Posted: 3/6/2015 4:19:42 PM EDT
| You know how a fan dipole will switch to the proper elements on the antenna when you change bands, well my question is will a horizontal loop do the same thing . Say if you have an 80 meter loop with a smaller loop say a 40 meter loop inside of that loop? |
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If you have an 80 m loop you don't need a separate 40 m loop. The 80 m loop will do 40, 30, 20, 17... etc, all the harmonic
frequencies with some touchup from the tuner. You can feed it with coax to a 2:1 or 2.5:1 balun, or 450 ohm window line directly to the loop, coming back to your shack tuner. |
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Figure your "skywave loop" circumference with the formula
1005 / Freq in Mhz = Length in feet. Yes, this is a little longer than twice the halfwave dipole length, but that is due to end effects of the wave. So, 80 m is 1 wavelength, 40 m is 2 wavelengths, 30 m is just below 3 wavelengths (but that is what tuners are for), 20 m is 4 wavelengths, 17 m is 5 wavelengths (or close to it), 15 m is 6 wavelengths, 12 m is 7, 10 m is 8 wavelengths. Cut the wire for low in the 80 m band, 3.500 to 3.550 mhz. The most low loss way to feed a skywave loop is with 600 ohm ladder line or 450 ohm window line leading back to the shack. The two wires either tie directly to the open wire terminals on the back of the tuner, or to a 4:1 balun and then a short section of coax from balun to tuner. Alternatively, you can use a 2:1 or 2.5:1 (better) current balun at the loop's feedpoint, and 50 ohm coax back to the tuner. Such a loop tends to be in the 112 - 120 ohm range at resonance. Don't worry if you can't make it exactly square. Actually best would be round, but that is not practical. 3 sides, 4, 5 or more sides, and not all the same is fine. What you don't want is to have it long and very narrow, in which case it becomes a folded dipole. You may find these articles by Don Keith N4KC helpful. Don is also the author of a number of naval warfare books. www.donkeith.com http://www.donkeith.com/n4kc/article.php?p=22 http://www.donkeith.com/n4kc/article.php?p=12 http://www.donkeith.com/n4kc/article.php?p=13 http://www.donkeith.com/n4kc/article.php?p=32 http://www.eham.net/articles/16690 |