KB9VBR Antennas

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Building an Off Center Fed (OCF) Dipole Antenna

Posted: 07 Jul 2020 11:12 AM PDT
https://www.jpole-antenna.com/2020/0...dipole-antenna



For this years 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. Youll forgo one band‚ but youll 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




Support Ham Radio Q&A by shopping at Amazon: http://amzn.to/2kO6LH7





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