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W2DU balun, damaged?




 
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Author Topic: W2DU balun, damaged?  (Read 1093 times)
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K6JEK
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RF in the shack


« on: May 08, 2017, 08:03:25 PM »

Can you cook a W2DU choke balun by (accidentally) pushing high power into it with very high SWR, i.e, wrong position on the antenna switch? If so, how can you tell it's dead?

As you will recall, the W2DU choke balun is a string of mix 73 beads strung on Teflon coax, RG-303. I suppose the beads could overheat and crack. Is there any other kind of failure? Does the ferite transform into un-ferite?

75M dipole is acting squirrely. SWR is up, resonant frequency is down a bit. I can't see anything obvious. I have yet to pull it down.
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AB2EZ
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« Reply #1 on: May 09, 2017, 11:56:17 AM »

If the ferrite cores get too hot:  they can crack (like a ceramic cooking bowl). Separately, if the ferrite material is heated to a high temperature (specified for a specific ferrite material as the "Curie Temperature"), the material will lose its ferrite properties... which will not be restored after if cools. For Type 43 material, the Curie temperature is 130C.

Without knowing the specific core material and the size (length, outer diameter, inner diameter) of each core, and the number of cores.... or the inductance specified for the balun at a specific frequency... it will be hard to tell if the balun is fully functional (i.e. no cracked cores).

But, you can make a rough check for catastrophic damage (when you have access to the balun) by measuring the common mode impedance of the W2DU balun (input connector center pin to output connector center pin) at (for example) 3.5MHz. The input-to-output common mode impedance should be much larger than j50 ohms.

Alternatively, you could measure (roughly) the common mode current flowing through the coaxial feed line at some point near your transmitter. The amplitude of the common mode current flowing through the coaxial feed line should be much less than the amplitude of differential mode current... i.e. much less than 1.414 x the square root of [the RF input power to the feedline / 50 ohms].

You could make this measurement using a snap-on core and piece of insulated wire and a 50 ohm non inductive resistor as follows:

Make a 10:1 current transformer by winding 10 turns of insulated wire onto the snap-on core, and placing a 50 ohm 1/2W (or larger) non-inductive resistor across the 10 turn winding. Snap the snap-on core onto the coaxial feedline. With about 10W of RF flowing into the feedline, measure the amplitude of the RF voltage across the 50 ohm resistor, using an oscilloscope. If the W2DU balun is doing its job of choking the flow of common mode current in the feedline, then the amplitude (not the rms value) of the RF voltage across the 50 ohm resistor should be much less than 1.414 x (50 ohms/10) x the square root of [10 watts / 50 ohms] = 3.16 volts.


Stu
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
wa1mtz
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« Reply #2 on: May 09, 2017, 03:44:00 PM »

I once had a W2AU 1kw  4:1 balun that dramatically cooked  in about 2 min. of 150w carrier ! apparently the load I was going into with the open wire line had hi swr, I had to saw off one end of the pvc housing,to open it up, and the solder on the inside of the coax connector melted and the ferrite rod was cracked in half. Not saying yours is...but maybye solder connection inside ??
73, mike, 'mtz
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K6JEK
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« Reply #3 on: May 09, 2017, 09:17:17 PM »

Thanks, guys.
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KD6VXI
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« Reply #4 on: May 09, 2017, 11:32:20 PM »

Ditch the W2DU balun.  Download Jim Brown's choke cookbook, spend 25 dollars one time and be done with common mode.

Walt's design was great, as long as you have a LOT of beads.  That was the problem with the commercial variants.  Between 1 and however many beads, and lean this number towards profit.  Don't use enough, and they start overheating.

I qso with Walt more than a few times on a feed point choke for a serious qro problem.

Newer mixes of bigger cores mean 5k ohms of choking impedance or better. 

For practical use, a pair of Jim Brown style chokes at the feed point will handle the power of the 3cx3000A7 I built.

5 to 7 turns through a single core will have a LOT more choking impedance than a string of beads does unless you spend a LOT more on the quantity of beads.

--Shane
KD6VXI 
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