For this year’s ARRL Field Day event I was looking to put up a homebrew antenna that was easy to construct‚ easy to deplay‚ and give me multi-band support. I settled on an 80 meter Off Center Fed Dipole antenna. Watch the video to see how I do it:
The OCF or Off Center Fed Dipole antenna is much like a conventional dipole antenna in that the formula you use to calculate the length of it is the same. That is: 468 / Frequency Mhz will give you the overall length of the antenna. For a conventional antenna I would normally split this wire in half and place a feed point in the center. Doing so will give you classic antenna with single band coverage.
The benefit of an Off Center Fed antenna is that if you move the feed point away from the center‚ you can find a spot that will allow the the antenna to resonate on multiple bands. So if we move our feedpoint so that one leg of the antenna is 1/3 long and the other is 2/3 long‚ the antenna will now resonate on 80‚ 40‚ 20‚ 10 and 6 meters usually without a tuner. Using an antenna tuner will give you the other bands: 30‚ 17‚ 15‚ 12 with not a lot of losses. Normally as you move the feed point of the antenna the impedance will change‚ so you will need to use a 4:1 current balun with this antenna.
Finally‚ if an 80 meter antenna is too long‚ as it is 133 feet in this example‚ you can use the same formula to build a 40 meter version which will only be about 67 feet long. You’ll forgo one band‚ but you’ll end up with an antenna that is easier to deploy.
Links/items mentioned in the video:
MFJ 913 4:1 balun: https://amzn.to/3hplfbn
Isotip butane soldering iron: https://amzn.to/3e1QWFJ
Rigexpert AA-600 antenna analyzer: https://amzn.to/3e2RoUa
Weaver Leather Throw Weight: https://amzn.to/2CigRek
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