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Author Topic: Balun or not balun, that is the question  (Read 991 times)
KB3PIW
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Posts: 2




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« on: November 09, 2007, 08:01:41 AM »

Hi guys,

I am putting up a G5RV this weekend and I've been reading a lot about how to home brew one.  I think I got most of the details straight, but one thing I cannot figure out:  some designs / discussions on the web (including the MFJ-1778 manual) say that a 1:1 balun is needed between the bottom of the 34 foot ladder line and the coax to the tranceiver, while other websites proclaim NO BALUN!....What do you guys think?  To B or not to B?

Mario KB3PIW
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WW5AA
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Posts: 2088




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« Reply #1 on: November 09, 2007, 12:10:41 PM »

Mario,

It all depends on what is being called a G5RV. A properly designed G5RV should not require a balun. However as said, these days anything thats about 105' long fed with twin lead or ladder line and a piece of coax is being called a G5RV. Poor Mr Varney.

73, de Lindy
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WW5AA
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Posts: 2088




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« Reply #2 on: November 09, 2007, 12:17:51 PM »

and by the way, the original G5RV was fed with 525 Ohm open wire feeder (34') and then 70-75 Ohm parallel line or coax. Good luck!

73, de Lindy
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N3OX
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Posts: 8852


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« Reply #3 on: November 09, 2007, 01:16:29 PM »

Mario,

The one thing I can say
 is that if a website or other source tells you you MUST NOT USE A BALUN on some antenna, then they are telling you YOU MUST ALLOW THE FEEDLINE TO RADIATE for the antenna to work properly.

I have found personally that using a good 1:1 current (also called choke) balun at the junction of unbalanced coax and balanced line or a balanced antenna has some real benefits.  In my location it tends to reduce noise that is otherwise conducted over the feedline onto the antenna!

My 15m dipole picks up 2-3 S-units less power line noise since I slipped a foot long string of #43 ferrite beads over the coax at the feedpoint to form a 1:1 choke balun.

When I started in ham radio in 1995, my parents' house was very electrically quiet and put up dipoles all over the place without baluns and had no problems.  I'm sure if I had measured the radiation pattern of the antenna it would have been something other than a nice clean dipole pattern because the feedline was a radiating part of the antenna, but that can help sometimes!  

At my current noisy house, I would never, ever put up a balanced antenna without a choke balun at the feedpoint.  It's not even worth experimenting with here ;-)

I would use good coax and put a 1:1 current/choke balun at the junction between the coax and the twinlead matching section.  

You can use a coil of coax on PVC pipe as an easy cheap choke balun, which a lot of people call an "ugly" balun.

Coax wound on a ferrite core can be a more broadband, higher impedance balun... but you have to get the right core.  

This document has a lot of very good information on this stuff, including balun and choke construction:

http://www.audiosystemsgroup.com/RFI-Ham.pdf


73,
Dan



 
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
W7AIT
Member

Posts: 487




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« Reply #4 on: November 12, 2007, 12:16:36 PM »

Should I use a balun on my antenna?  QUESTION AND ANSWER

I DO recommend a balun be used on any antenna HF, VHF, for the following reasons:

1.   Balances the antenna currents and voltages.  
2.   Makes antenna pattern uniform.
3.   Provides impedance match.
4.   Reduces & eliminates RF radiation from coax feedlines and helps reduce TVI, RFI.
5.   Provides balanced to un balanced function.  Most rigs are coax connector unbalanced outputs.  Most wire antennas including dipoles are balanced antennas.  A balun is required to match unbalanced transmitter circuit to balanced antenna circuit.  Gets all the power into the antenna evenly.
6.   Helps match impedances.  Baluns are wonderful impedance transformation devices and by doing this, gets all the power from the transmitter into the antenna.
7.   Conversely, direct feed – taking a unbalanced coax cable and attaching to a dipole – is awful because:
a.   Currents and voltages are uneven
b.   Pattern is all screwed up
c.   Impedances are mis matched and power is lost
d.   RF on the coax shield, increases TVI and RFI
e.   Rig case can be RF hot, RF burns to operator
8.   Many types of baluns.  Can be ferrite beads, coiled up coax around a coke bottle, a W2AU balun, etc.
9.   Come in different matching ratios.  1:1, 4:1, 12:1 etc.  Pick the proper one.
10.   Most handle full KW.
11.   VHF / UHF – still use balun & probably more important than ever at these frequencies.  Baluns for VHF/ UHF / Microwave are different than HF baluns.
12.   Not using a balun is one of the biggest reasons hams either get poor performance or no performance out of antennas (my antenna doesn’t work!).  If your antenna isn’t working, use a balun! (Also make sure the counterpoise is there too! See separate write up)
13.   Repeat after me “USE A BALUN!”.  
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K8AG
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Posts: 345




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« Reply #5 on: November 13, 2007, 12:32:36 PM »

I have been running a ZS6BKW for several years with several coils of coax at the end of the window line.  Before I put it on, the TV, phone and other equipment was badly disturbed by me keying up.  After the coils, no problem.

It keeps RF from the shack.

My 2 cents.

73, JP, K8AG
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KE3WD
Member

Posts: 5694




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« Reply #6 on: November 14, 2007, 08:14:24 PM »

Use the 1:1 air choke balun at connection point of coax to twinlead.

Disadvantages are none.

Advantages are that if you are in a situation where RF could radiate back down the coax, you will never know it because the balun will stop that.  


There's a period at the end of that last sentence, no matter what else someone will try to tell you on this subject.  See, they are wrong.


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WA7NCL
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Posts: 625




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« Reply #7 on: November 21, 2007, 09:03:07 AM »

Should have a balun.

But... since this is ham radio, and you are home brewing the antenna.  Why not try it both with and without.  You will learn something.

My guess is that it will be better with.
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K3DC
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Posts: 88




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« Reply #8 on: November 25, 2007, 06:23:18 PM »

I have a question in regards to the 1:1 air choke.  I'm planning on building a J-Pole and some documents say to use a choke and some say no.  I'm going with the choke.  What I've read is to use 4-5 turns at 5" diameter.  I count that as about 25" of Coax.  If I use the "Ugly Balun" technique are there any strict requirements that at least need to be met?  I'm building a j-pole for 2M so how much length of coax should I wind and does the diameter of PVC I use really matter?

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KB3GDD
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Posts: 4




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« Reply #9 on: January 28, 2008, 08:33:35 PM »

I can't give you precise measurements, but my 2-Meter copper J-pole requires a choke.  Mine is made of 5 turns of coax at 5" (rough estimates -- I didn't exactly use a ruler).  I used at various times zip ties or duct tape to secure it.  I put an in-line coax connector at the end so I could leave the choke connected but disconnect the feed line for convenience.

Two things were mandatory for solid transmit:  (1)  Use the choke.  (2) the placement of the center conductor and the shield makes a difference.

I can't remember which way to connect the center conductor and the shield, but there are only two options, so try both and see which measures or works better.  I think in my case it was the opposite of the written plan I used, so don't necessarily trust the plan you find on the internet.

I've read that the messy balun (I think that's the name?) is not efficient, and it is better to have the wire coiled side-by-side.  It made sense to me that the form would be important in making an inductor, so I kept the wire wrappings as close to side-by-side as possible.

Here's a plan from "Now You're Talking" (c) 2000, p. 9-5:  "At the antenna feed point, coil up 10 turns of coax into a roll about 6 inches in diameter.  Tape the coax turns together.  The inductance of the coiled turns tends to choke off RF currents on the shield."

My 2-Meter J-pole is fantastic with the choke and almost useless without it.
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KB3GDD
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Posts: 4




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« Reply #10 on: January 28, 2008, 08:35:07 PM »

"Ugly" is the name, I just couldn't see your post while I was writing mine!  :-)
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