[TowerTalk] Fixed Quads

[L. B. Cebik]

On Sat, 29 Jan 2000 n4kg@juno.com wrote:
>
>Re:  Full  Wave Loop antenna elements   de N4KG
>
>Chapter 4 in the W2PV  Yagi Antenna Design book
>discusses loop antennas.
>
>Table 4.5, estimated gain and self resistance vs shape,
>is especially interesting.
>
>Shape   Sides    Gain     R (ohms)
>		  dBi
>
>Triangle   3	 2.70	105
>Square	    4	 2.99	120   (same as diamond)
>Pentagon   5	 3.10	126
>Hexagon    6	 3.16	129	
>Octagon    8	 3.22	132
>Circle	   oo	 3.28	135
>
Out of curiosity, I modeled free space loops of the main entries above to
see how well NEC-4 compared to Jim's estimates.  Everything is #12 copper
wire.  The remnant reactance can be pruned out of the table, but that
would not change anything significantly.  Everything is fed bottom center,
but that makes no difference in free space.
Shape       Perimeter in ft   F-S Gain dBi    R+/-jX
Triangle      145.2'            2.85          116.5 - j 4.8
Square        144.0'            3.18          125.7 - j 6.8
Diamond       144.3'            3.10          128.7 - j 2.7
Hexagon       143.4'            3.39          134.5 + j 5.0
Octagon       143.0'            3.45          137.4 + j 7.7

There will be a very slight difference in gain and impedance if you feed a
"shape" at the corner rather than mid-side, but that difference will not
make a performance difference.  

Note that for maximum gain with horizontal polarization, a single loop
should have the form of a vertically tall rectangle fed on the shorter
horizontal side.  The impedance will also come down closer to 50 Ohms.  On
the lower HF bands, the max gain shape and the 50-Ohm shape are not far
apart.  Free-space gain may reach the 4.0 to 4.5 dBi range for a single
loop.  Of course, structural feasibility at lower HF is always a concern.

For most purposes, then, Jim's table remains a reliable guide.

-73-

LB, W4RNL

L. B. Cebik, W4RNL         /\  /\     *   /  /    /    Tel: (423) 938-6335
1434 High Mesa Drive      /  \/  \/\     ----/\---    
Knoxville, Tennessee     /\   \   \ \   /  / || /     http://www.cebik.com
37938-4443     USA      /  \   \   \ \       ||      e-mail: cebik@utk.edu




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