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[OT] "Backward" Baluns

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alorona

[OT] "Backward" Baluns

A 4:1 balun is always shown and talked about with the low impedance 50 ohm on
the unbalanced side and 200 ohm on the balanced side.

Can it be connected backwards: 50 ohm unbalanced on the high impedance side, and
12.5 ohm balanced on the low impedance side?

Al W6LX
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Guy, K2AV

Re: [OT] "Backward" Baluns

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.

Alas, there is no simple answer to your question, and in every case
depends upon the nasty and subtle details. "Understanding, Building,
and Using Baluns and Ununs, Theory and Practical Designs for the
Experimenter", by Jerry Sevick, W2FMI (SK), is a good reference that
will get you sufficiently muddy in the details.

73, Guy.

On Fri, Aug 6, 2010 at 10:57 PM, Al Lorona <[hidden email]> wrote:

> A 4:1 balun is always shown and talked about with the low impedance 50 ohm on
> the unbalanced side and 200 ohm on the balanced side.
>
> Can it be connected backwards: 50 ohm unbalanced on the high impedance side, and
> 12.5 ohm balanced on the low impedance side?
>
> Al W6LX
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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> Post: mailto:[hidden email]
>
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Alexey Kats

Re: [OT] "Backward" Baluns

In reply to this post by alorona

I don't see why not - just like a transformer, if it was not
reversible it wouldn't work for receiving signals properly. After all
when you receive through it the antenna impedance is scaled back from
200 Ohm to 50 Ohm, is it not? If this is the case it'd scale 50 Ohm
down to 12.5 Ohm just fine.

But... If your antenna has 12.5 Ohm of resistance, what is its
reactance? If it is way too high it will not work efficiently no
matter what balun you use. And out of those 12.5 Ohm, what part of it
is radiation resistance?

On Fri, Aug 6, 2010 at 7:57 PM, Al Lorona <[hidden email]> wrote:

--
Alexey Kats (neko)
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Alexey Kats

Re: [OT] "Backward" Baluns

In reply to this post by Guy, K2AV

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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W8JI

Re: [OT] "Backward" Baluns

In reply to this post by alorona

>A 4:1 balun is always shown and talked about with the low impedance 50 ohm
>on
> the unbalanced side and 200 ohm on the balanced side.
>
> Can it be connected backwards: 50 ohm unbalanced on the high impedance
> side, and
> 12.5 ohm balanced on the low impedance side?

A voltage balun normally can be used in reverse. Frequency response changes
and moves lower.

A current balun using transmission lines has a little line balance problem
and much more limited high frequency response when reversed. How it works
depends on how long the transmission lines inside are, in particular one of
the lines (on the upper non-inverting TLT) and what you accept for SWR.

73 Tom

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Guy, K2AV

Re: [OT] "Backward" Baluns

In reply to this post by Alexey Kats

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)
>

Jim Brown-10

Re: [OT] "Backward" Baluns

In reply to this post by alorona

On Fri, 6 Aug 2010 19:57:20 -0700 (PDT), Al Lorona wrote:

>Can it be connected backwards

A MUCH better question is, EXACTLY WHAT ARE YOU TRYING TO ACCOMPLISH?

73, Jim K9YC

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