Let's Make an OCFD

Today we will make an OCFD, also called a "Windom" (sic). This is not really a Windom, but that's what many call it. What this really is, is an Offset Center Fed Dipole, or OCFD.

So, why not just make a dipole? This IS a dipole, it is just not a center fed dipole. The problem with the center fed dipole is that it will only work on the fundamental frequency and odd numbered harmonics.  That is, it will work on F x 1, F x 3, F x 5, etc, but not F x 2, or F x 4. So, a coax fed 80 m dipole will not work 40 meters (F x 2) nor work 20 meters (F x 4). If you do get it to tune with a tuner there will be very high losses. The only place you get more than one band is with a 40 m dipole, which, with a tuner, will work 15 meters. ( 7 mhz x 3 = 21 mhz )

The reason, there is a very high impedance node right in the middle of the 80 m dipole for all even numbered harmonics. The OCFD gets around that problem by moving the feedpoint off center. The trick is to get it off that center node, and avoid nodes of other frequencies/bands.

Please refer to:  https://www.ar15.com/forums/outdoors/Centerfed-Dipole-vs-OCFD-Windom-sic-/22-690849/

The two sides of an OCFD are therefore unequal. One leg is longer than the other. Some think these two elements work two different frequencies, like a fan dipole. No, it is still a full length dipole and has the same efficiency as as a center fed dipole. That is, if a center fed dipole cut for 3.55 mhz is 132', an OCFD cut for 3.55 mhz is also 132'. So it is the same overall span.

Many divide the overall span 1/3 & 2/3, or a 33.3% feedpoint. This works well for a 40 m or 80 m size OCFD, but 15 meters will be unworkable. This is for the same reason that the even numbered harmonics won't work on a center fed dipole -- there is a node right on the feedpoint. 15 m is unworkable, but the SWR will be good and very even from band to band on all of the other ham HF bands.

We can fudge that a little by moving the feedpoint over a bit -- from a 33.33% feedpoint to a 36% feedpoint, regain use of 15 meters, and work all of the bands from 80 to 10 m, with the exception of 60 meters.

So, here we go.

First, you need to use a CURRENT balun, not a VOLTAGE balun. In most cases a 4:1 balun ratio should be used. Some use 5:1 or 6:1, but this is rarely necessary. 95% of the time a 4:1 balun is the right choice.

A balun IS needed. Right in the middle of a center fed dipole the impedance at more normal heights (not "free space") is about 50 ohms. The ends of a dipole, where no current can flow, have an impedance that approaches infinity. Would you not agree that somewhere in between the center and either end there will be found a feedpoint that is about 200 ohms? That point is at about 1/3 and 2/3 along the half wavelength wire. The 200 ohms of the offset feed point is transformed by the 4:1 balun to 50 ohms.

The favorite balun among experienced OCFD builders is the "Guanella type" current balun. This is actually a dual core design. The two cores are both 1:1 current baluns but connected in such a way that gives 4:1. This type of balun keeps RF off the coax shield and up in the antenna where it belongs. A favorite balun, and proven good performance, is the Balun Designs 4115ET. Yes, I know there are several others that are labeled OCFD, etc, but just buy the 4115ET anyway. Humor me. The ET means there are eye bolts for wire strain relief and hanging the balun.

Dual core!!! A balun with two cores stacked and wound as if one core is NOT a "dual core" balun. The Guanella type dual core balun has two toroidal cores with different windings on each toroid.

Use the dimensions given. Do not "trim" this antenna. Attempting to adjust tuning for one band will probably adversely affect one or more other bands. Use your tuner. Ignore any "works all bands without a tuner" advertising hype.

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You will need:

Balun - Balun Designs 4115ET or 4115OCF, or other good quality "Guanella" type balun. (Not the 4114OCF, which is two stacked toroids wound like a single core.)

UPDATE - The Balun Designs 4115ET and 4115OCF are the same balun.

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4 ea Split Bolts. These are to secure the ends. I had previously used tiny saddle type cable clamps, but another member here has convinced me that the copper split bolts are better. For one, they don't corrode and freeze up. Copper is non-inductive. Easy to use. These are the smallest size my local electric supply stocked.

2 ea Snap Hooks. Hardware store item. Like on a dog leash. Get the smallest size.

2 pr Anderson Powerpoles. These are needed only if this antenna is to be used for portable operations.  For permanent installation you may attach the wire ends directly to the balun's terminal lugs.

Wire -- for an 80 m OCFD (80-40-20-17-15-12-10 m), you will need about 140' of wire. For a 40 m OCFD (40-20-17-15-10 m) you will need 70'. Use 14 ga stranded insulated THHN or MTW. THHN tends to be stiffer, and coil and kink when handled. It is better suited to permanent installations. MTW (Machine Tool Wire) is more flexible, the insulation softer, relaxed, and the wire is much easier to handle.  Portable antennas are easier to handle when made with MTW.

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I like the plastic egg type end insulators from Univeral Radio. These are their C150 insulators.

http://www.universal-radio.com/catalog/antsup/4818.html

They are inexpensive, and when used for portable antennas, they don't chip when banged together like porcelain. I don't know if these get brittle in extreme cold up north in Wisconsin, Minnesota, Michigan, etc.

To measure length of wire measure out to the farthest reach of the wire in the insulator. Place a split bolt close to the insulator and cut off excess wire leaving about 1".

This "C150" insulator is actually manufactured by Parmak, "Baygard by Parmak" PM-522, and available in bags of 25 insulators.

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On the other end has a snap hook for strain relief. This is anchored with another split bolt.

For portable, leave a 4" tail sticking out. You can put an Anderson Powerpole here. On the balun, put another 4" pigtail and Powerpole. For permanent installation, just leave an 8" tail sticking out for now. Later it will be soldered to a ring terminal on the balun.

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Then when time to attach to the balun, clip the snap hook to eyebolt, and connect the two Powerpoles.

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The snap hook and Anderson Powerpole may be skipped if you are doing a permanent installation.  This was shown to provide you with a good way of fast assembly and disassembly for portable installations.  The same may be done with centerfed dipoles.

If the antenna is to be used for a permanent installation the Anderson Powerpoles need not be used. Just leave 8" of tail on the wire leg, and solder to the ring terminal lug on the balun.

For an 80 meter OCFD overall length will be 133'. This will tune the lowest band to 3.55 mhz. DO NOT try to tune the lowest band to 3.8 or 3.9 mhz for the phone portion. That will push all of the higher bands up out of the ham bands, making this antenna unusable for those higher bands.

Make one side 85', and the other side 48'. These measurements are from the insulator, as shown previously, to the split bolt on the snap hook, plus the 4" tail on the wire leg, and 4" pigtail coming from the balun. So, from the balun terminal to the insulator will be 85' on one side, and 48' on the other.

Note that both the short and long wires of this 40 m OCFD have gray Powerpole housings, and black is used on both sides of the balun. By using the same color Powerpole housing on both short and long 40 m wires I know those two wires are to be used together. I might use orange housings the 80 m pair of wires. In that way I don't mix up the short 48' wire from the 80 m version with the long 41' wire from the 40 m version.

(EDIT: Rick at Spiderbeam suggested this change in dimensions for the 40 m version.)

For a 40 m OCFD, no, it isn't half the dimensions above. The feedpoint is different. For this you will use 39'-10" on one side, 27'-2" on the other. This OCFD (an excellent Field Day or other portable use antenna) will work 40-20-17-15-10 meters.

You WILL need to use a tuner.

Enjoy your OCFD and Good DX!

Edited - related threads you may want to read.

Let's Make a Dipole

https://www.ar15.com/forums/outdoors/Let-s-Make-a-Dipole/22-690847/

Centerfed Dipole vs. OCFD

https://www.ar15.com/forums/outdoors/Centerfed-Dipole-vs-OCFD-Windom-sic-/22-690849/

If you tune to the middle of 80 m, that is 3.75 mhz, correct? Then the harmonics are

2 x 3.75 mhz = 7.5 mhz

4 x 3.75 mhz = 15.0 mhz

5 x 3.75 mhz = 18.75 mhz

6 x 3.75 mhz = 22.5 mhz

7 x 3.75 mhz = 26.25 mhz

8 x 3.75 mhz = 30 mhz

Whereas, if you had tuned low in the 80 m band, at 3.55 mhz

2 x 3.55 mhz = 7.10 mhz

4 x 3.55 mhz = 14.2 mhz

5 x 3.55 mhz = 17.75 mhz (still close enough to 17 m band to be low SWR)

6 x 3.55 mhz = 21.3 mhz

7 x 3.55 mhz = 24.85 mhz (close enough to 12 m to be low SWR)

8 x 3.55 mhz = 28.4 mhz

So, if tuning at 3.55 mhz all the upper bands are in good shape and if any tuner touchup needed, it is easily done.

That still leaves the 80/75 m band at about 2.5-3.0:1 around 3.9 mhz, and around 3.5:1 at 4.0 mhz. That's OK,
because if you must have high SWR, you want it as low freq as possible to minimize loss in coax.

Worst case, 100' of RG-8X, at 4.0 mhz with 4:1 SWR.

http://www.arrg.us/pages/Loss-Calc.htm

Inserting those numbers into this loss calculator, we get, with 100 watts out of the radio, 78 watts out the antenna,
which is only 1.1 db loss. Not bad at all, considering an S-Unit is 6 db. No one would ever hear this loss.

Even if we could tune the antenna on the 80 m band such that there was only 2:1 SWR at 4 mhz, the loss would
only be 0.7 db... a difference of 0.4 db... again, you'd never hear it.

Tune your OCFD to 3.55 mhz.

I have used a 66' overall length OCFD sometimes for special events. In the past it had the 22' & 44' (1/3 - 2/3 or 33%) split, but a few years ago I trimmed the 44' segment back to 41' and replaced the 22' piece with 25'. I did not simply splice on a few more feet the splice, soldered or not, will always be a weak spot. The 22' can be used to make a 15 m dipole.

I have also made an OCFD with 29% split, 39' & 93'. This one is still high on 15 meters, but not as bad as a 1/3 - 2/3 split. It has low SWR on 40, 20, and 17 meters, being between 1.4 and 1.6:1 SWR. 12 and 10 meters are around 1.7:1.

80 meters is a little high, 2.5 or so, and up in the 75 m range it is 3-4:1 SWR. But at that frequency coax loss is not bad at all and a tuner easily corrects that problem.

So, you still need a tuner, but only two wires and coax... not bad for a low visibility antenna.

It's great to see that article again, Jup. I built two of those 40m versions back when your posts first came out a couple years ago.  One had the 44/22/ split, and the other the 41/25 split.  One will tune on all bands except 17m using the tuner in the FTDX3000.  The other will tune all bands except 30m using the tuner in the radio.  I sometimes have one up in the air, sometimes the other, depending on what bands I am interested in at the time.  The other one stays in my go-kit antenna box for portable ops and field days.

I have no towers or tall trees, so my OCFD is only up about 20 feet in the air.  But it is my primary antenna and I have worked all over the world with it using the 100 watts from my transceiver.  Recently finished 5BWAS and 120 DX countries so far.  

I love the OCFD, and recommend this design for anyone that wants a relatively simple, low profile, but high performance antenna for multiband HF.

For a Go Kit, a 40m and up OCFD is just about ideal.  Covering 40 m regional mornings, afternoons,
and evening, and 20 m dx during mid day.  And other bands.  Only 66' long.  A nice, simple,
compact antenna.

Glad you are having such good results!

Jupiter,
  its threads like this,....from guys like you,.....that make this the best ham radio form on the web

Thanks, guys!

+1, and the dog says "Hey".  

Hey, BigDaddy!  Scritch the pooch behind the ears for me.

bump

Will crimping the ring terminal on the wire work as well as soldering?

Yes. However, you solder this connection to prevent water intrusion, since it'll be out in the elements.

Excellent first post by the way.

A proper crimp (like, really proper, with the correct dies for that lug size and everything) is as good, or better, than a solder joint.  I don't solder connections unless I have a really good reason to.

I'd solder them and use heat shrink and tape over that for weathering plus UV protection for the heat shrink.

ETA: Great article BTW. Somewhere I have 100ft of coax.

I built one of these 2 years ago when I got back on the air - 100 watts and I can work a lot of stations well. I have it installed as a sloper, high end about 70 feet, low end about 15.

I also made my 4:1 balun - it was a little bit of a challenge, but I am cheap and like to make things. I am sure it only cost me double what the recommended commercial versions are!

Thanks for the post back then Jup!

Lost access to the Photobucket pictures, but I had the whole thing in a folder in my ham antenna folder.  I copy/pasted all my
writing, and put the pictures back like it used to be.  It took some time, but got it back up as it was.

Thanks, guys, for all the nice comments.  Glad you find this useful.

Not OST'ing this for when an open weekend and access to my radio gear come together.
Nope.
Not at all.

Now that you mention it, my post about a fishing line dipole and tape measure 1/4 wave have gone fubar thanks to Photobucket. I've just started re-collecting those pics and putting them on imgur. I should go back and re-do those posts too.

bump

It's funny you bumped this because I was just referring back to this post
I have a 75' length of #14 super flex and a 4:1 balun just looking for a new home.
I was considering a New Carolina Windom
The New Carolina Windom By Len Carlson, K4IWL
but I don't have short lengths of coax for the vertical radiator or to make the choke lying around.
I'll cut at 25' and 41' and string up a simple OCFD and I can always modify it later.

Just built an OCF using these plans, but my legs were 45/90, works well on 80, barely tunes on 75, great on 40, 20, 17, 12, 10, and 6.

Very happy with it!

Hey Jup, thanks for posting this tutorial. I made an 80m ocfd version and it works great. Thanks again.

OCFD's are the wrong impedance on odd multiple harmonics.
Yes, due to the mathematical offset of 80m from the higher bands, you have to tune an OCFD to the bottom of the band to take advantage of the harmonic relationship.

When I built and tuned my first OCFD, I found it much easier on the analyzer to look at 40 meters, got that in tune and all the higher stuff lined up right, and just let 80 fall where it may. Got 40 pretty close and then would check 20 and use it for very fine tuning, then recheck 40. It was an interesting exercise.

If you build/tune/analyze an OCFD you really need an analyzer that can show you resistance and reactance separately, and that preferably you can field calibrate to nullify the effect of the feedline. It'll be an exercise in futility if you only look at SWR.

The real problem harmonic is 3 x Fundamental, as well as 6 x Fundamental, but 5x and 7x are not bad at all.

5 x 3.55 mHz is 17.75 mhz, and close enough to 17 meters that it works well on that band.

7 x 3.55 mhz is 24.85 mhz, and again, close enough to 12 meters.

6 x 3.55 mhz is 21.3 mhz, and while right in the middle of 15 meters, the feedpoint is right on a node.  If you
move the feedpoint off that 33.3% - 66.7% point, one way or the other, you may gain use of 15 meters, but
loose 17 m or 12 m.

So, the 29% feedpoint works well, and a tuner will take care of any mismatch.

Bump to the top for a great thread.

OCFDs are extra tricky because they are very sensitive to their surroundings, particularly to the permittivity of your ground (not electrical ground, but the dirt it lives over).

IMHO, the best way to tune an OCFD involves leaving a lot of extra wire in a tight, 6" loop at either end. Set your initial lengths to the book values, add/subtract (roll/unroll) evenly at both ends to get it tuned for the lowest band of operation. Then comes the hard part: see how the other bands are doing and roll one end/unroll the other end evenly to move the feed point back and forth to get them to a place where you want them. You will never get all the bands, just some of them. Even then, you will likely have to use a tuner on every band with an OCFD for optimum performance.