Posted by
Guy, K2AV on
Aug 07, 2010; 1:40pm
URL: http://elecraft.85.s1.nabble.com/OT-Backward-Baluns-tp5382873p5383608.html
Some true baluns are not transformers, but are parallel feedline
devices, transmission line transformers (TLT). Since they are wound
around toroids, the velocity factor is low. That means if the
frequency is high enough, the wound feedline can approach a quarter
wavelength. While HF use will not get to a quarter wave in most
designs, it's important to know the worst case to understand what is
happening as the frequency increases.
Suppose you DID attach 12 ohms to the (parallel) 50 ohm side of a
50:200 dual 100 ohm transmission line true balun, and 50 ohms on the
other (series 200 ohm) side. ALSO assume that you were at a frequency
high enough that caused the 100 ohm line to be an electrical quarter
wavelength. Work backward from the 50 ohm load on the "high" series
side. This will place 25 ohms on each of the feedlines. 25 ohms will
be transformed by a quarter wave of 100 ohms to 400 ohms. On the
"low" side two 400 ohm transformed loads in parallel will be 200 ohms.
Not exactly a good match for 12.5.
Stated differently, if you go high enough there is a frequency where a
50:200 transmission line toroid balun will transform 50 ohms to 200
ohms fed in the 200:50 direction. At lower frequencies than this
"flip" frequency, this 1:4 transformation will gradually drop down
through 1:1 to 4:1, but very reactive on the way, until at a frequency
low enough, the reactive component minimizes enough to not be an
issue. In these middle ranges there are balance problems, leading to
core heating, and inefficiency, and possibly core failure at high
power levels.
The question is how much of this perverse behavior you invoke at a
given frequency by trying to use in a way it was not designed for.
That is why wound transmission line 1:4 baluns are intended to be used
at 1/2 of the INTERNAL transmission line Z to twice the internal
transmission line Z and only that. Further, they do not do well with
transformations of highly reactive impedances and can become quite
lossy.
A common true TLT balun in use these days is the 50:450 for feeding
*well-matched* 450 line (three 150 ohm lines in parallel/series within
the balun). A popular use for this combo is very long runs where
feedline loss is crucial and long runs of 1 1/4 hard line are
out-of-budget. A TLT balun has excellent rejection of high common mode
currents from long runs.
Don't try to use one of these backwards to match 50 ohms to 16.7.
You can do baluns in quite some number of ways. They are not the
same. The devil is in the details.
Read Sevik's book. He lists designs for a 50:12.5, suitable for that use.
73, Guy.
On Sat, Aug 7, 2010 at 5:53 AM, Alexey Kats <
[hidden email]> wrote:
> Ouch, right, I was mostly thinking about QRP. On high power one has to
> start keeping track of what current each side of a balun was supposed
> to handle as well. Sorry for missing that.
>
> On Sat, Aug 7, 2010 at 2:49 AM, Guy Olinger K2AV <
[hidden email]> wrote:
>> It depends on what "balun" design you are talking about. Some 50:200
>> baluns take a pair of 100 ohm feedlines wound on ferrite toroids to
>> choke off common mode current, connect them in parallel on the 50 ohm
>> side, and in series on the 200 ohm side. This design will not do
>> 50:12.5. Others will not work well because the 50:200 assumes the
>> lower current on the 200 side at QRO, and a different design 50:12.5
>> is specifically wound with QRO high current of 12.5 in mind.
>
> --
> Alexey Kats (neko)
>
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