[Elecraft] Balun for KAT2

[Bill Coleman]

On 1/12/02 9:54 PM, Don Wilhelm at w3fpr@peoplepc.com wrote:

>I may be opening a can of worms here because there are a lot of 'balun
>believers' out there, but I would start with no balun at all.

There's a lot of bad infomation about baluns out there.

>It WILL work,

Yes, it will.

>but there is a possibility that you may get some 'RF in the shack' -

Which is exactly what any balun is designed to prevent -- RF currents 
flowing on the outside of the coax.

> should
>that happen, I would recommend an RF choke on a short section of coax (use
>big coax for best efficiency).

The size of the coax doesn't matter, but if you coil coax, be sure to 
respect the minimum radius of the type of coax.

>You could use a coil of coax, but even
>better is the W2DU type of balun (a bunch of ferrite cores over the coax
>shield) - make your own with 12 to 20 FT-50-43 cores slipped over RG213
>coax.  You need more cores at lower frequencies because it is the choking
>inductance that does the job.

Coiled coax, W2DU baluns, or toroidial core current baluns (Guanella or 
W1JR types), all work on the same principle. So long as there is 
sufficient choking inductance, they work.

There's a few caveats. First, you must have sufficient inductance, but a 
coiled coax balun can have TOO MANY turns. The distributed capacitance 
can make the coil resonant on the higher frequencies. Lots of turns tends 
to move that resonance lower, which is exactly what you don't want.

6-8 turns on a 4-6" form is sufficient for 10-30 MHz. 9-12 turns on the 
same form works for 3.5-25 MHz. 16 turns for 1.8-7.0 Mhz.

Second, the ferrite core baluns have a wider frequency range, but the 
characteristics of the core come into play. This usually isn't too much 
of a problem for QRP operating, but QRO operators need to beware they 
don't heat the cores.

Third, the W2DU baluns depend on the size of the coax, type of material 
and the total length of the beads. The size of the beads isn't really 
important. Typically, a 7-30 MHz balun can be constructed with about a 
12-18 inches of beads. For lower frequencies, you need more inductance -- 
hence you need to double or triple the length.

Bead material is important. Type 43 material has maximum impedance at 200 
MHz, so it isn't very appropriate for HF baluns. (However, I have such a 
balun using FB1024-43 beads on my A3S -- works great) You're better off 
using type 77 material for an HF balun. You'll end up needing fewer 
beads. Power handling may be a problem for QRO operators with type 77 
beads, but it isn't an issue for QRP.

Forth, Guanella and W1JR baluns are just as effective as W2DU baluns, and 
can be made a lot cheaper. A FT240-77 core cost $9.00 from Amidon, 
whereas the beads required for a W2DU balun are closer to $18.00. If you 
buy commercial baluns, however, the W2DU types tend to fit well with your 
typical dipole installations, and can be lighter.

>The baluns made by winding bifilar (or tri-filar) turns on a core can be
>very lossy when used on a feedline having input impedances far away from the
>25 to 300 ohm impedances that these baluns are usually designed for and they
>are not very happy with reactive loads.

This is wrong on three points.

First, I believe you are talking about voltage-type baluns. These 
typically use powdered-iron cores instead of ferrite. Such cores have 
1/100 of the permeability of ferrite cores. These cores are not suitable 
for making current baluns.

Second, not all toroidial baluns are the voltage-type. The Guanella 
design uses a bifilar winding for a 1:1 balun. The voltage-type 1:1 
requires a trifilar winding. The W1JR consists of a short length of coax 
(usually a very small diameter type -- often teflon insulated -- RG-142, 
RG-303, RG-316) wound 3-9 turns on the core. There's no electrical 
difference between the W2DU, Guanella or W1JR balun.

Third, none of the discussed baluns (with the exception of the coaxial 
coil) performs well in the presence of reactive loads. Such loads can 
cause core saturation, heating and additional loss, unless accounted for 
in the design of the balun. 

However, you are correct that current-type baluns provide better balance 
to the load than voltage-type, regardless of the reactance present. The 
ARRL Handbook no longer provides descriptions of voltage-type baluns. 
Current-type baluns are recommended.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901