Hi Bob,
See comments interspersed.
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.
- 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