[bars] Construction of an 80M OCF antenna

[Paul Pellegrini]

I like OCF dipoles because they offer the ability to bemultiband installations which can be fed with a single piece of 50-ohmcoax.  I have a one-acre suburban lot, soI have room for an 80M OCF dipole.  Thehalf wavelength for 80M is about 126 feet, so you need a straight run of thatlength.  The objective of thisinstallation was to have as many resonant ham bands as possible with a singleantenna and a single coax feed line.  TheOCF is usually cut in a length ratio of 2 to 1. For 80 meters those lengths are 84 feet and 42 feet.  Here are some of the experimental results forinstallation of my OCF.

·      Make the dipole for the low end of the 80M band.  Cut for 3.6 MHz one gets resonances for 7.2Mhz,14.4MHz and 28.8MHz.  It also works on18.1MHz.  These bands can be workedwithout an antenna tuner.  Most otherbands can be used with an antenna tuner.

·      4:1 baluns are commercially available and aresuitable for a 200 ohm feed point.  Theimpedance of an OCF is about 150 ohms which indicates a 3:1 balun which is notgenerally available.

·      Using a 4:1 balun means you must find the 200ohm feed point on the 126 foot wire. Finding the correct lengths for the shorter and longer sections involvesexperimentation.  My final lengths were40 feet and 88 feet.

·      A current balun is required immediately afterthe 4:1 balun.  The OCF is not asymmetrical antenna, so there will be rf current trying to go down the shieldportion of the feed line.

If you want a good multipurpose antenna for the principalham bands this is a good starting point. If you only have 66 feet of distance on your property, dividing thenumbers shown above by 2 will give you a 40M OCF.

Thanks to Steve for starting this conversation.  His was an interesting and in-depth analysisof OCF dipoles.  It got me to thinkingabout my 80M OCF and how I needed to make the theory work at the K1VK qth.

73 de K1VK

  

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