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In Reply to: DIY FM antenna posted by cdb on December 9, 2006 at 09:53:18:
The diagram given in "DIY FM antenna" page is very misleading, doesn't show the proper construction of this antenna, and has incorrect dimensions for the full band (commercial) version (should be 30 inches square). Go to the original WEB page by Ethan Funk, the developer of this antenna at.
The antenna is made of 2 square loops, the larger one being 30 inches square, the smaller one 15 inches square nested in the lower right (or left) corner of the larger square, the whole comprising a single continuous strand of wire. One lead of the 300 ohm twinlead is connected to the bottom inner corner of the smaller loop, the other lead to the center of the bottom strand of the larger loop. If this is confusing, spend some time looking at the animated rotating 3D image of the antenna referenced in the above web page. I plan to build one, and see if it outperforms my Antennacraft FM-6 log/yagi.
Follow Ups:
Somehow the web page address didn't get published in my message -Go to www.thefsb.org/antenna.html
This gives correct dimensions and rotating 3D view of this difficult-to-portray antenna.
...
Dave
Later Gator,
Crank up your talking machine, grab a jar of your favorite "kick-back", sit down, relax, and let the good times roll.
Dale, thanks!The feedpoint connections are missing in the graphic on the link I provided.
3/8" copper tubing might be a good material; though for initial test, I'd go with heavy gauge bare copper wire. I suspect the brace frames might not be needed with #2AWG.You'ld need little insulating spacer sleeves where the wires cross. Solder a balun on at the feedpoints, and you be good.
Be aware that element diameter vs element length is critical per frequency & feed impedance. Construction with tubing vs wire will neccesitate shortening the length & make 300-ohm twin-lead not an ideal feed line. Consult an ARRL Antenna Manual for particulars & math formulae. And insulated wire was specified for construction because the overlaping loops must not short to each other. When constructed of non-insulated material, the air gap required between adjacent loops may alter the feed point impedance whereas 300-ohm twin-lead is no longer a good match, causing a loss of gain. The animated drawing is for clarity, it is not to scale.
Ron:I don't see a spec for any AWG on the 'insulated wire' in the article. Having a bit of familiarity with 'wire', this could be anything from #22 or less to 1000 kcmil.
Unless you have suggestions, the article description is vauge and really meaningless. There certainly is 'insulated wire' w/ a diameter above and below 3/8" tubing.
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