[Elecraft] Toroids frequency range and baluns

[Jim Brown]

Hi Bob,

See comments interspersed.

On Mon, 29 Sep 2008 09:51:11 -0500, Solosko, Robert B \(Bob\) wrote:

>Jim,

>	I've been reading through your tutorial, and, if I understand
>it, the best approach is neither a plain wound coax balun nor a string
>of ferrite beads strung on the coax but is a balun made with  coax wound
>around a ferrite core in a way the minimizes capacitance - is that
>correct?

It depends mostly on the frequency range and the ferrite material, and to a 
lesser extent, on the power level. Think of the choke you're winding as any 
other inductor that has VERY low Q (typically around 0.5), and thus a very 
broad resonance. Like any inductor, we vary the number of turns, their 
diameter and spacing, and the core material to hit the desired resonance. A 
choke of #31 material can provide a strongly resistive impedance over a 
frequency range of roughly 4:1, so we can wind one to cover 160-40 meters. 
That choke would use closely spaced turns, because we need the additional 
capacitance and mutual coupling between windings to move the resonance down 
to about 80 meters. On the other hand, a choke to cover 20-10 meters needs 
wide-spaced windings, because we only want to move the resonance to about 21 
MHz. Power level enters the equation only to the extent that the choke must 
provide sufficient common mode impedance that it reduces common mode current 
to the extent that the P=I*E is small enough that it does not overheat the 
coax or the core. The tutorial shows that's an easily achievable objective 
once you realize that it's a key design parameter.   

>	But I still have several more questions:

>	- it appears that material 31 is the best material to use for
>baluns. 

It's the best material to use for a COAXIAL CHOKE that needs to work below 5 
MHz. #43 is equally good on 40M, and slightly better above 40M. If you're 
only stocking up on one material and buying in quantity for the best price, #
31 is the best choice. 

>In addition to my transmission line balun, I also have some
>problems with power supply birdies on 160m, and to a lesser degree on
>80m. Is material 31 still the optimum material for adding additional RFI
>filtering to my power supply (along with parallel capacitors)? 

These chokes kill common mode current on the cable you wind around them, but 
there can also be differential mode coupling that a capacitor ACROSS the line 
(that is, plut to minus) can suppress, and there can be direct radiation from 
insffficiently shielded circuitry. No external filtering will kill (or 
change) that direct radiation. 

>	- My transmission line balun is serving two purposes: as a
>current balun to reduce the RFI in the shack problem that I have had,
>and as a 4:1 transformer to better match the ladder line from my antenna
>to the short length of coax to my rig. (My 4:1 balun is made from 2
>separate cores as is the BL2, but they're much larger cores to minimize
>saturation and heating problems.) It seems to me, if I understand your
>tutorial, that the characteristics of the cores used for transformers
>(low resistance) and those used for suppression (high resistance) and
>mutually exclusive. Thus, does it make sense to have a single balun
>serving these two purposes, or is it better to optimize the balun for
>the transformer application (material ??) and have a separate balun
>optimized for suppression?

That's a very perceptive question. For the first part of the answer, study 
the photo of the high power DXE 4:1 balun in my Power Point presentation -- 
select it from 

http://audiosystemsgroup.com/publish  

That "balun" is essentially two chokes wound with parallel wire transmission 
line (that is, bifilar) around what could be #31 or #43 cores. On the 50 ohm 
side, they're wired in parallel, on the 200 ohm side they're in series. 
Because the chokes are bifilar, there's a lot of leakage flux in that core if 
you're running power. The choking impedance isn't very high either. A far 
better design would use coax, #31, and a lot more turns. That "balun" would 
have a much higher choking impedance, and would also be a lot more efficient.

The second part of the answer is to study the DXE catalog -- they sell a 
separate product that they call a common mode choke! When you study my 
measurements (in the Power Point) for three of their high power "baluns," 
it's obvious why -- they have very poor common mode rejection. 

So the short answer to your question is, YES!

Another point of clarification. Both the DXE two-choke series/parallel combo 
and the one I described wound with coax are NOT transformers -- they are NOT 
coupling signal through the core, they are using the core to form a choke. 
They ARE doing impedance transformation and balancing. So it is correct to 
call them "baluns" but incorrect to call them transformers. 

73,

Jim K9YC