Topband: Flag - Mixed diameter conductors
n6ry at arrl.net
Thu Oct 25 18:57:59 EDT 2007
At 09:33 AM 2007-10-10, Tony, K1KP barockteer at aol.com wrote:
>I'm thinking about putting up a flag antenna for 160 and 80 meters. I
>had an idea that I might build the vertical sections out of some 3/4 inch
>copper pipe I have lying around. The horizontal wires would be 14 ga.
>copperweld. I modeled this with 4nec2 and find that the increased
>diameter of the vertical sections results in a reduced feedpoint and
>termination impedance, down around 623 ohms, give or take. (This is for
>the bottom wire only 2 feet above ground).
Go for it! I need to put one of these up, too.
I modeled this in EZNEC+ 5.0.10 with the classic 29' by 14' flag
dimensions mounted 2 feet above "average" "real/high accuracy"
ground. This showed the terminations for zero reactance at the
feedpoint were 701 ohms at 1.8 MHz and 717 ohms at 3.6 MHz. (Using
bare #14 for all wires, the loads were 960 and 964 ohms.) The gain
is about 1 dB higher when using the copper pipe sections, although
that isn't really significant.
My model has 15 segments for the vertical wires and 30 segments for
the horizontal wires. I used 0.875 inches for the O.D. of the copper
pipe, which is what my "3/4 inch" copper pipe measures.
The pattern looks fine on both bands with a deep null at about 35
degree elevation off the back. The lower feedpoint impedance should
help reduce the effect of inter-winding capacitance in the matching
NB: NEC-2 based models may be somewhat inaccurate when mixing
conductor diameters, especially when they meet at an angle, so it's
possible that the termination resistance may need a little tweaking
for best F/B. See "Stepped Diameter" in the EZNEC User Manual (help file).
73, Terry N6RY
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