AWESOME 80-METER OCFD – 7 BANDS FROM THIS BIZARRELY MAGIC ANTENNA

A practical 7-band antenna designed by KK7MA

This has become my favorite antenna for camp trips and Field Day. Due to its offset design it can also be a great solution for getting the feed point where you need it on small lots within city limits.

80M Off Center Fed Dipole (OCFD)

Hung in a mostly horizontal fashion, this antenna tunes up throughout the 80M, 40M, 20M, 17M, 12M, 10M, & 6M bands using only the internal tuner of most modern transceivers.

To gain 15M, we have had limited success by lowering the legs closer to the ground for more of an inverted-V configuration, but this does degrade performance on the other 7 bands.

Using an external tuner to pull in 15M is not recommended for any length of time as it seems to overheat the balun resulting in a dead antenna for several hours if not longer.

If I felt the need to add 15M, I would instead hang an additional small dipole specifically for that.

This antenna is roughly 135′ long and fed at 33.44% with 86′ 5″ and 43′ 5″ legs.

Using a fishing pole, 3-oz weight, and braided fishing line, we typically hang these antennas between trees at 50′ – 100′ height

The 4:1 BalUn

Many designs can be found for the 4:1 balun. KK7MA based his design off one by OH1AYR whose site no longer seems to be working, but the PDF version is here.

This design is simply a pair of 1:1 Guanella Current Baluns (common mode chokes) connected together to form a 4:1.

On the antenna side, an opposite pair of wires are connected together to feed through the second choke, the remaining pair splits off to each leg of the off-center antenna.

This design has the benefit of choking the common mode current in addition to providing a great feed match on multiple bands.

Parts List

  • 1 – Junction Box 4.5″x4.5″x4.5″ from local hardware store
  • 3 – 2″ x 1/4″-20 Eye Bolt
  • 2 – 1″ x 1/4″-20 Bolt
  • 14 – x 1/4″-20 Nut
  • 6 – x 1/4″-20 Wing Nut
  • 2 – 1-1/2″ x 1/4″-20 all-thread
  • 2 – 240-31 (or 240-43) Toroid
  • 16′ – 14 AWG Magnet Wire
  • 1 – Teflon Tape Roll
  • 1 – SO-239 connector
  • 10 – 1/4″ 12 AWG Ring Terminal
  • 2 – Fence Insulator
  • 3 – Carabiner Clip
  • 135′ – 12 AWG THHN copper stranded wire

Build Steps

  1. Wrap toroids with a layer of teflon tape. (This step helps prevent the magnet wire coating from scratching off during the winding process)
  2. Cut 4 pieces of magnet wire long enough to wrap toroids in a 7 turn + 1 crossover + 7 turn manner. (For 14 AWG, 48″ leaves plenty of extra tail)
  3. Wrap the toroids with magnet wire as shown to create a pair of identical chokes.
  1. Stack the chokes so they are lined up identically on top of eachother.
  2. Add a thin insulating spacer so that the magnet wire of one toroid does not settle between the wires of the other.       ( I like to use a hole saw on the cheap $1 store cutting boards about 1/8″ thick)
  3. Identify and strip the coating from the two wires that make the connection between toroids on the antenna side. Strip back enough that you can twist & solder the two together close to the toroid. Be sure to get all of the coating off where soldering as it does not make a good conductor.
  4. Connect the balun wire pairs on the coax side (red-to-red, and blue-to blue)while leaving enough length to attach to the lid terminals. I leave 2-1/2 to 3″ for this. Connect the antenna side with the single blue from the bottom to the single red from the top. The remain red and blue wires will connect to each antenna leg.
  5. Add the four ring terminals to the stripped ends of the balun wires.
  1. Drill (4) 1/4″ holes in the corners, (4) 1/8″ holes in the sides, and one 9/16″ hole in the center of the lid.
  2. Install the SO-239 to the lid of the box.
  3. On one side of the lid, install the two pieces of all-thread with lock nuts/washers so that they are evenly spaced inside & out. With wing nuts, these will connect the antenna side of the balun to the antenna terminals.
  1. On the other side, using lock nuts/washers, install the two 1/4×20 1-inch bolts with the bolt heads on the outside of the lid. With wing nuts, these lugs will be used to connect the coax side of the balun to the SO-239 connector.
  2. Using ring connectors, install two short pieces of 12 AWG stranded through the 1/8″ holes to bypass the all-thread and connect the balun to antenna from inside to out.
  1. If using the face mount SO-239 connector run a short wire from the outside of the box to the inside shield terminal of the balun. If using the panel mount, simply run both wires from SO-239 the balun on the inside of the box. (Both types are shown here)
  2. Using wing nuts complete installation of the balun to the lid. Take care to match up the antenna side of the balun with the outside antenna terminals.
  1. Drill the remaining 1/4″ holes in the box body. Two on the sides near the natural bottom of the box, and one in the natural bottom. One for each leg, and one to hang the assembly from.
  2. Using lock nuts/washers, install the three eye bolts to the box body.
  3. Double check all of the connections, then screw the box lid down. The lid of the box is now the bottom. Your feed point is now complete!
  1. Cut the two antenna legs at 89′ and 46′
  2. On each leg tie a loop knot about 10″ in for your carabiner to hook to the eyebolts, leaving roughly an 8″ tail to tie into the antenna terminals.
  3. On the other ends tie on a fence insulator for hanging at 86.5′ and 43.5′, then wrap the remaining wire tightly back to itself.
  1. For best results, connect the longer leg to the center conductor terminal.
  2. Pack in an old soft tackle box.

Performance

Hanging at 50′

80m-75m
40m
20m
17m
12m
10m
6m

Comments

2 responses to “AWESOME 80-METER OCFD – 7 BANDS FROM THIS BIZARRELY MAGIC ANTENNA”

  1. that is a different antenna, I sent it to a friend in texas who is getting his station together and has some antenna restriction.

    1. Different for sure. I have seen many OCFD designs with big claims about performance, but none of them worked for me as advertised. This one does, regardless of where I hang it.
      I forgot to log the SWR charts when last measured, but next camp trip, I will be sure to log them then post back here.
      75M has the worst SWR at near 3:1, but the internal radio tuner seems to handle that nicely. Most of the other bands are 2:1 or better.