[Elecraft] Toroids frequency range and baluns

[Don Wilhelm]

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
>   
>