[TowerTalk] Sorry feedpoints

[Tom Rauch]

> 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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