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Thread: Common mode currents on an OCFD

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  1. #11
    Join Date
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    While not necessarily THIS brand (Grumpy Old Guy "BUCK" at BUXCOMM 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)!

    http://www.packetradio.com/catalog/i...oducts_id=1938

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

    [ATTACH] http://www.packetradio.com/catalog/i...oducts_id=1938[/ATTACH]


    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

    B2KLISO:
    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 - "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! 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 Images Attached Images
    Last edited by WD5JOY; 12-30-2011 at 05:39 PM.

  2. #12

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    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...
    Charley K1DNR

  3. #13
    Join Date
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    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 by K8JD; 12-30-2011 at 10:44 PM.
    73.....JD, FISTS #3853,cc 455,SKCC # 1395,tribune #12,
    Official US Taxpayer

  4. #14
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    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.gifStrange 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.

    The Carolina Windom .... The Bux Windom .... Jim Bobs Improved Windom.... Walts All-Band Windom .... A Better Windom!
    Last edited by WD5JOY; 12-30-2011 at 11:08 PM.

  5. #15

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    Quote Originally Posted by WD5JOY View Post
    One of the 'guru's' says:

    In .....1945-1949, a BALUN was unheard of...;...
    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.
    Charley K1DNR

  6. #16

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    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.

  7. #17

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    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.

  8. #18

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    Quote Originally Posted by M0ZAI View Post
    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.
    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 by K1DNR; 01-14-2012 at 10:53 PM.
    Charley K1DNR

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