[TowerTalk] Voltage or Current Balun?

[Tom Rauch]

Hi Bill,

I hope no one reaches their curie temperature....but......

> Someone else suggested in a direct e-mail to go to the Home Depot 
> electrical and buying a can of Plastic Dip. A bead balun could then be
> successively dipped in the Plastic Dip to seal and protect it.

I never worry about keeping beads dry. The only parts that are 
water problems are the connectors. Better to have air over the 
beads than to insulate the poor things. They have low enough 
thermal conductivity as it is, and if the beads reach the curie 
temperature the balun is no longer a balun unless it cools back off.

I haven't found a balun yet that does not reach curie at rated power, 
although I'm sure no one knows the difference unless they are 
running a multi-multi station or measuring common-mode current in 
the system.

I dug out some test data sheets on baluns I did about 5 years ago, 
here are the times it took baluns tested at 14 MHz to reach curie 
temperature (where the beads quit being beads) CW with a 70 
degree F starting point. I won't post all the data, but here are two 
typical devices:

W2DU "Big Signal" 40-10 meter balun@ 375 watts 1 minute 50 sec
W2DU "Big Signal" 40-10 meter balun@1500 watts 37 seconds

HyGain BN4000D 4kW balun@375watts infinite time
HyGain BN4000D balun@1500watts 1 min 30 sec

I roll around on the floor holding my sides when I read "Big Signal" 
and see the balun reaches curie in 2 minutes with 375 watts. 

I have yet to find a bead balun or core-type balun that lives up to a 
manufacturer's claims in a worse case test. That case is where 
you transpose the ground and center at the output of the balun in a 
standard test fixture, subjecting the balun to a common mode 
voltage of just under 300 V RMS @1500 watts output.

That's why, despite the fact I have boxes and boxes of sample 
baluns and cores, all of my antennas have air-wound chokes.

Now in fairness there might be a choke balun or two out there that 
will pass a worse case test at rated power. I just haven't seen one 
yet.

> Seems like a W1JR balun might be a candidate for such treatment. 
> (Although the plastic would certainly increase the inter-winding 
> capacitance and lower the series-resonant frequency.)

It also lowers the self-resonant frequency, which can be a good 
thing. Roy Lewallen W7EL uses that scheme in his scramble-
wound choke baluns.

Maybe I'm reading this wrong, but it almost seems that we 
sometimes think any capacitance across a balun or inductor is a 
"bad thing" and always decreases impedance.

Extra capacitance in parallel with an inductor increases the 
impedance unless we wind up operating at 1.414 times the self-
resonant frequency or higher. At that 1.414 times the self resonant 
frequency, it is the same reactance as if the choke had zero 
capacitance in parallel....except the sign of course is opposite.

As long as we are debunking core-type choke baluns, moving the 
balun from the output of a "floating" tuner to the input does 
absolutely nothing to improve balance. Look at how many articles 
claim it does make a difference.
73, Tom W8JI
W8JI@contesting.com 

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