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80m OCF - Balun and tuning mysteries

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G0KDT, Sep 13, 2020.

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  1. G0KDT

    G0KDT Ham Member QRZ Page

    Any suggestions?

    I attempted an 80m OCF.... easy so many posts and websites all babbling about OCF dipoles, plus I I'd had it recommmended as a good antenna to build.

    The reality has been far from good. There are so many arguments between ops on designs both on the element lengths (ratio 33% 66%.... what happened to the last 1%) and baluns 4:1 or 6:1 .... ok some people may use 75ohm coax... but most transceivers are setup for 50ohm. [I am deliberately ignoring ladder line 300 or 450 ohm as I don't have more than a built in ATU and for other reasons].

    Then there are arguments about current or voltage baluns and using two ferrites for guanella baluns etc.

    All in all a mind boggling confustication but we're told that these ocfs work well and will tune up well too on many bands.

    Not in my experience. I have tried several times to this one to work.

    I built a guanella balun and tested it as best I could using a nanovna across the hf bands with a 200ohm resister connected to simulate the antenna elements (load). A vswr sweep through the bands looked promising but not brilliant. Vswr of c. 1.1:1 at 80m through a peak of 1.67:1 at 10m. I had thought it should be better than that.

    The balun was made with bifilar wire, un twisted as wound, with 7 windings up one side before crossing and winding 7 windings up the other side of each ferrite ft240-31s. Pairing up the two ferrites and connecting the windings.

    I have tried a wide range of offset split lengths (I lost count) for the flexweave elements but the Vswrs for the primary hf bands are pretty poor. More often than not hitting figs greater than 2:1 and therefore meaning the AtU would have to do a lot of work. On a TS590SG internal ATU that isn't really a great prospect.

    Testing and tuning is difficult as a 1 man op, making the elements in length steps of 3 feet, getting height, running up and down to tweak lengths by folding back, twisting the fold back, untwisting the foldback.

    Putting the feed point at my rooftop chimney and the elements over the roof to either side of the house was hopeless....

    At the moment the antenna (if you can call it that) is tossed in the corner of the workshop.

    So what say those of you that have built these with success?

    Is it the balun... more bifilar turns or less? I don't have a web account to link a pic or I would.

    All electrical connections were thoroughly crimped and soldered and best quality ferrites and stainless fixings used.

    I am utterly puzzled.

    Phil
     
    Last edited: Sep 13, 2020
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Phil,

    Pictures can be inserted in-line on QRZ with no third party hosting required...

    About the OCF:

    How can you deploy it?

    How high?

    What is around it? (within a few meters)

    Straight or InvV?

    How long the coax run?

    Type of coax?

    An idea of earth properties?
     
    KU3X likes this.
  3. KU3X

    KU3X Ham Member QRZ Page

    First of all, make your 4 to 1 Guanella current balun using two cores. You can use two each FT 240-43 cores. Next you have to make sure you bifilar windings are actually 100 ohms. My guess is you are using #14 THHN wire. As long as you use that balun into a pretty good matched load, you can get by with it. Twisting the wires helps keep the wires tightly together. You should be able to achieve a 1.1 to 1 match on 10 meters. 1.6 to 1 is not good at all.
    Next, "Never feed an off center fed antenna with parallel feeders !" That antenna is not a balance antenna and the feeders will radiate. That you can take to the bank. If you are going to make a multi band antenna, good for 80 meters on up and use parallel feeders, feed it in the center of the antenna. Balanced line, balanced antenna.
    What happened to the 1%? If you make it 33/66, you are close enough, you're splitting hairs.
    My OCF80 has the apex at 55 feet and I get a 1 to 1 match at one point on every band except 80 meters. The one end of the antenna is about 10 feet in the air and the other end is about 20 feet in the air. I've set one of these up portable with one end up 10 feet in the air and the other end up 8 feet in the air with the apex at 22 feet. I got the same SWR results. The reason I don't get a perfect 1 to 1 match on 80 meter is the feed point is less than 200 ohms. I don't need a tuner on the CW portion of 80 meters, but it's just not a perfect match.
    I use insulated wire, #12 THHN. Here are your cut lengths. Cut one wire 45 feet 6 inches long and cut the other wire 89 feet. This will allow for 5 inches of strain relief for the end insulator and 7 inches for the balun.
    DO NOT use this antenna on 15, 30 or 60 meters.
    I've already feed my OCF40 with 75 ohm coax, but I cut it so it's one wave length long on 40 meters. I would not suggest using 75 ohm coax if it's random length. That is, if you want the lowest SWR.

    First focus on your balun. If it's not right, you can't expect a good SWR on the antenna. No climbing needed to build a good balun.

    I am sure other people have their versions that work just as well. I am just sharing what works for me.

    Barry, KU3X
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    To add to what Barry said:

    There are two approaches to building OCFs: My least favorite is what Buckmaster (commercial maker) does: Use a voltage step-up transformer (I refuse to call it a balun) with no Common Mode current blocking, let the coax radiate, and call that a feature. That puts RF voltage into the shack and house wiring. That makes that antenna susceptible to RFI originating in your house. That restricts you to routing the coax in free space (you cannot route the coax parallel to a metal mast or tower). If you add a CM coax choke at the input to the transformer, the antenna stops working. IMO, a totally deficient design!

    The other approach is to design the antenna around a 1:4 current-mode balun which intrinsically has good Common Mode current blocking. This will work on fewer bands, but avoids the pitfalls above... An alternative would be to use a voltage-mode 1:4 transformer with a Coax CM Choke right near the transformer from the get-go. The difference between 33/99% and 25%/75% seems to be which bands each one works on...

    I went down this learning curve, and if I needed yet another multiband 80m-on-up wire antenna, I would opt for a horizontal 80m full-wave loop or a ZS6BKW.
     
    K7JOE likes this.
  5. G0KDT

    G0KDT Ham Member QRZ Page

    Well thank you for the replies Barry and Michael. Both of you chaps seem to have huge antenna setups from your pages.

    To cover the questions and clarify as best I can;

    1. Deployment has two options; (a) Over the house with Balun mounted on Tv antenna pole on house chimney at C. 45 ft, short element dropping at 45 degrees to front garden and C. 5ft above ground with long element to a 20ft pole at the top of the garden behind my house or (b) Balun affixed to weatherboard at the end of my house with the short element dropping at C. 45 degrees to garden fence dividing my drive from my neighbours and the long element again going to the 20ft pole at the top of the rear garden. The rear garden element is virtually horizonal but as the garden slopes upwards the height does in ground effect terms drop.

    2. There is nothing much around the wire elements save the roof slates in option 1 for a few feet. In option 2 pretty much free space coming away from the end weatherboard of the house.

    So the antenna would effectively be Inverted given the 45 drgree drop on the short leg and rising slope of the rear garden.

    Coax runs being kept to a minimum would be a bout 40feet of either Ultraflex 7 or Mini8.

    Earthing is a conundrum as the 'shack' (don't let mrs G0Kdt hear it called that ) is our loft room, getting independent earthing over household earthing is not straight forward (near impossible) given the location. In addition I get totally bogged down reading the arguments on independent earthing vs other impacts like earthing and hum loops etc.

    The guanella balun I made is alleged ( as you seem to also seem to imply Michael ) to have 'some' common mode choking capability and being made of two ferrite toroids. there was a lot of stuff varying between FT240-31 or Ft240-43 with things coming down on the 31 as more suited to the HF bands, plus I had some. With two core flex I used to wind these was two core high grade and power speaker flex with copper cores, same sort of size as the rig 30A psu cable. Being paired meant that it was easy to prevent it twisting and the K9YC choke and balun cookbook (which seems pretty comprehensive) says 'don't twist'. True he uses coax and other mateirals to wind and I do have some teflon coated silver single strand stuff I could use, but the balun idea needed to be tested first or its a waste of expensive cable.

    Now to Barry's 100 Ohms for the balun windings I haven't seen that dictated in anything I've read (maybe I missed it with so much stuff I've waded through), indeed the variety of balun windings were rather vague to say the least. I will try to measure what I have but that will mean a complete dismantle of the balun and are we saying 100ohms for each wire on each toroid or some other measure? I have to say, that I have got bogged down wading through webpages and arguments about what windings, materials and I've not found anything that explains in basic principals the build and testing process, most just dive in with some detailed Zo and Inductances etc. leaving the reader who is not familiar or at home with such detail baffled.
    Phil
     
    Last edited: Sep 13, 2020
  6. KU3X

    KU3X Ham Member QRZ Page

    A lot of information on building 4 to 1 Guanella Current Baluns came from W2FMI's, "Building and understanding baluns and UnUn's.
    Years ago Jerry set the bench mark for building baluns and UnUn's. Jim Brown really refined chokes/baluns. Jim really did not focus much, if anything on 4 to 1 baluns and Unun's. He may have? If he did, I missed it.
    You can twist parallel wires when building a 4 to 1 balun. Do not twist more than 3 turns per inch. If you do, you will change the impedance of the parallel feeders. The key to achieving 100 ohms is the proper spacing between the wires. I don't think you have the proper teflon tubing laying around with the correct enamel wire? Most people don't so they use #14 THHN paired. Pairing those two wires is just about on the money for 100 ohms. The only problem with using THHN is the voltage between the two wires. It's rated at 1200 volts. That's 600 volts per wire paired is 1200 volts. Now that's not the breakdown voltage, that's just the rated voltage.

    100 ohms? How do we get that?

    50 ohms times 200 ohms = 10,00. The square root of 10,000 is 100.

    When you test your balun, make your leads as short as possible. Don't hang 6 inch long leads off of your load resistor and expect a low SWR on the high bands. They must be as short as possible to get accurate reading. Even the leads coming from the core and connecting the SO 239 can effect the SWR.
    It's not super critical when it's on the antenna since the antenna is not exactly 200 ohms on every band. But on the test bench, you want to get it as low as possible so you know you have a good balun.
    You are not talking about using the OCF on 160 meters so 43 cores will work just fine. If you have 31 cores, you can use them as well.

    Barry
     
  7. G0KDT

    G0KDT Ham Member QRZ Page

    Ok hear what you are saying Barry but not clear exactly on what you mean about spacing. At present the two core bifilar is flat two section and wound without twisting through the toroid, 7 turns then crosses the toroid and winds another 7 turns on each ferrite. Spacing the windings, well they just neatly fit an FT240-31 with a small gap at the top and at 6 O'clock (starts posn) and 12 O'clock (crossover to start second 7 turns from 6 O'clock ) to wind the second 7 windings back to 12 O'clock. (Wish I could add a pic).

    I have black and blue lengths of this https://www.hfkits.com/product/ptfe-insulated-silver-plated-winding-wire-blue-18-awg/ but not used it yet wanting to test balun winding.... The box and fittings used also came from HFKits so you can get the idea. They don't have a 4:1 Guanella balun kit.

    Ok on the voltage ratings. I 'm pretty condifent that the wire I have would cope with that and certainly the HFKit Balun wire if I switch to it. THAT SAID, I'm still not clear on where or how you are measuring the 100 Ohms


    It seems to me that you are drawing a relationship between 50Ohm Transceiver Impedance (or the coax feeder) and the 200ohms that the OCF dipole elements impedance are meant to be … I think. THis was also the reason I figured that we use a 4:1 balun to bring 200 ohms to 50 ohms to match the transceiver required antenna impedance or as near as possible.

    It seems to me that you are suggesting another relationship that I haven't understood and still don't.

    So how should I go about measuring the 100 Ohms, between which points on the balun?

    There is virtually no lead length off the toroids to the SO239 in the current build, it was a devil of a job to fit that side. Clearly within the box there is about 2 to 3 inches of wire to that allow them to wrap around to the external flexweave element fixings.

    Hope this all makes sense Barry. I'm happy to learn but the grey matter has become slow and easily gets bogged down or certainly does trying to read the article on the subject.
    Phil
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    The 100 Ohms Zo for the wire pair is a goal, but is not particularly critical... This online calc lets you estimate Zo
     
    KU3X likes this.
  9. G0KDT

    G0KDT Ham Member QRZ Page

    Hmmm Ok Michael but you jumped into Zo etc... I go back to my earlier comment using such terms when you are familiar and comfortable with them and their context and meaning is great. Not so if you aren't ... i passed an exam for radio years ago but don't eat sleep and breathe electronics and theory so it means pretty much nothing to me.

    Also why I'm asking about measuring the 100 ohms. So sorry to say but the calculator means nada to me.
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Zo is shorthand for "characteristic impedance", as in the Zo of RG-8U (Belden 8237) is 52 Ohms...

    The calculator is the goto resource whenever you are building a parallel-wire transmission line, be it for making ladder line or for creating paired wires to wind a balun.

    When making a 1:4 balun using paired wires to wind a current-mode device, going from 50 Ohms to 200 Ohms, the rule-of-thumb is that the paired wires should have a Zo = sqrt(50*200) = 100 Ohms.

    The calculator shows that I can use #16awg speaker wire and come pretty close (excuse the US units). I just wound a choke this past week using speaker wire and #31 toroid.

    upload_2020-9-13_16-44-52.png
     
    Last edited: Sep 14, 2020

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