Let's Make an OCFD

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

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 but for a 40 m or 80 m
size OCFD 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 it over a bit -- from the 33.33% feedpoint to 36% feedpoint, regain
use of 15 meters, and work all of the bands from 80 to 10 m, with the exception of 60 meters.

Please take a look here:  

http://www.ar15.com/forums/t_10_22/674537_Center_Fed_Dipole_vs__Offset_Center_Fed_Dipole_OCFD_aka_Windom__sic_.html

So, here we go.

First, you need to use a CURRENT balun, not a VOLTAGE balun.  In most cases a 4:1 balun
ratio needs to 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 halfwave lengthwire.  The 200 ohms of
the offset feed point is transformed by a 4:1 balun to 50 ohms.

The favorite 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 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 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.  Adjusting one band will probably adversely affect
one or more other bands.  Use your tuner.  Ignore any "works all bands without a tuner" advertising hype.

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.



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 with MTW.

Watching.....

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".

UPDATE!!!  I had been getting these egg insulators from Universal Radio as per above.  This same
"C150" insulator is manufactured by Parmak, "Baygard by Parmak"  PM-522, and available in bags of 25 insulators from
KV Supply as "97104  Corner Insulators 00522 (pkg 25)" for $8.95 + $5.00 shipping.  That will last a while.

http://www.kvsupply.com/c-150-heavy-duty-corner-insulators-bag-of-25

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.



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



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.

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.

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 terminal lug on the balun.

(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!

Some men just want to watch the world burn.

Great post, great antenna!



I built the 40M version with Jup's instructions and it has worked great for me.  I've even gotten Australia and New Zealand with it just strung up over the house on my tiny suburban lot.  I've even got another one dedicated for "portable" use.



Seriously guys, it really is as easy as he says.  No need to buy a pre-built antenna.  And once you build your first, well, like the drug dealers say: "the first hit is free..."



Mods: Can we tag this in the "important threads" sticky?  I feel like a lot of new hams are missing out because they are intimidated by their first HF antenna, and this is a cheap and EASY one to build that will get you on the air on most bands in an afternoon.

You bet!  No more difficult than a center fed dipole.  Same length of wire, and using a 4:1 balun instead of a
dipole center insulator you get a lot more bands.  Takes the same space.

Compare to:

http://www.ar15.com/forums/t_10_22/677525_.html

I still have to build mine.  Have all the parts sitting here.  

Thanks for this and help in the past.

I love mine. However it is a bit more difficult to tune than the average dipole. They are very sensitive to ground conductivity and other surroundings. Tune overall length for resonance on 80M, usually around 3600 or maybe a bit lower, then adjust the feedpoint location until it works on the combination of bands you want. Somewhere between 25 and 35% it'll work, again depending on ground conductivity, etc. Best to leave a lot of excess wire at both ends for tuning until it all works.

I put a remote tuner within a couple of feet of the balun and I can get mine to work on 80, 40, 20 and 10 with no tuner, and 60, 30, 17 and 15 with the tuner. Couldn't make 12 happen. Performance is excellent on all bands except for 10.

I have a 160m OCF up and it is now my go to antenna.

Dead flat on the whole 20m band.

Tunable everywhere else except 40m.  Not sure what's wrong there.

Do it...I love this antenna.

Posted Via AR15.Com Mobile

hey my closet just barfed up one of those!





admittedly mine is not as pretty as yours, but it's hanging in the trees now hopefully for a few days at least before that paracord gives up the ghost.

What are the measurements of the short and long legs?

Uh... perhaps you should take a few minutes to review these videos.  

http://workshop.lifehacker.com/coil-your-extension-cords-like-a-roadie-with-the-over-u-1706890537

awesome! I'll be making the 40m one. Most of my stuff I ordered should arrive this week, but I still have another week (I'm guessing) for my callsign to get posted.

haha I actually did watch those when they were posted a couple weeks back and have since been teaching all my colleagues.  this antenna looked much better when I put it away two years ago, I swear!

Note to self:  Go to the dollar store and get some extension cord keepers...

Jup - what do you think of this balun for those of us with small back yards where antenna's will most likely be <20 ft away from  houses?

http://www.balundesigns.com/model-4114ocf-4-1-balun-1-5-54-mhz-for-ocf-dipole-5kw/

ETA: I just saw their note on power.  It seems like they suggest this one actually if you're using <700 watts.

http://www.balundesigns.com/model-4115ocf-4-1-for-ocf-dipoles-1-5-54-mhz-5kw/


ETA 2:  is the 4115ET and the 4115OCF basically the same balun?  clearly I got confused somewhere along the way.

The 4114 is not a Guanella type Balun.  It is a single core balun wound on stacked cores.

Use the 4115ET.



I don't know the difference between the 4115ET and 4115OCF.  The ET was the one recommended.

Noted - I will get that one when I'm ready to build something.  thanks

OK, got the answer... the 4115ET and 4115OCF both are the same.  The OCF suffix was to aid customers
looking specifically to find a balun for OCFDs.

I kind of figured that. I will order the ocf to be edgy and disobey. You're not my supervisor !

I have made 2 of these cut for 80m, tuned it to the middle of 80 with an antenna analyzer. i can tune with the internal tuner in my 480 sat all the bands except for 1 if i remember correctly.

Embarrassingly, I do not know for sure.

Here's the plot of it as it is now vs the G5RV.  OCF is the yellow and G5RV is Red.

Possible stupid question: Are those measurements from tip to clamp? Or from tip to balun connection? I'll be building my first antenna here in a few weeks to get on the air, so I'm getting everything together.

On a side note: From my many hours of research into antenna, that is by far the best way I have seen to attach to eye bolts for strain relief! Gotta make that a must!

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 assume by "tip" you mean the end of the wire at the end insulator.  If so, yes, from the end of the wire at the insulator to the balun connection.  

Ignore what is doubled back in the end insulator.  Ignore the little loop through the clamp and around the eyebolt.

You need not use the Anderson Powerpoles if this is going to be a permanent installation.  Just make the tail coming past the snap hook and clamp
8" long and solder the end to the terminal on the balun.

In the case of the club OCFD, we have a lot hung off the tower.  Up top on the rotator stinger is a Moseley 10-15-20 m Tribander, a 2m beam, and
a 2m vertical (IRLP).  There are arms coming horizontally from the tower with pulleys hoisting up the center insulator for the 75 m dipole, and another
to hoist the balun for the 80 m OCFD.

When certain tower work is done we must lower the 75 m dipole and OCFD out of the way.  The hoist line going to the OCFD has a snap hook on the
end that clips to the top of the balun.  We unclip and remove the two wire legs of the OCFD and roll them up out of the way, and store the balun in the
shack.  

To do this we have the Powerpoles for that final connection by the snap hooks.  Since we want reliability from these connections we squirt dielectric
grease into the Powerpoles before plugging them together.  Then we wrap with first Teflon tape (plumber's tape) and then with Scotch 130C butyl rubber
tape (like Coax Seal) to keep water out.  This works well, and we have had no problems with the Powerpole connections becoming intermittent.

What about using thimbles on the loops at the balun end. I realize you won't be able to get the split bolts as close, but it would eliminate the strain on the wire. Thinking two or more years down the road that looks like the weak point.

Even if someone didn't use the snaps and just used the eyebolts, I think the larger radius of the thimble will ease the strain instead of having single point contact rubbing.

Haven't had any problem with rubbing through on either end, even with 7-8 years of use.  I've built
quite a number of wire antennas, dipoles, fan dipoles, OCFDs, for myself, club, and friends.  Never
a problem.

Very interesting!  I just ordered the balun.  Looks like a great antenna.  Can't wait to build it and try it.

Good!

But let me stress one more time... just build it to dimensions given.  Do not "trim" or "tune".  That's what you
have a tuner for.  

If you trim for one band, you mess up one or two other bands.

The lowest band should be at the bottom end of the band.  That is, for the 80-40-20-17, etc meter version, the
lowest band should be resonant in the 3.500 - 3.550 mhz range.  The 40-20-etc version should have the 40 m
band in the 7.100 mhz range.  

This will have the lowest band the one furthest out, needing the most touch up from tuner, and having the lowest
coax loss due to SWR due to frequency.  And it will cause all of the higher bands to fall close to desired resonance.

Don't be tempted to tune the 80 m band to the middle of the band, or to 3.900 mhz.  This will screw up all of the
higher bands, I promise you.

If your 80m OCFD is low, say, 3.495 mhz and you want to get that up a tad, for every inch you trim from the short
end, trim 2" from the long end.  If you have it anywhere from 3.520 mhz to 3.580 mhz leave it alone!

And enjoy working lots of HF bands with one wire antenna.  

Jup

May I ask a (stupid) question?

Why pay $100 for the balun when there are a boatload of sites that show how to wind one of these ourselves and even source the torroids and wire?
Am I missing something?

Well, in my case, my time is more valuable than the money involved.  I would rather pay for the device than take the time to gather all the materials, tools, box, connectors, hardware, sealants, etc. and build it.  I have also found that it generally takes fabrication of a few items to get the "unknowns" in the fabrication process worked out.  I'd rather buy one from someone that has built one a few (hundred) times before, thanks.  

As far as building things go, I have been a ham, technician, and engineer for 50+ years.  I've built a few things.  Been there, done that.  Now, I'd rather play with the antenna.

Sometimes you can buy the complete antenna cheaper than you can build it for.

I saw a pretty stout looking Windom on eBay for about $60. Even with $10 shipping you may come out ahead if you consider everything. Gas for chasing stuff adds up pretty fast. So does shipping and postage.

No, not a stupid question at all.  

1 - 90% of the sites showing how to wind a Guanella type balun are WRONG.  Some work poorly, and some don't work at all.  This perhaps explains
inconsistent results among some builders.

2 - The Balun Designs baluns are wound with good toroids and teflon coated wire, which resists insulation melting at high temperatures.

This is a good balun and gives proven results.

Can you post a link to a site that shows the correct way to wind one?

This fellow, Rick DJ0IP / NJ0IP is quite the antenna expert, and a real "guru" on the subject.

http://www.dj0ip.de/   and    http://www.aerial-51.com/

Rick is currently the Customer Support Manager for Spiderbeam GmbH.  "I run the Online Shop,
mind the company's telephone, fax and Info-email, do sales support and tech suport - World
Wide, and when I find time, I also help develop and test new antennas."  

Specifically, if on the German site you click "Balun Stuff" at the top, then on the balun page,
click "Single-Core Vs. Dual-Core 4:1 Guanella Balun"  which brings you to:

http://www.dj0ip.de/balun-stuff/1-vs-2-core-baluns/

This page shows how to make a 4:1 Guanella balun correctly.

Rick has done extensive tests on baluns, the results of which are posted in the Windom Yahoo
Group.

On that page linked above he stated "THE BAD NEWS:  ALL of the companies manufacturing
OCFD antennas commercially are still using the single core 4:1 Guanella,"   Note that he said
"manufacturing OCFD antennas" not "manufacturing baluns".

Rick knows that Balun Designs produces a dual core balun made as he suggests is correct.

I have also communicated with Rick, and he has been most generous with his time with me and
my questions.

Also, read

http://www.dj0ip.de/balun-stuff/bad-baluns/

And he backs up his writing with links to other experts.

I'm just trying to make it easy on you guys... buy the Balun Designs 4115ET or 4115OCF and
don't worry about the details.  This works.  There are many posts on the Windom/OCFD group
to the effect, "I swapped out my (other brand) balun for the 4115ET and now my antenna works
just fine!"  

If you want to experiment and adjust for several months, that's OK.  But a new ham with a new
license and new radio wants to get on the air today, or at least, as soon as possible.  This antenna
gives a lot of bands for its size.  While not "all band" it is certainly multi band, and no more difficult
to make than a plain dipole.

Jup

Cancel

Inspected by #32

Hey Jup, do you know the guy from Balun Designs? Apparently he's local to me. Just heard a few guys on the repeater talking about his stuff with nothing but good things to say.

No, but hams I know that do know him have nothing but good to say about his baluns.

UPDATE!!! I had been getting the egg insulators from Universal Radio. This same "C150" insulator is manufactured
by Parmak, "Baygard by Parmak" PM-522, and available in bags of 25 insulators from KV Supply as "97104 Corner
Insulators 00522 (pkg 25)" for $8.95 + $5.00 shipping. That will last a while.

http://www.kvsupply.com/c-150-heavy-duty-corner-insulators-bag-of-25

Will a RadioWavz b14cf balun work ? It is a 1:4 farrite balun rated for 1500 watts ssb. Their website doesn't say if it is dual core or not but claims it is a guanella type.

Balun designs, I just bought a 9130 9:1 from them, that I'll be putting into service with an end fed on Saturday.    Looks pretty well built.    I didn't know it at the time but they are over from me 150 miles or so at a place called Alto NM.   As this area goes that's pretty local.

I was confused about their location.  They list Alto NM on the site, but it's a Denton, TX phone number.  not sure now.

Again, a balun with two toroids stacked and wound as if one big toroid is not a "dual core" balun.  

The Dual Core balun is a true current balun that will suppress RF on the shield.  This is important because an OCDF is inherently
unbalanced.

I have built, successfully, OCFDs with simpler 4:1 baluns but had to add a 1:1 choke below the balun, that is, a series of ferrite beads.

From the description at Radiowavz it sounds right.  At the most, if you are getting certain indications, such as SWR changing if you
touch the shield, or an SWR dip in the 75 meter range (when tuned properly down about 3.5 mhz) then add some snap on ferrites to
the feedline just below the balun.  I'd start with 10 beads.

Lots of folks here asked me why I put a 1:1 below my 4:1 on my OCF.

It's nice to know I didn't read that information wrong after all.

If I do it again, I will probably roll my own dual.

The Carolina Windom uses the vertical shield as a radiator to fill in the dipole's radiation pattern. Pretty clever, and mine seems to work well.

I recently had a belated AHA! (or maybe more like DOH!) moment regarding multi-core, bifilar wound, Guanella style current baluns.  

What finally dawned on me was that if I wanted a flat SWR response for the finished product, I needed to pay attention to the impedance of the transmission line pair used in the windings (i.e. 50 ohm pair for a single core ~ 100 ohm pairs for a dual core, ~150 ohm pairs for a three core). With that done, I could check myself while working on the individual core windings by using a proper load (resistor) for that particular transmission line pair impedance and doing a sweep on the analyzer. If the variation is small enough, changing spacing on the turns can sometimes be enough to tweak things to fit.  Once that finally settled in I found it a lot easier to get a product that would be at or below 1.3:1 SWR for large band ranges like 80m (or 160m) through 6m.

Don't know why it didn't sink in before...

Nick

I already have a 1:1 1113ET Current Choke from Balun Designs, now I have a 4115ET 4:1 on the way.
Just a couple of things to gather until it arrives.

A little digging on their website revealed a 4:1 designed for OCFD's that run close to a metal roof. It's like they looked in my back yard and built a balun for me. Got one on order, and if I get really motivated I can try it out side-by-side with my present Carolina Windom.

Also ordered an unun for portable endfed use.

4114ocf, and 9130 if anyone's interested. LOL that their QRP units are rated to 300 W. These guys are serious.