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Old July 31st 05, 03:42 AM
Chuck Olson
 
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Default 40 meters added to 5-band Yagi without increasing size or weight

If you haven't read about the "SPIDERBEAM" yet in QST for July 2005, you
might want to check out a copy from the library, or join the "spiderbeam"
Yahoo! group and read about the latest developments. The kit comes out of
Germany www.spiderbeam.net , and is sold in the U.S. by www.thewireman.com .

So what's this about adding 40 meters? Let's take the basic 3-band
Spiderbeam - - 3 elements on 20m, 3 elements on 15m and 4 elements on
10m - - no traps - - nice, huh? It turns out since everything is made of
wire strung between fiberglass poles, we can connect the center of the 20m
director to the center of the 20m reflector with a wire about 10m long and
the overall result is a dipole with V-shaped capacitive hats at each end,
resonant at about 6500 KHz. At 7150 KHz, the dipole has a measured inductive
reactance of about 171 ohms, courtesy of my MFJ-260 Antenna Analyzer, and it
would need about 130pf in series to resonate it. A balanced feed would
require two 260pf capacitors. The manufacturer who discovered this resonance
felt it best to connect the dipole to the center support plate to ground the
center and feed the dipole with a "Gamma Match" configuration to control the
feeding resistance at resonance. Further investigation kept the center
ground but changed the feed to a balanced "Delta Match" to be compatible
with the balanced feed to all the other band antennas. So that's it - - 40
meter dipole - - no increase in size, only by adding a total of 14 meter of
bare wire - - one 50 ohm feed through a current balun - - 4 bands.

The Spiderbeam is also available in a 5 band kit adding 2 elements on 17m
and 2 elements on 12m, which becomes a 6 band antenna with the 40m dipole.
And the required 250pf 15KV capacitors (for full power operation) can be
mounted in the weatherproof balun box, with two added terminals for the
Delta Match wires. I found 6KV capacitors locally which seem to work okay
with my rig's 200 watt maximum output.

Okay, so the added wire adds a little weight, but it's small. Enjoy!

73, Chuck W6PKP


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