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Common mode currents on an OCFD

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0ZAI, Dec 28, 2011.

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  1. M0ZAI

    M0ZAI Ham Member QRZ Page

    Hi all,

    I've recently bought an OCFD for 20m-6m excluding 17m. Running it barefoot into my FT857D is producing very high amounts of feedline radiation even at 20m where it exhibits a SWR of 1:1. The common mode currents are enough to reset the rig even at 20w. A few turns of coax helps with the common mode problems but raises the SWR.

    Is high common mode current on the braid a feature of OCFD? I would have though the balun would stop this.

    Has anyone heard of this before or have any suggestions as to how I can cure it?


    Matt M0ZAI
  2. VE3EKJ

    VE3EKJ Ham Member QRZ Page

    First off. let's assume it's not a Carolina design with a vertical radiator portion of the feedline.

    With a traditional OCF dipole, you absolutely need to be using a current balun or you will have RF on the feedline. Also with any OCF it's important to bring the feedline away trom the antenna at right angles, for as far and as best you can. If' it's dressed too close to on side element or the other you will also have RF on the feedline problems.

    The "few turns of coax" ugly balun trick is NOT wideband enough to cover all bands! It tends to only work well on one or two bands at a time. You can google "ugly balun" and you'll find lots of stuff.

    In your case, things are bad enough that I would consider a true choke balun, where the feedline is run through large ferrite beads. A sleeve of about 10-12 " long of such beads will usually block RF from flowing any further. It's a bit heavy but you could mount it outside, before the feedline enters the shack.

    Some hams have reported success when using smaller feedlines like RG58 or RG8X by using a large toroid a couple of inches in diameter, big enough to wind a few turns of the feedline right on the toroid, a few inches to a foot below the feedpoint balun. Such a trick can be easily weatherproofed with tape and doesn't have any connections to corrode anyway.

    Still, you seem to have an excessive condition. I would be looking at my initial couple of tips first, like how the feedline is dressed, if I were you.
  3. KU3X

    KU3X Ham Member QRZ Page

    You need a good 4:1 Guanella Current Balun. Here's one that will do the trick.
    Search, " gu4-hf-5kw" in Google. You not only will find the balun but you'll find a video of it on
    I would not bank on your OCF20 to be resonant on 17 meters. The OCF40's are not but the OCF80's are.
  4. K1DNR

    K1DNR Ham Member QRZ Page

    Just to say it again - SWR and transmission line radiation have nothing in common...

    Transmission lines radiate because of current imbalance. When you feed an *off center* antenna, you instantly have an issue with balance.

    The issue may get better or worse depending on a variety of factors including the length of your transmission line, etc.

    This is a very, very common problem with off center feed dipoles. The common mode issue may or may not be easily correctable to your satisfaction.

    I thought these things typically came with 4:1 current baluns. Maybe its a voltage balun. If you already have the 4:1 transformer at the feed point, then you might try putting a second 1:1 choke balun in line as well. Do not put two 4:1 baluns in line. Then your SWR really will get bad!

    If you had no transformer you would not be getting a 1:1 SWR. So I'm going to assume you already have the 4:1 transformation and need a good choke that does not transform the impedance any appreciable amount.

    A good 1:1 current balun might solve the problem.

    Or replace the existing 4:1 with this current balun, which has two cores and a very high common mode impedance
    Last edited: Dec 29, 2011
  5. N0AZZ

    N0AZZ Ham Member QRZ Page

    On my 270' coax fed OCFD I have a 6:1 balun and do not have that problem only bands need tuner on are 15,30m and band edges on 160m. The rest if any VSWR is present < 2:1 the internal tuner in my amp takes care of it.
  6. K4SAV

    K4SAV Ham Member QRZ Page

    What model antenna did you buy? Most OCF antennas have a high SWR on their fundamental frequency, however there is at least one commercially made antenna that intentionally uses common mode currents on the feedline to obtain a low SWR on the fundamental frequency. It has no choke at all but only a 4 to 1 voltage mode balun.

    Jerry, K4SAV
  7. K1DNR

    K1DNR Ham Member QRZ Page

    Just out of curiosity have you ever measured the common mode current anywhere on the coax? The op is complaining about RF in the shack which has nothing to do with SWR, although that seems to be a commonly held belief.

    As K4SAV mentioned, sometimes the coax is intended to be part of the radiating system. This is usually how end fed designs work - then they place a choke at some point down the coax from the antenna. If its a long length of coax, and mostly vertical it might be part of your success!

    How bad the RF is in the shack depends on many factors, which likely differ from your particular setup.

    I believe the standard OCF design presents ~200 ohms on even harmonics - which would match 50 ohm coax with a 4:1 transformation.
  8. KA4DSK

    KA4DSK Ham Member QRZ Page

    HI I have a homebrew OCFD 133 feet long with a 4-1 homebrew current balum. Can use 80 to 2 meter and top end of 160 with out a problem. The tunner in my icom746 PRO will bring all to 1-2 or less. I found that by moving the feed point several inches toward the long or short side of the antenna made things much better. In others words the long side was not one third of the overall length which was 133 feet and placing the balum feed with the correct side of the coax (the center not the out side shield) to the long side of the antenna shield to the short side worked best Using a 50 foot coax feed straight to rig.
    Check out this site " The Windom Antenna Handbook,by Buck Rogers K4ABT"
    Get some wire and build your own all band windom, i used #12 solid insulated home depot house wiring.
    Best luck have a great new YEAR and i hope to hear you on the air Frank
  9. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    OCFD Madness

    Yes ,CM is the main drawback to the OCFD.
    Many times the balun supplied at the feed point will be a 4:1voltage type balun. These are less effective at reducing CM on the feedline.
    A 4:1 current balun at the feedpoint would be more effective,but you still may need another 1:1 current balun further down the feedline.Both of these need to be high impedance baluns over the bands used.
    Something else to consider may be to ground the coax outer shield outside the shack. This may or may not be a help,you could try it and see.
    What-ever you do ,make only one change to the antenna system at a time, then test thoroughly and take notes.This way you can identify what works,and then later learn why it worked.
    Hope this helps.

    Here is some links for some theory and practical solutions.
  10. N7WR

    N7WR Subscriber QRZ Page

    I have a homebrew 270 foot long OCFD (180 on one side and 90 on the other) Fed at 90 feet on the tower, ends at 55 ft. 6:1 current balun and an 8 inch diameter coax choke balun just below the 6:1. Outstanding performer on all bands and no CM on the feedline.
  11. WD5JOY

    WD5JOY Ham Member QRZ Page

    While not necessarily THIS brand (Grumpy Old Guy "BUCK" :(at BUXCOMM:(:(:mad::mad: even at his "customers") -- I believe this would resolve the issue (perhaps): My brother uses one on his "end-fed 160-meter long-wire" with the result being elimination of current on his microphone and other inside the shack equipment. It worked for him - and looks like it should solve your issues as well (IMHO). He runs up to 1,000 watts without ANY issues whatsoever (well, maybe not-so much 'whatsoever' - other than neighbors and their stereo amplifiers bringing his melodious-tones to their living quarters)!

    "Coaxial in-Line isolator, 160 TO 2 METERS LINE ISOLATOR EASILY HANDLES 3000 HF."

    Catalog Number: B2LISO

    160 TO 2 METER LINE ISOLATOR EASILY HANDLES 3000 watts average power or 5000 watts SSB.;)

    We (Buck - the Grumpy-Old-Man;) who proudly tells folks over and over of his exploits at Cal Tech (Yes, the one in California. Where else do they have a Cal Tech?) ... true as they may be) included, enough ferrite cores in the B2LISO for effective choking impedance. Our Line Isolation B2LISO BALUN is conservatively rated. It can easily handle 3000 watts RMS, and 5000 watts SSB. The ferrite material we use for the cores is made to our precise, wide band standards to provide optimum performance across the Amateur bands 160 through 175 MHz, No other line-isolator or UNUN can provide this level of isolation at these frequencies.

    BUXCOMM B2KLISO coaxial in-Line isolator (B2LISO) tm is isolated by multiple, ferrite cores transformer. 1.5 to 175 Mhz. Made with Multi-Mix
    Ferrite cores for wide bandwidth, and Ultra-low-loss (0.5dB @ 10 mHz) coax. See illustration and recommended application, this page:

    The feedpoint BALUN or antenna UNUN may not balance the currents in the coax section feeding the antenna. However, lower down the coax line, closer to the RF output, a BUXCOMM B2LISO line isolator is added to reduce the common-mode current on the coax shield, thus reducing the radiation from the feedline between the isolator and equipment in the Ham Shack..

    * Includes RG8X Coax Jumper.
    * 5000 watts SSB, 2000 wts CW/FSK/PSK Line Isolator LISO Regtm BUXCOMM
    * Input, connector, SO239
    * Output, connector, SO239

    An Isolator is installed at the output of the Radio, and/or if you are using a Linear Amplifier, it should be placed at the output of the amplifier.

    A Line Isolator is exactly what it says: ..... The B2KLISO Isolates the Coax or transmission line from the Radio to perform two functions:

    1) Prevent stray RF that may ingress into the coax shield from the nearby antenna radiation, And (2) Reduce spurious re-radiation or secondary radiation that may be a product of the random length of the transmission line.

    As a result of number 2, many parasitics that cause TV Interference are reduced. Only a BALUN is to be used at the Antenna feedpoint. DO NOT use a Line-Isolator at the antenna feed-point.

    When vertical antennas are installed above radial system, the feed-point impedance is approximately 44 ohms. When they are install without radials there are losses that enter into our equation that defeat the gain/band-width factor of our vertical antenna(s).

    In addition, the feed-point impedance is no longer (approximately) 44 ohms. Some reduced VSWR may be noticed, however, the gain of the antenna suffers as a result of the antenna efficiency loss.

    There is a reason for this effeciency loss; . because too often there is (RF) current introduced on to the feedline. To circumvent this problem with RF being induced into the shield of the coaxial feedline, we isolate the coaxial feedline from the RF currents from the antenna radiation, by inserting a "Line ISOlator" at the base or feed-point of the vertical antenna.

    The BUXCOMM LISO (line isolator) inhibits undesired RFI by preventing feedline currents and re-radiation. In turn, the LISO forces all the RF energy from the transceiver, tuner, or amplifier, into the antenna. The BUXCOMM LISO2K does not require any further tuning.

    The LISO2K works great with Windoms (ANY OCF), Dipoles, Bazookas, Sky-wires, and Loops. The BUXCOMM LISO2K is very broad-banded, and is effective from 1.5 to 175 mHz.

    In all fairness :eek:- "Buck" was very instrumental in the research and development of what we now call 'HDTV'! A ham since age nine (as in "9") - Buck is at the magic age of '77' and has earned the right to be a bit 'brash'. He has managed to stay in business for many years while insulting customer after customer, many who return year after year to line his pockets by buying his well engineered, well made and sometimes 'crazy' wares! I like the old fart - he makes me laugh. He does, however, love to refer to what he considers the only accurate information on amateur radio products - HIS INFO - time and time again in the course of ANY conversation!:D He also has been awarded the "Most Domain Names for a Single Company" (fourteen) three decades in a row). No "Hi-Hi-OM" here folks -- just let out a REAL chuckle! BUCK would!:)

    Attached Files:

    Last edited: Dec 30, 2011
  12. K1DNR

    K1DNR Ham Member QRZ Page

    Personally I'd rather see my current in the antenna, and not as a common mode current I have to convert to heat - in order not to have so much RF in the shack that it resets the radio...

    At some point, I think one is "throwing good money after bad", and if the common mode issues are so bad - one should be looking at why, and fix the problem!

    If it is "part of the design" - that's one thing. Different strokes for different folks, and if the application meets the need, form follows function - so whatever.

    If its just a bad design, improperly installed, or whatever the case may be - I think one ought to investigate, fix it, or pick a different antenna...
  13. K8JD

    K8JD Ham Member QRZ Page

    I could never figure out the reasoning behind taking a good Windom design and messing with it to the point of invalidating the original design just to get an "all band " antenna without traps of stubs to tune it.
    The original Windom was Off center fed only to get a 600 ohm feedpoint to match a halfwave dipole to 600 ohm ladder line!
    Anyone remember that concept?
    I am trouble free with single band dipoles and loops fed with the appropriate 1:1 balun, in some cases, and coax line.
    Last edited: Dec 30, 2011
  14. WD5JOY

    WD5JOY Ham Member QRZ Page

    One of the 'guru's' says:

    In .....1945-1949, a BALUN was unheard of...;... my Elmer's described, a means of connecting the coax to the coax fed Windom antenna using a lossy, nine (9) turn coil of the coax feed-line at the feed point. This coil of feedline coax formed a "de-coupling" loop. The de-coupling loop provided a crude BALUN of sorts that would prevent some RF from being re-radiated off the coax shield. In later years I learned about something called a "BALUN."

    In 1958 I read more papers by Gillette Guanella which referenced a “current” type BALUN. then I came across Thomas O’Meara’s papers, “Analysis and Synthesis with the Complete Equivalent Circuit for Wide-Band Transformer.” This is when I made some design changes to the original Windom antenna. In 1968, I met Lew McCoy W1ICP (SK).

    We talked about the Windom antenna and how we were building them. Lew had some ideas that I felt had enough merit to give them a try…
    I made the changes and… !! walla, almost like magic, Lew's current type BALUN design gave us the bandwidth that we needed to turn the WINDOM into an eight (8) band, plus antenna (even adding some VHF bands).

    "And they still work today ..." The Windom antenna is an Omni-directional, off-center-fed, wire type antenna, that when installed at 35 to 45 feet above ground will exhibit a feed point Impedance near 200 ohms (MOL, slightly more). For most Windom users, we install our BUXCOMM 802134 Windom in the Horizontal or "flat-top" plane as we would with any other Multi-band antenna. Where real-estate is limited, the Windom may be installed as an inverted "Vee", or as a sloping antenna (mutt and jeff) style; that is, long (tall) end higher than the (short) end.

    img2B.gif Ta-Da! Now we can use coax to feed it instead of the ladder-line AND make big bucks off the idea!
    img5.gif Strange that all the gurus say essentially the same things about the Windom. Yet, they all make it sound as if the idea was their original one - even though they admit to simply making modifications to the real-deal.:D

    The Carolina Windom:rolleyes: .... The Bux Windom:cool: .... Jim Bobs Improved Windom:confused:.... Walts All-Band Windom:eek: .... A Better Windom!;)
    Last edited: Dec 30, 2011
  15. K1DNR

    K1DNR Ham Member QRZ Page

    So were the Internet, cell phones, antenna analyzers and injection molding of modern plastic.

    It doesn't matter what the gurus say. It is what they measure.

    Measurements don't lie. Balance doesn't lie.
  16. 4L4AGU

    4L4AGU Ham Member QRZ Page

    It should be checked, wherever there are some wires running in paralel to antenna, that come back to shack. They may also bring RF back to shack, and feedline radiation may not be an issue.
  17. M0ZAI

    M0ZAI Ham Member QRZ Page

    Hi all,

    Many thanks to all of you for your comments and advice
    It's an OCFD made by a fellow amateur, with a 4:1 voltage balun.

    I have solved the CM problem with an "ugly balun" coax choke on 20 and 10m.

    I wasn't aware of CM being a "feature" of OCFDs and was unsure if there was something being with it.

    Seems to be ok, but it is only resonant on 20m and.
    10m, not 20, 15, 12, 10 and 6m as advertised.
  18. K1DNR

    K1DNR Ham Member QRZ Page

    The ugly balun is not broad band. Not as broad band as a Guanella type current balun.

    I would replace the 4:1 voltage balun with a 4:1 Guanella current type balun (examples already given).

    If you were to measure the impedance to common mode current on an ugly balun, you would find that it works well on the frequency it was wound for, but not well on other frequencies (not broadband). Air wound coax baluns can be effective if wound for a particular band - but they don't work well as a common mode choke over many bands.

    The Guanella type current baluns suggested provide a high impedance to common mode current on the bands you use.

    *make sure the current balun is on the coax (toward the shack) side of the voltage balun if you are going to use both. I would use only one, a 4:1 current balun at the feed point of the dipole to minimize losses from using two baluns.
    Last edited: Jan 14, 2012
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