[Elecraft] BL-2 Balun Heating

[Paul Christensen]

Gary,

What frequency of operation causes the balun overheating?  Assuming the 
balun was constructed correctly, I would begin looking at trying to optimize 
line length if you favor that band of operation.

For example, when using a 100 ft. dipole at 3.8 MHz and 1/4-wave in height 
above ground, the optimal feedline length of 450 ohm transmission line 
occurs at roughly 27, 143, or 260 feet from the antenna feed point.  The 
50-ohm RG-213 or 1:1 balun would couple at one of these distances.  At these 
lengths, the 50-ohm line VSWR on your RG-213 would approach 1:1 while using 
any 50-ohm line length although the balanced open line VSWR is always 
determined by the line to antenna mismatch and not considering line loss, 
VSWR on that section of the line is constant irrespective of line length.

When coupling the 50-ohm line (or balun) to the balanced line at a 
line-current maximum point, a tuner is generally no longer necessary. Worst 
case is that the tuner is used as a touch-up device to ensure full band 
coverage but at least the balun is no longer seeing wildly outrageous Z 
values since the change in line length functions as a variable RF 
transformer to achieve Z values reasonably close to 50-ohms at the balun.

Paul, W9AC


----- Original Message ----- 
From: "Gary Winkelman" <thewinks02 at gmail.com>
To: <elecraft at mailman.qth.net>
Sent: Thursday, November 19, 2009 10:02 PM
Subject: [Elecraft] BL-2 Balun Heating


>I recently built a BL-2 Balun.  It is connected between my Palstar AT1KM
> tuner and my doublet antenna.  The antenna consists of a 100' wire up
> 25' and fed by 95' of 600 ohm open wire feeder.  I'm using the 1:1
> setting on the balun.  There is a 4" section of RG-213 between the balun
> and a coax input of the tuner.  I'm driving it with 125 watts from a
> TS-850.   After no more than a few minutes of CW or RTTY transmission,
> the balun is too hot to touch, the SWR rises and the power goes down.
> The balun is obviously saturated.
>
> Does anyone have an idea how I can reduce the overheating?  I've tried
> adding a heat sink but it doesn't make any difference.
>
> Tnx for reading!
>
> Gary  N2UM
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html