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Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44

Michael Sanchez (N2UJN) on March 20, 2017
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Building a 6 (Amateur) Band, 2.5 kw, OCF Dipole for $44


Michael Sanchez, N2UJN

Genesis:  Participate in the K2QZR 80m Net

As is common, several “elmers” factor into my entry into the realm of Amateur Radio in 2012.  K2QZR (now K2ZX) factors fairly prominently in my entry into HF Amateur Radio.  It was at that 2m net that I heard K2FX advertise his FT-1000MP; a wonderful and fortunate purchase that led me into HF.

Then, from Aug 2013 until June 01 of 2014, I listened intermittently to the venerable K2QZR 80m net on Wednesday evenings, but could not join the net due to lack of 80m band access via antenna.   So, as warm weather onset, I resolved to stop just listening to the 80m net and do something to participate in Joe’s excellent 80m net. 

What Antenna?

Clearly, I could build a dipole and string it on the same pole as my 40m dipole.  However, my yard would not support 66’ in two equal directions…it is significantly too short on the east side.    

The “Buckmaster OCF?”

By random chance I ran across a reference to the “Buckmaster OCF (Off Center Fed Dipole)”.

The “Buckmaster OCF” claims to give coax fed access to 6 Bands, including 80m….my desired target.  This seemed like a dream antenna yes.  For me, if I achieved only two bands:  80m and 40m, I would be ahead….assuming it worked on those two bands. 

However, the band support is only one aspect of what caught my eyeThe Buckmaster OCF has a leg length of 90’/45’, ideal for my backyard + side yard combination.   See Figure 1.

I saw the dimensions on the website, and, immediately went outside to measure from my already installed Rohn push up mast to the tree on the east and then to the edge of my side yard on west where another tree lines up.   A geometry almost perfect for an OCF.  90’ on the west side with 8’ to spare.    45 feet on the tree for the east side with 8 feet to spare.  See Figure 1.  Here was an antenna, in terms of leg length, designed exactly for my backyard+sideyard.

Figure 1:  Google Earth View of Backyard with OCF 90’/45’ Design.  Perfect fit. 


The Price – Maybe I Should Build It!

I read more about the Buckmaster.   eHam reviews were universally positive.

Then I read the price.  $299!   (now $330).

So, this is an expensive antenna.  

I ran across a second reference to an OCF which utilized a purchased 4:1 Comtek Jerry Sevick W2FMI from DX Engineering ($60 at the time of reference build).   Total cost around $100.

This still seemed expensive and I could not find exact design details of balun on web. 

4:1 Current Balun for OCF at 35’

The two preceding references prompted an email to Joe, K2ZX. 

“Have you ever heard of an OCF?”. 

Joe’s prompt email return clarified quite a bit à [Yes, he said, I have tried a couple of OCF.  OCF are always fed, as far as I know, with coax.  And, if installed at 65’, need 6:1 Balun, but, if installed at 35’, need 4:1.  ] (thanks Joe).

From Joe’s note, I now knew I needed a 4:1 Balun.  My Rohn mast only goes to 34’.  But, what, precisely was a “Balun”, what do they do?

Importantly, how to build one??  Initially, I went off to learn about them.  I am busy, and, sometimes build, sometimes buy, mostly depending on how much I might learn.  

4:1 Current Balun Design – Learning First

W8JI article

A google search returned many, many links of which three were worth reading, and, re-reading.   The first is an article from W8JI.

In this article, two significant and important learning’s are available after multiple readings to sort out the root message.   Here are two messages from that article relevant to OCF dipole:

a)       A good 4:1 Current Balun is both an impedance transformer (50 ohm to 200 ohm), AND, a common mode choke (prevents current on the coax shield back into the house).

b)       In order to be a completely effective common mode choke for BOTH legs of the OCF dipole, a dual toroid design is absolutely required.  Since no two halves of a dipole, especially an OCF, have identical common mode currents, each must have a dedicated toroidal transformer/choke.

N1IW (sk) 4:1 Dual Toroid Current Balun Construction Information

With the above message from W8JI, I proceeded to locate one of the all-time best powerpoint slides in the history of power point on the web by N1IW (sk).

In this very photographically detailed Balun/Unun construction document, one can, after several readings, figure out the following:

To build a good, dual core, 2.5kw, 4:1, Current Balun, one needs:

a)        Two Amidon  FT-240-61 toroidal cores.

b)        Some 14 or 12 gauge wire.  (my personal favorite: Home Depot THHN 14 gauge solid)

c)         Some glass tape (also sold at Amidon).

d)        Some Teflon tubing for wrapping wire (Debbie Supply). 

e)         $24 + $10 Shipping.     

Paradoxically, N1IW construction steps do not include clear wiring method for the dual toroidal transformer after winding is complete.    Nor does the document indicate the best way to test the wiring to make sure it is functioning before spending time putting it in the air.

4:1 Dual Toroid Wiring for Transformer Operation

Recognizing that I did not know how to translate the electrical diagram for what is called the Guanella 4:1 transformer to actual post constructed dual toroid wiring, I found the above reference which is fairly clear.  As we shall see, fairly clear is not crystal clear.

Figure 2:   Wiring a Guanella Dual Core Transformer



The Choice To Build the OCF

Learning about the 4:1 Dual Toroidal Core Transformer/Balun/Choke was interesting, and, building one costs about 10X less than the antenna from Buckmaster and building the balun is ½ price of buying and I would learn how to do it.  Plus, I had purchased 500’ of 14 Gauge THHN solid copper wire from Home Depot ($48) the previous August and, for my 40m Dipole, that wire had survived the winter looking new.  I could just extend that to the wire lengths of the 90’/45’ OCF’ with some of the additional THHN.    No additional cost.

The main challenge would be building the Balun, but, the work seemed reasonable.

So, I decided to order two toroids and glass tape from Amidon….$34 including shipping.

One other factor affected my choice to build.   In my limited experience with antenna systems, I understand that the performance of the antenna is local.  By that I mean, what Buckmaster was able to get working, may, or may not work at my location. 

So, because the entire OCF experiment might fail or sub-perform, I preferred to spend $34 finding that out rather than $299.

Building the 4:1 Guanella Balun (Transformer/Common Mode Choke).


I followed the steps and procedure shown in the N1IW after studying the information closely enough to know that the dual toroid built was indeed a 4:1 Current Balun.  

Step 1:  Wrap Each Toroid in Glass Tape

Wrapping the metal toroid in glass tape prevents the sharp edges of the toroid from piercing the insulation or tubing around the wire.  I studied the pictures, and, followed along….making my own pictures to document so, if I need to build again, I have a reference.

Figure 3:  Glass Taping the Toroids


Step 2: Strip the THHN insulation

After some experimentation with stripping the THHN wire I found a rapid, safe method using a razor blade.  I had to strip 4 lengths of about 3.5 feet or so (did not measure).  Used a string to get the length by pre-wrapping 8 winds on the toroid, then, cut the wire longer than string for ends.  

Step 3:  Route the bare wire through clear Vyton tubing.

From Debbie Supply, a great local store, I garnered clear Vyton tubing to flexibly insulate the wire from the toroid and from each of the windings.   Vyton is not as temperature insensitive as Teflon, but, it was available and cheap and I don’t have an amp.   I chose an inner diameter that resulted in wire not getting stuck due to friction half way through by taking the cut wire with me and trying successive diameter tubing until I found the right one.

Step 4:  Bifular wrap the toroid.

A dual wire winding is referred to as a Bifular wrap.   Fortunately, I used soft, solid, copper wire because I had to do it a few times to get the ends even on each end (input/output).

Figure 4:  Winding the toroid.

Step 5: Wiring and Testing the Transformer

This step was confusing for me.   I had a hard time translating the electrical diagram to actual hookup, but, found another reference for wiring the two toroids into one input/output transformer.   To insure proper transformer operation after wiring, I hooked two 100 ohm transistors from Radio Shack to represent the “antenna”.  Ideally, this would be transformed to 50 ohm between the feed and ground input to the transformer.   The two 100 ohm resistors were actually 193 ohm, and, also, suffered capacitive and inductive reactance at frequencies higher than 1.8 MHz and even some at 1.8 MHz.  But, at that frequency, the transformer worked nearly exactly as a 4:1 transformer.  So, the wiring was correct!  For now anyway. 

Figure 5: Testing the Wiring and the Transformer for 4:1

Step 6: Finalizing the Toroidal Configuration and Wiring for Install.

The first time I completely wired up the toroids for install in the home built housing, I marked each wire, then, folded the toroids over and I was too confident and soldered all connections before testing the system with the 200 ohm “antenna”.  When I did test, the SWR was infinite.   This was a couple hours of careful work.  

At that point, honestly, I almost punted the project.

But, my wife and kids were at the local Greek festival, so, I did have some more time.   I pulled the entire assembly apart, and, when re-wiring the second time, tested at every single step.   Also, when I finished, I did NOT solder all connections.   I just looped the soft, solid wire, and, “crimped” the wires together.  Works fine and easy to disassemble if need be.

Figure 6: Final Toroidal Configuration Ready for Install in Housing.



Installing the Balun in Custom Built PVC Housing.

I had a 3 inch piece of PVC pipe leftover from a toilet exhaust repair some years back.   I went to Lowes and found 3” PVC caps, and, bolts and already had an SO-239 connector…..just needed small bolts for securing.

Once the housing was built (to be shown), I installed the top wiring first (see Figure 7).  Then, at the bottom, I routed a wire through one of the SO-239 holes to the external frame of the SO-239 for ground and soldered.   Additionally, I soldered the center feed with a solid piece of THHN 14 emanating from the connector.   These were then connected to a “spring” like winding to enable me to connect the bottom of the housing to the Balun, now inside the main part of the housing, and, compress the wiring into the bottom of the housing.  


Figure 7: Installing the wiring leading to the antenna terminals. 


I used the nuts and two wraps of the THHN around the bolt, and tightened hard.  No solder. 

Final Balun Installed in Housing, Ready for Waterproofing and Up in the Air!

Testing the wiring at each and every step is critical to ensure that, upon hoisting into the air, it actually works.    This is the last step except for water proofing, and, still transforming 193 ohms to 49.6 ohms….good to go.    Note, I could reduce weight by cutting the caps shown below.   But, only thought about it at the end of the day.    So, this 2.5 kw balun beast weighs 2.4 lbs (toroids weigh a bit).   I only use 100W….so…..a little overdesign.  J

Figure 8:  Completed Balun Ready to Put in the Air.

OCF SWR Antenna Performance at 30’ Center, 8’ Ends.

I added wire to my 40m Dipole to make up exactly 90’ and 45’ on each leg.   Then, I connected the balun to the wire, and, hoisted the assembly into the air for testing with the RigExpert Antenna Analyzer.   Honestly, I literally ran to the basement to hook up the coax and run a scan from 1.8 Mhz to 29 Mhz.

The results were striking, but, showed resonant frequencies, on all bands, too low (SWR minima below each band).

So, I returned outdoors and cut 2’ off of each end of the wire, and redid the measurement.  Results are below, and, frankly, for me, impressive. 

Below, in Figure 9, from the SWR data, and counting 17m at SWR of 3.0:1, there are 7 usable bands.

Note, my total time invested in learning and doing was around 18 hours of research, building, testing, and confirming.    This includes building the balun, the housing and hoisting and testing and waterproofing when done.   Note, if I had not wired incorrectly the first time then the total time would be around 16 hours of investment.  So, cheap in dollars ($50), but, not in time.  The trade is learning.    Note, in the region of 12m to 10m I do think that, perhaps, the nearby location of the vertical matching section influences the SWR.  See Figure 10.  


Figure 9: RigExpert AA-54 Scan 3.0 MHz to 29.0 Mhz …. 6 Amateur Bands.


View of the Balun Implementation and OCF Installed

I use a Rohn 9H50 push up mast to support my Solarcon Max 2000 and, added the Off Center Fed Dipole (OCF) to the base of the Solarcon.  This replaced the 40m dipole.

Note, at age 54, the push up mast keeps me motivated to work out.  Pushing up the last leg is not much different than farm work.   J

Figure 10:  The OCF Installed and On Air.



I installed the OCF in the Spring of 2014.  Now, almost 3 years later I have hundreds of contacts on 80m, including a recent one from Scotland at 4am.   I enjoy 80m and leverage it during contests.   The SWR moves a tiny bit when it snows, but, still stays in the band just fine.

I plan to publish a simulation showing a model of the OCF and comparing that model to the SWR here and, also, defining the very, very complex 3D propagation pattern…which prompted modifications to enhance 40m. 


Member Comments:
This article has expired. No more comments may be added.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by K0UA on March 20, 2017 Mail this to a friend!
A very nice detailed write up. I like it.

I have been using an 80 meter sized OCF for several years now up 35 feet. Mine also works quite well on 6 meters, but does not work well on 17m.

Again thanks for the article.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by K9JCS on March 20, 2017 Mail this to a friend!
I believe this is the finest article I have read on EHAM since I started visiting this site about 12 years ago. I want to build this antenna so badly, but don't have the real estate available to make it work. I might try a 40 meter version, though.

It is interesting, clearly written, logically arranged and fully supported with photos and a graph. Sometimes I have trouble following some of the articles written by others. I'm not criticizing other authors. Any one who takes the time to write an article for EHAM has my appreciation. They all add to my enjoyment of our hobby.

Any one wanting to write a future article should read this one and consider using it as a template.

Thanks for an interesting and easy to understand article.

Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by WB4M on March 20, 2017 Mail this to a friend!
I read this with interest as I constructed an OCF about 3 years ago also. Mine covers 80-10 meters, except for 30, and even the low end of 6 meters! I was able to get the feed point to about 70 feet, with both ends sloping down to about 30-35 feet. I have great SWR readings on all bands; it has been a great performing antenna.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N4DBC on March 20, 2017 Mail this to a friend!
Great Article! However, you didn't factor in the cost of the antenna analyzer (which ain't cheap). Perhaps the antenna could be built without using an analyzer, but you obviously did use one. Even if you didn't purchase it just for this antenna, many hams can't afford one.
I don't mean to be overly negative, but I see so many articles purporting to "do something cheap", but are really not once you look into the details. 73, Dave N4DBC

Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N8ME on March 20, 2017 Mail this to a friend!
Why did you strip the insulation and then use the Vyton tubing? Could you have just left the insulation on that parts of the wires that needed insulation?
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2EY on March 20, 2017 Mail this to a friend!
A question and a comment:


What is the 2:1 SWR bandwidth on the various bands? IOW, how much of each band has SWR of 2:1 or less?


It's spelled "bifilar".


Great article! Thanks!

73 de Jim, N2EY
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

Thank you for the kind feedback.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

Thank you for the good feedback. It was fun to do and write. Am glad eham supports publication.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

Thanks for the input on the instrument cost.

Correct, I purchased the RigExpert AA-54 and did not include that in the cost of the antenna.

So, although I have many local ham ops from whom I could have borrowed one for free, I will factor in this cost just for curiosity:

At the time I bought the AA-54, I paid $180 for it. So, when I add that to the $44 I get a $224 antenna, still, about 30% below the purchase price for a commercially built one.

And, now, I have a good instrument and can build antenna combos.

However, let's say I could not have afforded the instrument....there are at least 4 I could borrow within about a 6 mile radius.

Thanks for the input again.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by W7VAQ on March 20, 2017 Mail this to a friend!
Have you tried a NVIS Antenna?
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

Good question, why strip the wire just to enclose it back into vyton?

Well, my original intention was to strip the wire and go buy teflon tubing which is much higher temperature rated than just the THHN wire insulation. But, when I arrived at Debbie Supply, they did not carry Teflon tubing but had Vyton. Which, is probably about as temp resistant as the housing wiring insulation. So, answer, for low power operation I did not have to strip the wire. I now have high temp insulated wire ready for my next build.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

For 80m, when the SWR minima is in the center of the band, and, with snow the minima is not centered, almost the entire 80m band is within the SWR 1:2.0 bandwidth. I have measured high resolution responses for each band 80m, 40m, and 20m and will find them and send via email. Overall, response is better than I thought it would be by far, and, a lot of time, when the minima are centered, I don't need a tuner at all for those 3 bands.

However, when there is a foot of snow, the minima shifts down (I tuned to the center of the band on a not snowy day)....and...the SWR can go to 2.7 on the upper part of the phone band.

I am using RG-8X coax, about 60 feet, from dxengineering. The losses through the Guanella transform "should" be small. But, I don't have a way to measure those losses (well, I probably can figure out a way with my RigExpert, but, have not done).
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

The OCF I built, when used on 80m, is only up at an apex of 30ft with 9 ft ends or so. So, it is an NVIS antenna on the 80m band.

On 40m, because of the current null, the antenna has no NVIS component at all. It radiates off of two lobes parallel to the antenna direction.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

Also, thanks for the spelling correction. I wish I could say it was "autocorrect" or some tool, but, I just misspelled it and I will correct the article.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by VA7CPC on March 20, 2017 Mail this to a friend!
This article is a great model for how a project _should_ be documented --

... Thank you!

. Charles
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KJ4DGE on March 20, 2017 Mail this to a friend!
Excellent article and photos. I love antenna articles and this one is one of the best. I use a OCF 40 I made with a 1:1 ugly balun. It works great on 40 and 20 but no other bands. Your attention to detail should be commended. Thank you for the write-up!


RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 20, 2017 Mail this to a friend!

Good work on your OCF. I would be curious to know if you are using a 6:1 balun at the 70 foot height??
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N4DBC on March 20, 2017 Mail this to a friend!
I'm glad to hear that you have so many people who would be willing to loan out delicate/expensive equipment. Some of us are not so lucky.
Still, you've spent some quality time & effort on this project and article. Congratulations on a job well done.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by WD4AOG on March 20, 2017 Mail this to a friend!
I recently had the need to install an antenna that would work 40 and 80 meters and be coax fed. (Transmission line had to run through metal conduit so I didn't think ladder line would work.) I ran across another article on the OCF dipole and built one. At 35 feet on surplus military masts with a Wireman 4:1 balun, it did the job and, as advertised, did not require a tuner!

Wish I'd had your fine article to follow when I was building mine. Good work!
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KE4ZHN on March 21, 2017 Mail this to a friend!
One thing that always puzzled me about OCF's. Why do you use a balun on an unbalanced antenna fed with unbalanced line? Coax is an unbalanced feed antenna with unequal length legs is also why not use an unun? Won't a balun burn up with an unbalanced load attached to it? Not trying to be a smartass, just always wondered how this works without frying the balun.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KE6EE on March 21, 2017 Mail this to a friend!

Whether or not a device is acting as a balun or an unun does not depend solely on the design of the device, but on the operation of the device within an antenna and feedline system.

Similarly a “balanced” antenna is likely not to be a balanced load because of the real situation of its installation--proximity to objects in the environment, etc.

The way a balun- or transformer-type device works in a system is explored in some depth in the W8JI article cited above by the article’s author.

What seems clear is that the “balun” built by the author is working satisfactorily as an impedance transformer and a part of the transmission line. Whether or not the device is suppressing common mode currents in the feedline or balancing the currents in the antenna proper are other questions.

Bottom line is that this well-written and thoughtful article answers some questions and stimulates many others. That is one of the great things about building one’s own antennas and sharing experiences.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KB2HSH on March 21, 2017 Mail this to a friend!
I am in agreement with K9JCS. Well done!!!

RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2UJN on March 22, 2017 Mail this to a friend!

Thank you for the feedback and the question about the transformer. First, I do want to say I leveraged existing knowledge about transformers/common mode chokes and am not an expert myself. I read carefully the noted references, in particular the reference from W8JI on transformers with unbalanced common mode currents. In that case, W8JI strongly observes that a dual core will function as an effective common mode choke with the proper number of turns within that core. So, I used a dual core transformer/common mode choke. I don't have any signs of RF back in the shack at all. And, I do know what those signs are because I had a 40m dipole without a common mode choke. Router gets knocked out, LCD screen blinks out, etc. So, I think the dual core I built is working as both a transformer (SWR is as it should be) and a common mode choke as specified by W8JI. Thanks for the question.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by VE3TMT on March 22, 2017 Mail this to a friend!
Thoroughly enjoyed reading this article, very well documented. I have often wanted to try an OCF but just don't have the real estate available. Using a DX-B sloper for now.

Thanks for the great write-up, will make a great reference for anyone wanting to build one. One of the better articles to grace eHam in quite a while.

RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KE6EE on March 22, 2017 Mail this to a friend!

I appreciate your comment on the common mode issue and am glad to hear that the antenna is working well.

Your article is definitely a generous gift for those (including myself) who think that making antennas is one of the most enjoyable aspects of our hobby.

You have made the details of the project very clear and very simple. Thank you for your effort.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by W4AUE on March 23, 2017 Mail this to a friend!
Outstanding article - well done!

This one - and the article on rewinding the FT101 power transformer are some of the best I've seem on Both represent one of the most attractive aspects of Amateur Radio - hands on learning! Thanks guys for sharing!

For those concerned with the cost of an antenna analyzer - check out the Fox Delta Kit. I added one to my bench last year - Best $50 bucks I've ever spent.

The swr scan of my Windom looks very similar to that shown in this article. Performance is also about the same.

OCF (or Windom) is a great antenna for a beginner on HF (or an ole timer)!

My FT101B rebuild project includes power supply upgrades and conversion to 6146's. just have to find the time to work on it.

Thanks to eHam for the platform!!

73 & CUL
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KE4ZHN on March 23, 2017 Mail this to a friend!
Interesting. Thanks for the replies. This is a great article and made me reconsider an OCF as a future antenna project.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by WB4M on March 24, 2017 Mail this to a friend!
I believe the 6:1 balun was reported to work better at lower heights, say 30 feet or less. I used the Balun Designs 4:1 model that is designed especially for use with OCF dipoles. Mine is coax fed all the way.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by KI4WCQ on March 25, 2017 Mail this to a friend!
What is your experience running power to the OCF? I've used them and found that 80m doesn't like more than 200 watts or 40m about 300 watts or the SWR goes off the charts. Higher bands seem to take power without a problem.
Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by WB2RCB on March 27, 2017 Mail this to a friend!
I built an OCF to use here in Florida using the design of KK1CW which is available from the Balun Designs website. Coupled it with the Balun Designs 4114 dual core (4:1 transformation & 1:1 choking balun in the same enclosure)

I did notice in your writeup that you trimmed 2' from each side of your antenna. In actuality, you should trim the ends proportionally meaning 16" from the short side and 32" from the long side to keep the 2:1 ratio intact.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by K1LI on April 1, 2017 Mail this to a friend!
The purpose of the Teflon(r) (PTFE) tubing is to create a two-wire transmission line with a characteristic impedance of 100 ohms. This is the optimum value for the transformation between 200 ohms and 50 ohms (square root of 200 x 50 = 100). Using a transmission line with a different impedance will produce a feedpoint impedance other than 50 ohms when the transformer is connected to a 200 ohm load.

I can't find any data on the electrical characteristics of the Viton(r) tubing you used. Even if the dielectric constant and tubing thickness deliver the desired transformation ratio, the material may be loaded with metal particles that could affect its loss performance. Note that Amidon carries Teflon tubing in sizes meant to cover standard wire gauges.

Very nice article. I recently helped a new ham in a DX location to build an OCF dipole using the W8JI approach. In an article titled, "Windom Off Center Fed," W8JI recommends feeding the wire 20% from one end. This provides usable SWR on the 80,40,30,20,15,12, and 10 meter bands. Compared to the traditional 33% feedpoint you used, this adds 30 and 15 meters but gives up 17 meters.
RE: Building a 6 (Amateur) Band, 2.5 KW, OCF Dipole for $44  
by N2EY on April 3, 2017 Mail this to a friend!
Thanks for the info! Very interesting!

If balun losses are high, and you are running power, it will get warm and eventually show signs of the warming.

73 de Jim, N2EY
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