used the same amount of copper.
So here's the rule.........
If both antennas have equal surface areas of the same material, the
loss is the same. In other words if you sliced the yagi's element in
half, and turned the same tubing halves into a quad, loss would
be identical.
If you took the quad conductor, and pulled out the sides and
compressed the upper and lower current areas together until a yagi
was made out of the same wire used in the quad, loss would be
about the same.
It's only when you get to use twice the surface area of identical
material in either antenna that loss becomes less in that antenna.
Here's a suggestion. Find the resistance of a unit length of
typical quad wire (at RF, not dc), and compare that to the resistance
of a unit length of the typically very fat smooth yagi material. I'll
bet the yagi, with many times larger surface area of aluminum
tubing at the highest current area, has much less integrated power
loss than the quad's thin wire.
Feedlines are a more complex issue, but impedance alone again does
not mean lowest loss. It is quite possible to have a 50 ohm line with
less loss than a 600 ohm line.
Looking at impedance and/ or current alone, when comparing
very dissimilar systems, will often give the wrong answer.
73, Tom W8JI
w8ji.tom@MCIONE.com
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