I just finished a complete rebuild of my OCFD over the Christmas holiday. My first attempt was pretty lame and if were not for an antenna tuner it wouldn't have worked very well. The rebuild has got the thing working
much better
After quite a bit of study, here is what I did for an 80M OCFD:
Step 1--buy a really,
really good balun. The long answer:
http://static.dxengineering.com/pdf/Choosing%20the%20Correct%20Balun.pdf. The short answer:
http://www.dxengineering.com/parts/dxe-bal200h10at.
Step 2--ground the balun! That's what the ground lug is for! Honestly, it pains me to say I don't know why you need to do this, because normally I'm pretty smart about these things, but damn if I know!
All I can say is without the ground it's junk!
Step 3--add a good ferrite choke on the coax adjacent to the balun. 6 or 8 Type 31 cores.
Step 4--put up more wire than you need on each end, enough so that you can move the feedpoint between the 20% and 35% points. Just roll up the extra wire in a small, 6" diameter loop at the ends. It will not have any effect on performance. Hang the ends so that you can lower them from the ground for easy, repetitive length adjustment. It needs to be easy, since you'll be doing it a lot during the tuning phase
Since I use trees for my end supports I just use enough 550 cord to allow this to happen.
Step 5--throw away an preconceptions you have on how long or exactly what feedpoint you "should" have. There is too much variability associated with ground conductivity and whatnot for the "book" numbers to be anything but an approximate starting point.
Step 6--start with an arbitrary 30% feedpoint location and adjust the
overall length, not the feedpoint, to achieve 1:1 VSWR at around 3600KHz for an 80M OCFD.
Step 7--take VSWR readings on all of the other bands other than 60, 30 and 12, those are throw-away's. Then start adjusting the feedpoint along the antenna without changing the overall length until 40, 20, 17, 15 and 10 all come in at under 2.5:1. There is a way to do this with an antenna analyzer that will shorten the process, sadly I don't have an analyzer and just did it by brute force.
Step 8--use your tuner to knock things down from 2.5:1 to 1:1. I'm using a remote tuner immediately adjacent to the balun (my feedpoint is the peak of my house) and this allows me to run 60 and 30 meters, although tuning 60M is very touchy. 12M does not work at all, though.
Step 9--enjoy your multiband antenna!
I love mine and get the best signal reports of any wire antenna I've ever had, but I spent two entire days tweaking it. OCFD's are twitchy beasts. Of course like any long dipole it will generally work better on the lower freq's due to take off angles and all that. If you are brave, you can trim the excess wire now, but I left mine in case I ever want to re-tune it.