[bars] Construction of an 80M OCF antenna

[Steven Schultz]

Thanks for replies.

Something I wanted to mention in my discussion but left it omitted was:

If a 200 ohm feed point impedance is targeted, then it seems a single 1:4
current balun could be used, instead of the method that MFJ uses and the
method I saw employed in one of ARRL Antenna Books where two devices are
employed in series, the first being a 1:1 current balun, followed by a 1:n
or 1:4 transformer.

It has been a while since I studied Sevick's work, but baluns with ratios
between 1:1 and 1:4 are discussed in his work.  I can try to locate an
example.

The simple, easy to build baluns will have turns ratios of 1:n where n is
an integer, 1, 2, 3, ... .  Thus you see commercially available baluns with
impedance ratios of 1:1, 1:4, 1:9 which are: 50 ohm : 50 ohm, 50 ohm : 200
ohm, 50 ohm : 450 ohm.

Paul, K1VK, talks about a feed point impedance of 150 ohms and as he said
this is a ratio of 1:3.  I will try to locate an example of a balun which
is in that range.  I think these are not commercially available, and this
is why hams live with the 1:4 balun.  A question I have is: If a 1:4 balun
is used, is it best to find the 200 ohm feed point on the dipole, rather
than the customary 1/3, 2/3 split?

In my second example in my discussion, the impedance transformation is 50
ohm to 139 ohm, which is a ratio of 1:2.8.
I am wondering if the method which employs two devices in series is favored
by comercial antenna manufacturers for some reason.  Maybe the reason is
that it is easier from a manufacturing standpoint to do it this way, rather
than manufacturing a 1:n balun, where n is not an integer.  You see this in
the third example I gave in my discussion, the OCF vertical dipoles by
Cushcraft and Hy-Gain.

73's
Steve
AC1EX



On Sun, Dec 6, 2020 at 11:11 AM <apizer at comcast.net> wrote:

> 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
>
>
>
> Sent from Xfinity Connect App
>
>
>
> ------ 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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