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Author Topic: 1: 1 Balun for a Dipole ??????  (Read 1618 times)
VK2FAMB
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Posts: 5




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« on: March 05, 2007, 05:41:39 PM »

Hi...
I would like to get peoples opinions wether to use a 1 : 1 balun on a 40m dipole to connect the coax the the antenna's arms or just solder the coax directly?  
Cheers
Joe
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W3ML
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« Reply #1 on: March 05, 2007, 05:47:24 PM »

I have a 1 to 1 balun on my dipoles.  It is easier to have a coax connector so I can change coax if needed.

Also, mine has a loop on top to attach the rope to hang it up as an inverted Vee.

John

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N0XMZ
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« Reply #2 on: March 05, 2007, 06:17:49 PM »

A lot of literature says it's not necessary. I think it depends on your situation. If your feedline exhibits common-mode currents (which often causes problems like RF feedback) a current balun can certainly help.
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K0RFD
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« Reply #3 on: March 05, 2007, 06:35:30 PM »

You're going from a BALanced antenna to an UNbalanced feedline.  It's what a balun is for.
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K4IA
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« Reply #4 on: March 05, 2007, 08:11:45 PM »

For many years we ran dipoles without baluns and no one seemed to care.  Give it a try.  I would say it is not necessary unless you find yourself with RFI issues.  Then a balun may help or you may need a choke type balun at the shack end of the coax.  
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N6AJR
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« Reply #5 on: March 05, 2007, 10:38:54 PM »

If I have one handy I use it, if not I just solder the wires and  dip in plasticote
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K0ZN
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« Reply #6 on: March 05, 2007, 10:39:38 PM »


Hi,

You can decide whether you will benefit from a balun at the feedpoint of your dipole. Here is the "deal".
At the lower frequencies, and 7 Mhz is really fairly low, the ratio between the length of the antenna and the diameter of the coax is a factor in how much unbalance you may get without a balun...and IF your coax feedline runs perpendicular to the antenna for at least 1/4 wave...about 33ft (~11 M)...then there will be a minor amount of unbalance. If your feedline runs off at other than 90 degrees, a balun may have more benefit on 40 M.

At 7 Mhz, the main benefit of a balun is a slightly cleaner/symetrical radiation pattern, and possibly a little better isolation of the antenna from the coax shield, which runs vertically, and tends to pick up more man-made radio noises. Possibly, you may see a little broader SWR bandwidth too. But NONE of these "benefits" will be significant, especially if your antenna is less than a half wave high. Many typical ham antennas are about 30 ft/10 M high...and this is electrically LOW on 40 M, so the balun will have minimal effect on radiation pattern because at that height the antenna is essentially non-directional due to ground reflection.

If your antenna is electrically high or a directional antenna such as a yagi, a balun is highly advised to maintain a cleaner radiation pattern.

I can tell you this....a station that is 700 miles away will likely find it impossible to tell if you do or do not have a balun at your feedpoint! In my mind, probably the best benefit for a balun in your situation will be possibly a slighly lower received noise level and a minor improvement in SWR bandwidth.
If you have fairly sigificant unbalance due to the coax not coming off perpendicular to the antenna, a balun is probably advised. Sometimes they can help reduce TVI, but most TVI is a problem with the consumer electronics and not the ham antenna.

Bottomline: the addition of a balun on your 40 M antenna will not HURT anything....and it may marginally improve a few things. (On the higher bands...20 M and higher, a balun has more benefit).

73,  K0ZN
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N3OX
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« Reply #7 on: March 06, 2007, 05:32:56 AM »

"In my mind, probably the best benefit for a balun in your situation will be possibly a slighly lower received noise level"

I've noticed that I can get 1 or 2 S-units improvement in noise level by putting a good 1:1 choke/current balun at the feedpoint of my antennas.  

Additional common mode chokes help somewhat as well.  I'm in a high powerline noise environment right now.

When I started out hamming back in high school at my parents' quiet, fairly rural house I never used an effective balun on any of my dipoles.  After a few years of living in an apartment and a noisy suburban house, I wouldn't go without one.

By the way, a lot of commercial baluns are NOT current baluns and won't really do anything.  Most "1:1" baluns for sale are voltage baluns.

You should probably make your own out of a proper coil of coax for a single band or out of coax wound through a big ferrite for a broadband choke.  I use ferrite bead baluns because I run low power and had a box of many hundreds of #43 material beads.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #8 on: March 06, 2007, 05:51:04 AM »

By the way, I even use a common mode choke at the feedpoint of my *vertical*

http://www.n3ox.net/projects/sixtyvert/comchoke_lg.jpg

It's not a balun since it's feeding an unbalanced antenna, but it does help a little with noise.

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W5DXP
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« Reply #9 on: March 07, 2007, 08:13:02 AM »

If the electrical path length down the outside braid of the coax is close to an odd multiple of 1/4WL, one has a natural choke built in since the low impedance at the transceiver will be transformed to a high impedance at the dipole. It acts somewhat like a 1/4WL sleeve choke.

If the electrical path length down the outside braid of the coax is close to a multiple of 1/2WL, that's worst case for common-mode current and one definitely needs a balun since the low impedance at the transceiver will be transformed close to the same low impedance at the dipole.

When a ham says that his coax-fed dipole works fine without a balun, he has probably picked a lucky length for his coax. Note that the velocity factor for common-mode currents is ~0.95, not the velocity factor of the coax, e.g. 0.66, when used as a transmission line.
--
73, Cecil, w5dxp.com
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N3OX
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« Reply #10 on: March 07, 2007, 08:18:41 AM »

"When a ham says that his coax-fed dipole works fine without a balun, he has probably picked a lucky length for his coax."

Or has absolutely no noise, tuned the dipole in situ, and doesn't care about the directive pattern.

Not a big deal to have radiating coax in a quiet, rural environment when you just want to do some casual operating.  A bit lossy because snaking coax across the earth isn't exactly a low-loss ground return...

Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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Posts: 13572




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« Reply #11 on: March 07, 2007, 06:58:30 PM »

On the other hand, I have known of one case where the lack of a balun
on a 40m dipole 12 feet up in an attic enabled the operator to work
stations a couple thousand miles away.  Adding a balun limited his
coverage to a few hundred miles.  That was, of course, because he
had a significant  vertically-polarized radiation component from his
feedline.  The problem was that when he plugged a piece of coax
into the unselected port of his coax switch the SWR jumped out of
sight.

Even though I'm out in the country I have a problem with RF noise
when I use any antenna fed against earth ground.  A balanced antenna -
even a coax-fed dipole without a balun - doesn't pick up the noise.
I was testing out an 80m DF receiver once and the noise nearly blew
my ears off so I tracked it - right to my incoming power wires.  (I can
hear a broken splice wire jingling on the power wire a quarter mile away.)

So using a balun will make things more predictable.  It may make your
antenna easier to tune, since changes to your station configuration
otherwise may change your SWR.  In my experience there is at least a
75% chance that the dipole will work fine without a balun, so don't let
the lack of a balun keep you from getting on the air.
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