QRP Club

QRP Station => Antennas => Topic started by: nx1p on July 04, 2014, 14:39:41 UTC

Title: LOCD (Linked Off Center Fed Dipole)
Post by: nx1p on July 04, 2014, 14:39:41 UTC
After being totally disappointed (extremely low efficiency) with buddipole I decided to research the more successful SOTA designs.  I discovered that the LD (Linked Dipole) was very popular in EU.  However, it was difficult to assemble in the field and required changing links on both sides of the dipole.  Although efficient, because of its 50 ohm input impedance, resonance was fairly sharp.  OCF (off center fed dipole) is slightly less efficient[80%] (but 5 times as efficient as a buddipole!) and broad banded. However, to make it multiband I had build a full size 80 meter OCF.   Enter the Linked Off Centerfed Dipole.  I began modeling for a more compact LOCF that could be used for 20, 17 and 10.  These are very popular SOTA bands (after all, you hike a mountain during the day!).  I now have a 3 band OCF with one set of links on the long side of the OCF.  A QRP 4 to 1 balun and a 1 to 1 choke balun at the center feed completes the design and I have a 1/2 wave on 20, full wave on 17 and 15 all with 250 to 300 ohm common feed point.  I have 80 to 90% efficiency compared to a pure dipole and I get less the 1.7 to 1 swr across the entire ham band (the advantage of a higher input impedance).  You can see the design on my blog page at www.nx1p.com (http://www.nx1p.com).

Have fun QRP!

Paula NX1P