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Author Topic: 1:1 balun for Inverted v  (Read 2364 times)
AC2CH
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Posts: 20




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« on: January 24, 2010, 04:54:29 PM »

I am building an inverted v for 40 meters. I plan on winding a 1:1 air choke balun using RG-58. How many turns at what diameter would i need to use to prevent coax shield from radiating RF?


Joe
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KB5YAU
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« Reply #1 on: January 24, 2010, 05:35:34 PM »

Hi Joe,
I used the very same thing for my 40 meter inverted V and used the measurements out of the ARRL Handbook and it works great.  The length of the RG-58 is a length of 15 feet wound into 6 turns which will be a bit of experimentation till you get it right or at least that is how it worked for me.  73 and hope this helps.  
Mike KB5YAU
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KD8GEH
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« Reply #2 on: January 24, 2010, 07:05:23 PM »

I use 20-22 feet of RG8X wound side by side on a 8 in pvc or plastic coffee can. Works well for me. Side by side winding works better then a plain ugly rough wound one.

Let me know if you need any help.

73,  Dave  KD8GEH
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AC2CH
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« Reply #3 on: January 24, 2010, 07:41:22 PM »

Mike KB5YAU,
Appreciate the feedback WRT the air wound choke. What type of VSWR does your set up yield at the antenna's resonant frequency? How high did you set the center feed-point? What is approx. apex angle? I know that factors specific to my antenna location will affect my SWR, but at least I will have a sound starting point.

My other consideration is the trade off between height and the horizontal space available. I feel that the height sure be sufficiently great so as to produce a good broad side figure-eight pattern. The problem is that the higher one sets the center feed-point, the more horizontal space is needed to produce a Right/Obtuse apex angle.

So I have two questions:

Does the transmission line have to exactly bisect the apex angle or could one angle be 40 degrees and the other be, say, 55 degrees?

When looking directly down onto the antenna, do the two leg of the V have to have a radial angle of 180 degrees, e.g., 9 0'Clock and 3 O'Clock or can I place it at say 120 degrees e.g., 10 O'Clock and 2 O'Clock? I would assume this would create a loop-sided figure-eight pattern, with one lobe greater than the other
 
Any help would be appreciated.

Tks,
Joe
AC2CH
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VK1OD
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« Reply #4 on: January 24, 2010, 07:53:26 PM »

I am building an inverted v for 40 meters. I plan on winding a 1:1 air choke balun using RG-58. How many turns at what diameter would i need to use to prevent coax shield from radiating RF?

Joe,

Your posting implies you are making a single band antenna.

Of of the problems with the type of balun you describe is that the choking impedance varies widely with frequency, and they are typically resonant somewhere within the HF range. Resonance is not bad, they are actually very effective near resonance, and not very effective well away from resonance. In fact, the most effective application of this type of balun is to a single band antenna when the balun is self resonant around that frequency range.

That is not hard to arrange, but you do need something to measure the choking impedance and to adjust the turns.

So, it is real cheap, and can be real effective, provided you have the knowledge, skills and equipment to adjust it.

There is an example of a self resonant balun of this type at An antenna for 7MHz local contacts. It was very effective, but it is not in use now because birds attacked it and it was replaced with something more bird resistant.

Owen
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AC2CH
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« Reply #5 on: January 24, 2010, 07:55:23 PM »

Dave KD8GEH,
I was thinking PVC, but the plastic coffee can idea saves me a trip to Home Depot, (but I got to tell you I could spend 6 hours wandering around that place.)
I would love to encapsulate the entire thing in shrink tubing, but I doubt they make it 8+ diameter.

Burning 20~22 ft of coax for a choke does not make happy, but it is what it is. Would that length still applied when using RG-58?

Thx,
Joe
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W5DXP
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« Reply #6 on: January 25, 2010, 07:43:25 AM »

6 turns of coax doesn't give much choking impedance on 40m. The following article sheds some light on the ugly balun problem.

http://www.k1ttt.net/technote/airbalun.html

For 40m, it says:

            6 Turns
           4-1/4 in
          sngl layer
          ----------
Frequency  Mag Phase
7.00E+06   190  89.4

190 ohms is not nearly enough for an effective choke. 1000 ohms is a lot better. Some antennas (not a resonant inv-V) need a 10k ohm choking impedance.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5WSS
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Posts: 1742




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« Reply #7 on: January 25, 2010, 08:27:29 AM »

One can increase the choking effectiveness by adding a second choke of this design. One at the antenna and one at the equpment. Just a heads up. 73
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KD8GEH
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« Reply #8 on: January 25, 2010, 08:42:34 AM »

Cecil & Owen both have good points and are much more knowledgeable than I.  

I use my chokes generally on multiband di-poles, loops and inverted L's.  Check out the site Cecil mentioned, really good data there. They imply that straight 40 meters, 12 turns of 213 works well. That’s a pretty big choke, and as mentioned, without proper equipment, there is a bit of trial and error.  I have used the same method with 58 no known issues. I run up to about 600 watts no RF in the shack. With lower power, its less of an issue in my experience.

73,
Dave KD8GEH
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AC2CH
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« Reply #9 on: January 25, 2010, 08:55:11 AM »

Cecil W5DXP,

Thanks, I found the article to be just what I what I needed and even printed it out (I don't care what anyone says, reading off a computer monitor does not compare at all with reading from a piece of paper physically in one's hands.) Up to now the info I have found on air wound chokes has been based anecdotal evidence, not to  say it's incorrect, the article's author provided empirical results that I could more confidently hang my hat on.

I wish I could find a Java Applet calculator for air wound chokes.

Thanks again,
Joe
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AC2CH
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« Reply #10 on: January 25, 2010, 09:13:48 AM »

KD8GEH,

Dave, I was thinking that too. I have never worked with RG213, but from the specs - it looks like RG8. I am reluctant to use that length of RG8 for a choke instead of height. And I imagine struggling trying to roll that cable into 6 ~ 8 inch loops. Plus, the RG8 is definitely ruined after that; I would never trust RG8 that has been unwound from that tight a radius as transmission line again. The insulation has to take a real hit with that.
I  would like to use RG58 if there were no real discernible difference.

Joe
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K1BXI
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« Reply #11 on: January 25, 2010, 09:31:10 AM »

Here is one for Cecil and Owen....since his coaxial fed dipole is only for 40 meters, wouldn't he do just as well with a straight run of coax, since it appears that this coaxial wound choke is going to be of little choking impedance at that frequency,

Perhaps a length chosen to present a high impedance on the outside of the braid at the center feed point if he is worried about common mode on the outside of the coax.

John
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W5DXP
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« Reply #12 on: January 25, 2010, 11:01:06 AM »

> AC2CH wrote: I wish I could find a Java Applet calculator for air wound chokes. <

There's an inductance calculator that will probably do what you need done. Pay attention to the "self-resonant frequency". Hope you like metric. :-)

http://hamwaves.com/antennas/inductance.html
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W5DXP
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Posts: 3613


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« Reply #13 on: January 25, 2010, 11:24:09 AM »

> K1BXI wrote: ...since his coaxial fed dipole is only for 40 meters, wouldn't he do just as well with a straight run of coax, ... <

An inadequate choking impedance is better than zero choking impedance. IMO, what he needs is some high-perm ferrite helping out his air-core coil. Or, if it is a single-band 40m choke, simply coil up more turns. 20 turns would probably make a good choke on 40m.
--
73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
VK1OD
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Posts: 1697




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« Reply #14 on: January 25, 2010, 12:05:27 PM »

....since his coaxial fed dipole is only for 40 meters, wouldn't he do just as well with a straight run of coax, since it appears that this coaxial wound choke is going to be of little choking impedance at that frequency,

Perhaps a length chosen to present a high impedance on the outside of the braid at the center feed point if he is worried about common mode on the outside of the coax.


John, there are many different ways to implement a balun.

Yes, you could arrange the outer of the coax so that it offered a high impedance to common mode current. For example, a dipole of 20m length, at a height of 10m, centre fed with coax feedline which drops vertically to ground where the shield is connected to ground is likely to have very low common mode current because the quarter wave vertical line section (comprised of the outer surface of the outer conductor of the coax) has a high impedance at the point that it is connected to one side of the feedpoint.

Some folk might even cite such a configuration as proof that baluns don't do anything and aren't necessary. But, the configuration does include a balun, just that they haven't recognised it.

However, it might be better to use a lumped balun of some kind rather than to constrain the antenna dimensions to the balun described above.

If I were to use a coil of coax for a balun for a single band antenna, I would make it self resonant.

Otherwise, I would use something that I knew had a high common mode impedance.

There is not much point installing something that looks like a balun unless it works like a balun. (Its behaviour at 14MHz demonstrates that the feedline configuration is an effective balun at 7MHz and not at 14MHz.)

Several authors have measured the common mode impedance of 'ugly baluns' of various construction and have shown them to not have the usually desired characteristic of high common mode impedance over a wide frequency range.

Opinion on adequate choking impedance is moving (albeit slowly) towards favouring quite high values, Rule 500 doesn't stand the test, and baluns with a choking impedance of a couple of hundred ohms are pretty much a waste of time.

Owen
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