[TowerTalk] Air wound bauln?

w8ji.tom w8ji.tom@MCIONE.com
Wed, 18 Nov 1998 19:11:14 -0500

Hi Brian,

> I recently dissected a commercial (name witheld here) bauln.  It failed
> due to failure of the enamel coating between the trifilar windings.
> It was a 1:1 bauln supposedly covering the ham bands and rated "1.5Kw".

That means it was a voltage balun, the worse kind to use for insuring good
That also means full feedline voltage is applied across the thin enamel

Peak voltage is 1.414 times the sqrt of P times R, or 388 volts at 1500
watts into 50 ohms.
The peak voltage is 550 volts with a 100 ohm load.

> The last air wound design I saw was a 4:1 Heath-Kit design one.  It used
> two coils of about 12" long and 3" in diameter.

With air gap sufficient to stand at least 1000 volts between the windings. 
Normal insulation thickness for enamel wires is ten mils for single layer
enamel, normal voltage limit is 40 volts per mil or 400 volts. Proper
designs keep peak voltage below 75% of rating in dry room temperature
environments at low frequencies, and enamel has a poor dielectric constant
at radio frequencies. Further derating is necessary due to dielectric
heating of the insulation. since that is the among the poorest of RF

Knowing this, would you use it in a balun with a wire carrying the opposite
polarity next to it with no air gap between windings?

73 Tom

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