[bars] Construction of an 80M OCF antenna

[apizer at comcast.net]

I built the ocfd and balun from Mikes class and have been using it ever since. I run up to 500 w so I had to stack two cores to avoid overheating (thanks to Wally WA1LY for selling me his unused core from the class project). Back when Mike was still with us we used to have a sked whenever he went to Michigan and I seemed to strap everyone else out even without the amp so the antenna works. I used to have it as inverted vee with center at 60 feet. That branch broke so now it is only 45 or 50 feet at high point but still works great on 40 and 80. Works on 20 and 10 too but I have the hex beam for that (which N1IW helped put up along with N1HTS, WO1N, and K0TV(sk)).
Art
NF1A

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------ Original Message ------

From: apizer at comcast.net
To: W1hh, Paul Pellegrini
Sent: December 6, 2020 at 11:11 AM
Subject: Re: [bars] Construction of an 80M OCF antenna

Google n1iw ocfd presentation
He taught a class on this and also Balun construction at bars we built them as a class project
Sadly mike is now sk and and left us way to soon
RIP om mike

Art
Nf1a

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------ Original Message ------

From: Paul Pellegrini via bars
To: W1hh
Sent: December 6, 2020 at 10:33 AM
Subject: [bars] Construction of an 80M OCF antenna

I like OCF dipoles because they offer the ability to be multiband installations which can be fed with a single piece of 50-ohm coax. I have a one-acre suburban lot, so I have room for an 80M OCF dipole. The half wavelength for 80M is about 126 feet, so you need a straight run of that length. The objective of this installation was to have as many resonant ham bands as possible with a single antenna and a single coax feed line. The OCF 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 for installation 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 on 18.1MHz. These bands can be worked without an antenna tuner. Most other bands can be used with an antenna tuner.

· 4:1 baluns are commercially available and are suitable for a 200 ohm feed point. The impedance of an OCF is about 150 ohms which indicates a 3:1 balun which is not generally available.

· Using a 4:1 balun means you must find the 200 ohm feed point on the 126 foot wire. Finding the correct lengths for the shorter and longer sections involves experimentation. My final lengths were 40 feet and 88 feet.

· A current balun is required immediately after the 4:1 balun. The OCF is not a symmetrical antenna, so there will be rf current trying to go down the shield portion of the feed line.

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

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

73 de K1VK

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