Toroids frequency range and baluns

[Solosko, Robert B (Bob)]

Hello All,

        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 W1SRB
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[Jim Brown-10]

On Thu, 18 Sep 2008 16:20:56 -0500, Solosko, Robert B (Bob) wrote:

>toroids
>all have a specified frequency range but what happens when they're used
>outside of that range?

To gain a better understanding of ferrites study my tutorial.

http://audiosystemsgroup.com/RFI-Ham.pdf

Look at the measured data in that tutorial. STUDY the Fair-Rite data for
their products.

73,

Jim Brown K9YC


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[Don Wilhelm-4]

Bob,

To further add to the confusion, I will throw in my 2 cents worth too.  
Let me say that a balun wound on a powdered iron core is highly
dependent on the inductance.  The inductive reactance should be at least
5 times the feedline impedance at the balun's antenna side for it to be
effective, so how well it works depends on the antenna too.  Also as the
frequency gets higher, the resistive loss through the balun becomes more
significant and the interwinding capacitance becomes important.

For all those reasons (and a few more too), it is common to use a
ferrite core rather than a powdered iron core for a balun.  The
inductance per turn is much higher, so fewer turns are needed to satisfy
the inductance requirement at the low frequency end of the scale and the
wire resistance  and winding capacitance is less at the high frequency end.

The core for a balun can be lossy at the frequency of use.  You can
think of a current balun as a transmission line inside a choke (the
coil-of-coax type baluns are a perfect example).  The differential
signal travels on the transmission line (inside the coax) while the
common mode signal on the outside of the coax (the one you want to
suppress) encounters the choke impedance.  The advantage of a ferrite
core is that the transmission line length is shorter than a balun built
with a powdered iron core - fewer turns = lower resistance, lower
capacitance.

I know that did not answer your question directly, but it all depends on
the inductance of your balun, the antenna side actual feed impedance,
the wire resistance, and the capacitance across the transmission line
inside that balun.  I don't have enough data to provide a definite answer.

73,
Don W3FPR

Solosko, Robert B (Bob) wrote:

> Hello All,
>
> 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 W1SRB
>  
>

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