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Author Topic: When do I need a Balun for a Dipole?  (Read 14147 times)
G3TXQ
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Posts: 1515




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« Reply #15 on: March 15, 2011, 04:05:30 PM »

Excellent summaries from Dan and Dale!

I would only add this: from the information you gave, it's possible that your coax length may be in the region of a half wavelength long; if it's well grounded at that shack end, as Dale mentioned that's one of the combinations more likely to produce troublesome levels of braid current.

As Dan said, you can build an effective choke yourself at pretty low cost; so my attitude has always been to "play safe" and include one. On the other hand, the World probably wont end if you don't Smiley

73,
Steve G3TXQ
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N4JTE
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Posts: 1155




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« Reply #16 on: March 15, 2011, 09:35:09 PM »

BYU, nope not confused; a dipole only needs a balun if their is a chance of extrainious mutual coupling with feedline from other antennas or something nearby, as a single monoband dipole antenna there is really no need for a heavy balun sagging the line, not worth the cost or the resultant weight. A dipole is probaly the only antenna  out there that when fed by an unbalanced feed like coax will not exhibit unwanted stuff due to the natural closeness in ohms to the feedline.
TXQ, a true 1/2 WL line considering it's velocity factor, is in fact basically invisible on any feedline as it does the job of repeating what the antenna sees right back at the transmitter, good or bad.
As far as "grounding" a dipole don't have a clue what that means.
Bob
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G3TXQ
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Posts: 1515




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« Reply #17 on: March 16, 2011, 02:06:50 AM »

BYU, nope not confused; a dipole only needs a balun if their is a chance of extrainious mutual coupling with feedline from other antennas or something nearby, as a single monoband dipole antenna there is really no need for a heavy balun sagging the line, not worth the cost or the resultant weight. A dipole is probaly the only antenna  out there that when fed by an unbalanced feed like coax will not exhibit unwanted stuff due to the natural closeness in ohms to the feedline.

You appear to be thinking only of Induced CM currents and ignoring Conducted currents. At the feedpoint of a coax-fed dipole there is a Conduction path from the inner surface of the coax braid to one leg of the dipole AND to the outer surface of the coax, which behaves as an independent conductor. If the impedance looking into the CM braid path is relatively low, significant current will follow that path EVEN THOUGH THE DIPOLE IS PERFECTLY BALANCED AND THE COAX DROPS SYMMETRICALLY. In fact, the moment you connect coax to a perfectly balanced dipole you have unbalanced it by connecting a "third" antenna leg.


TXQ, a true 1/2 WL line considering it's velocity factor, is in fact basically invisible on any feedline as it does the job of repeating what the antenna sees right back at the transmitter, good or bad.
As far as "grounding" a dipole don't have a clue what that means.

You are confusing the differential-mode operation of the coax with its common-mode operation. As I described earlier, the outer surface of the braid potentially carries CM standing wave current and acts as another antenna wire, independent of the "power delivery" function of the coax. That "new" antenna wire is connected to one side of the dipole feedpoint and probably to ground at its other end in the shack. If its length is an electrical half-wave, and the shack ground is effective, the wire will present a very low impedance at the feedpoint and a significant percentage of the feed current will flow that route.

Hope that helps.
Steve G3TXQ
« Last Edit: March 16, 2011, 02:08:41 AM by G3TXQ » Logged
WB6BYU
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Posts: 13248




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« Reply #18 on: March 16, 2011, 09:10:37 AM »

Quote from: N4JTE
BYU, nope not confused; a dipole only needs a balun if their is a chance of extrainious mutual coupling with feedline from other antennas or something nearby...

...A dipole is probaly the only antenna  out there that when fed by an unbalanced feed like coax will not exhibit unwanted stuff due to the natural closeness in ohms to the feedline.


Dear me, more confused than I had thought.


Let me suggest you read W7EL's article on Baluns:  What They Do and How They Do It here:

http://www.eznec.com/Amateur/Articles/Baluns.pdf



Common mode current has nothing to do with reflected power or the SWR on the feedline.

The currents flowing INSIDE the coax are totally independent from those flowing OUTSIDE on the
shield - the latter are what we call common mode current, and the purpose of a balun is to
reduce these currents.

If you take a theoretical dipole and connect a long wire to one side of the feedpoint that
drops down to ground level, the dipole is no longer balanced, and that added wire will have
some amount of current flow that depends on the length of the wire.

This is exactly the situation you have when you feed a dipole with coax cable:  the outside
of the coax acts like the added long wire connected to one side of the dipole, unless you use
a balun to decouple it.

The comments about grounding have to do with whether or not the end of this added wire is
grounded - since it is part of the antenna, that will change the current distributions.

A perfectly symmetric dipole certainly can have common mode current (and no longer be
symmetric) once a coax feeder is connected.  This has happened to me several times, where
the SWR changes when I rearrange the extra coax, or the SWR measured with an SWR analyzer
doesn't match that with the rig connected (due to the added length of the power cable being
added to the antenna), etc.  As I tried to make it clear above, that doesn't mean that such a
dipole can't work, because "work" is a relative term.  I don't use baluns in my wire dipoles for
the reasons you suggest, but I also know how to recognize and deal with the problems that
I encounter due to not doing so.  Often the coax length is such that little common mode current
flows and the operator doesn't notice it, but a seemingly insignificant change in the station
configuration can change that.


In the immortal words of Mark Twain, "It ain't what you don't know that gets you into trouble.
It's what you know for sure that just ain't so.
"


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VE3QJ
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Posts: 21


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« Reply #19 on: March 29, 2013, 10:48:47 AM »

do i need a balun or coax choke at the base of a Cushcraft AV-5 vertical?

thanks
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KD2CJJ
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Posts: 369




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« Reply #20 on: March 29, 2013, 12:13:00 PM »

Ultimately they all do the same thing... I would recommend either a Guanella balun (if you dont build it they are expensive) at the base of the antenna OR a simple Ferrite Bead (Choke balun) on the feed line at the antenna.  I have used both - the Guanella Balun works a little better when there is significant common mode current OR near field issues.  --- Just stay away from the "ugly balun" (where you coil up the coax) there is always potential to throw off the tuning... I dont mind spending the little bit of $$ to not have to think if I have detuned the antenna because I didnt coil up the coax properly
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73

Mike
KD2CJJ
KB6HRT
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Posts: 106




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« Reply #21 on: March 30, 2013, 03:18:52 PM »

 Use a G5RV on a HF as one of my antennas i use, do use it on 3940 most every morning Rag Chewing, tuned this antenna so
it has very low standing wave on the part of 75m where I transmit most, made an air balum where the coax connects to the 300 twin lead using 8  6" turns of the RGX8 feed line then used my FS meter laided  it on the coax at the other end while transmitting an 25 watt carrier on 3940 while moving the FS meter antenna up and down the coax until I found the null.
Cut the coax an installed a connector there.  On 3940 radio see's a very low SWR on that frequency when transmitting an it hears well, Is there other ways to do this of course, but this cost me a few feet of RG8X that you can use to make some  jumpers, no RF in the shack an no tuner need when transmitting on 3940...........kb6hrt
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NV2A
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Posts: 115




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« Reply #22 on: April 01, 2013, 01:39:32 PM »

N3OX, very nice of you to take the time to put your thoughts together along with the sites you posted.  You helped me out quite a lot with your simple no BS explanation.  This is also a topic I've pondered over.

73's Ray NV2A
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WA8UEG
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Posts: 354




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« Reply #23 on: April 01, 2013, 07:15:53 PM »

 I have installed for others and personally used hundreds of 1/2 wave dipoles fed direct with 50 ohm coax since 1964, never used a balun never had a problem with any I used or installed for other hams. Just sayin unless you need a store bought solution to easily feed a dipole forget it and save the money.
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AD4U
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Posts: 2164




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« Reply #24 on: April 02, 2013, 05:50:11 AM »

In summary:

Do you NEED a balun to make a coax fed half wave dipole work well?  Emphatically NO!

Does using a balun on the above described antenna guarantee that the antenna will perform better than without one?  Emphatically NO!

Does using a balun on the above described antenna eliminate SOME possible issues with stray currents etc?  MAYBE.

Dick  AD4U

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W1JKA
Member

Posts: 1667




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« Reply #25 on: April 02, 2013, 06:53:17 AM »

  Interesting topic:I have never used a balun on dipoles(coax fed) so last week took down two of my dipoles and put a balun on each (one homebrew,one commercial).Long story short no detectable difference on receive or transmitting between 3-100W at my qth.Will be removing baluns today.
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