Toroids frequency range and baluns

[n7ws]

Bob W1SRB writes:

>    There have been several e-mail threads recently about ferrite
cores and about baluns and transmission line loss with high SWR. Related
to this is a question that I haven't seen addressed anywhere - toroids
all have a specified frequency range but what happens when they're used
outside of that range?

>   To be more specific, I have a multiband fan dipole fed with
ladder line into a 4:1 current balun that then connects to my rig
through about 5 feet of RG-8. I made the balun using a pair of T200-2
powder iron cores, which have a specified frequency range of 0.25 to 10
MHz. Since I'm using this single antenna from 160m to 10m, I'm way
beyond the specified frequency range of the cores - is the balun likely
to be very inefficient above it 10MHz? The antenna seems to work very
well on 40m and 20m and OK on 15m. Since the band conditions haven't
been particularly good in the last few years that I've been using this
antenna, I can't tell whether my lack of many QSOs on 15m and above is
due to inefficiencies in the balun or to band conditions? What do you
think?


Bob, you are mixing apples and oranges.  Current (choke) baluns (that use cores) are almost always built using ferrite cores, not powered iron.  If you wanted to tune one to parallel resonance at some particular frequency, you might use iron, otherwise, you want ferrite that provides a high impedance to common-mode currents over a broad frequency range.   A resistive component in this impedance is not a bad thing.


Roy, W7EL, has a good description of balun operation here:

http://www.eznec.com/Amateur/Articles/Baluns.pdf

Wes N7WS


     
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[AC7AC]

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[Jim Brown-10]

On Fri, 19 Sep 2008 10:21:44 -0700, Ron D'Eau Claire wrote:

>The balun doesn't have any effect on the efficiency of the antenna unless it
>involves lots of coax at high SWR such as the big coil of coax some Hams use
>for a "choke balun". (They're FB as long as the SWR is low but, like any
>coax, the losses go up with the SWR.)

WRONG! There are three fundamental cases of ferrite "baluns" or chokes. 1) A
choke wound with coax  2) a choke wound with parallel wires (bifilar)  3) a
transformer (voltage balun).

Case #1: Coax contains 100% of the differential power (transmitter feeding
antenna) within the dielectric. The ferrite core of a COAX CHOKE sees ONLY
the common mode voltage and current. If the impedance of the choke is high
enough, the current is very small, so the dissipation in the choke is very
small.

Case #2: A choke with a bifilar winding (that is, two parallel wires, NOT
coax) puts a significant fraction of the transmitted power in the core. This
is "leakage" flux from the bifilar winding (really a short length of parallel
wire transmission line). This leakage flux is typically 30% of the
transmitted power, and is NOT related to common mode current.

Case #3: A VOLTAGE balun is VERY different -- it puts 100% of the transmitted
power in the ferrite.

All of this is discussed in the tutorials, previously referenced. BTW --
earlier work and publications by W7EL, W2DU, and W1JR are all very good, but
they were done 30 years ago. My work builds on theirs, and is considerably
more advanced. Also, the #31 material that is so useful for HF chokes was
developed only a few years ago.

73,

Jim K9YC





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[Jim Brown-10]

On Fri, 19 Sep 2008 20:45:57 -0700, Ron D'Eau Claire wrote:

>"Balun" is an unfortunate, all-inclusive and misleading term. One so-called
>"balun" may be as different from another "balun" in its requirements and
>application as a motorcycle is different from railroad locomotive.

Yes, it really is.

>I stand by my statement when talking about a "choke balun" consisting of a
>string of ferrite beads on some coax or a coaxial line on a coil form such
>as I  described. The only losses caused by such a balun will be those of the
>transmission line itself, which cannot be ignored if the SWR is high.

Yes on all counts. BUT -- the chokes wound as a coil are vastly superior.

>So-called "baluns" that transform impedances are, typically, transformers --
>often transmission-line transformers. They're a whole different animal and
>can be very unpredictable, especially when they use ferrite or powdered iron
>cores and are exposed to a wide range of impedances.  

Yes. Again, poor use of words by the industry.

73,

Jim K9YC


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[n7ws]

Ron wrote:

> So-called "baluns" that transform impedances are, typically, transformers --
often transmission-line transformers.

There is a type of "choke" balun that is both a balanced-to-unbalanced device and an impedance transformer at once.

Connecting two ferrite-loaded transmission line baluns in parallel at one end and series at the other will yield a 4:1 transformation.  Higher ratios are possible with more series/parallel combinations.

Wes  N7WS


     
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[Solosko, Robert B (Bob)]

Jim,

        Why are choke baluns wound as a coil vastly superior to choke
baluns made with a sting of ferrite beads? With the coil type, you're
adding additional coax loss... What am I missing here?

73,

Bob W1SRB

 

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Jim Brown
Sent: Saturday, September 20, 2008 4:17 AM
To: [hidden email]
Subject: RE: [Elecraft] Toroids frequency range and baluns

On Fri, 19 Sep 2008 20:45:57 -0700, Ron D'Eau Claire wrote:

>"Balun" is an unfortunate, all-inclusive and misleading term. One
>so-called "balun" may be as different from another "balun" in its
>requirements and application as a motorcycle is different from railroad
locomotive.

Yes, it really is.

>I stand by my statement when talking about a "choke balun" consisting
>of a string of ferrite beads on some coax or a coaxial line on a coil
>form such as I  described. The only losses caused by such a balun will
>be those of the transmission line itself, which cannot be ignored if
the SWR is high.

Yes on all counts. BUT -- the chokes wound as a coil are vastly
superior.

>So-called "baluns" that transform impedances are, typically,
>transformers -- often transmission-line transformers. They're a whole
>different animal and can be very unpredictable, especially when they
>use ferrite or powdered iron cores and are exposed to a wide range of
impedances.

Yes. Again, poor use of words by the industry.

73,

Jim K9YC


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[Jim Brown-10]

On Mon, 22 Sep 2008 08:28:28 -0500, Solosko, Robert B \(Bob\) wrote:

>Jim,

> Why are choke baluns wound as a coil vastly superior to choke
>baluns made with a sting of ferrite beads? With the coil type, you're
>adding additional coax loss... What am I missing here?

Study my tutorial. http://audiosystemsgroup.com/RFI-Ham.pdf

73,

Jim K9YC


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