[TowerTalk] Balanced Transmission Line

[Riki, K7NJ]

Rick,

According to Sevick's "Transmission Line Transformers" (latest 5th edition), there is a technique for winding transformer cores with parallel lines in a way that doesn't create magnetic flux in the core(s) thereby reducing magnetic core losses (theoretically) to zero. Sevick shows how such windings are interconnected using integral multiples of them such as 2, 3 or 4 parallel conductor lines. The low impedance input is the ends of these lines that are all connected in parallel. The high impedance is obtained from the other ends that are all connected in series. The result is an impedance step up equal to the square of the number of such parallel lines which would be 4,9 or 16 (for 2,3,or 4 lines). For 50 ohms on the low impedance side, this technique yields possible high impedance step up to 200, 450, or 800 ohms (or even more, theoretically...). I've not been able to find a way for constructing a balun that has 50 ohm input and 600 balanced output that does not produce magnetic flux in the core.  Most likely, such baluns have some loss in the core.  But, a good design would minimize such losses to the extent that hopefully they are negligible. I am hoping to find such an excellent design for a 50 ohm to 600 ohm balun.

Granted, without the limitation imposed by the above technique, then many different impedance step-up ratios may be used, some of which are discussed in Sevick's book. I would welcome information on tested designs that includes insertion loss (as well as frequency coverage and power handling capability). Do you have data like this for any of the designs that you've mentioned?

73,
Riki, K7NJ

-----Original Message-----
From: Richard (Rick) Karlquist [mailto:richard at karlquist.com] 
Sent: Saturday, February 24, 2018 5:55 PM
To: Riki, K7NJ; towertalk at contesting.com
Subject: Re: [TowerTalk] Balanced Transmission Line

On 2/24/2018 10:49 AM, Riki, K7NJ wrote:

> My main concern with using 600 ohm OWL is the insertion loss for the 
> two 50 to 600 ohm baluns. From what I've read, transformers with 3:1 
> or 4:1 turns ratio (for 450 ohm or 800 ohm OWL) have negligible 
> magnetic flux flowing in the core, which doesn't seem to be true for a 50 to 600 ohm transformer.
> Therefore, it appears that a 50 to 600 ohm balun would have more loss 
> than either a 50 ohm to 450 ohm or a 50 ohm to 800 ohm balun of 
> similar  Comments, and especially information about proven designs for 
> 50 to 600 ohm baluns would be greatly appreciated.
> 
> 73, Riki, K7NJ


No problem!  If you use a 7:2 turns ratio, you get a 12.25:1 impedance ratio meaning 50 ohms converts to 612.5 ohms, which is only 2% more than 600.  Since you're making your own line, you can simply increase the spacing slightly to get it up to 612.5 ohms.

For ideas on how to make a 7:2 transformer, study 3:2 (2.25 to 1
impedance) transformer designs.

Another possibility is a 11:3 turns ratio, and 672 ohm line.  This lends itself to a balanced autotransformer design because
4+3+4=11.  You drive the center 3 turns out of the 11 turns.
This would be similar to my 50 ohm to 450 ohm balanced autotransformer where I drive the center 3 turns out of the 9 turns.  I think that design would work by simply adding the extra turns.

The line to line spacing has to be increased by a factor of 1.8 to get to 672 ohms, so that may be an issue for you.  With 10 AWG wire, you would be looking at 13.5 inch spacing, vs 7.5 inch for 600 ohms, vs 40 inches for 800 ohms.

73
Rick N6RK


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