[Elecraft] Balun for KAT2

[Bill Coleman]

On 1/18/02 7:58 AM, Charles Greene at W1CG@QSL.net wrote:

>At 04:46 PM 1/17/2002 -0500, Bill Coleman wrote:
>
>>Bead material is important. Type 43 material has maximum impedance at 200
>>MHz, so it isn't very appropriate for HF baluns.
>
>The 200 MHz pertains to coils, not to baluns; type 43 material works very 
>well on HF baluns.  I have some type 43 material current baluns that work 
>well from 700 KHZ through 30 MHz.  The balun made with type 43 material is 
>somewhat more lossy than the powdered iron cores ones and the type 77 
>material even more so, but losses still are reasonable for low power 
>operations.

If you read what I wrote, you'd see that I never said that type 43 
materials couldn't be used as a balun. In fact, read the parenthetical 
note that follows:

>>(However, I have such a
>>balun using FB1024-43 beads on my A3S -- works great) 

And the important point:

>>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.

Type 43 materials work, but you'll have to use more beads to get 
sufficient inductance. More beads means more cost. Why pay more when you 
don't have to?

>>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.
>
>Core saturation is a factor in coils, but is not a factor in current baluns 
>if the balanced current is maintained.  The currents in the coiled 
>transmission line are equal and opposite, and the net flux is zero.  

Yes, but you also have to deal with the fact that the choke impedance is 
shunted across half of the antenna impedance to ground through the coax 
sheild.  

This causes some current to flow, so there is some flux. If the choke 
impedance is high enough, this current is small.

When you deal with reactive loads, though, things change a bit.

>I have 
>done extensive testing of baluns into reactive loads with high SWRs.  The 
>loss increases, but the balun will work well into a SWR of 5:1.  The losses 
>are primarily dielectric and eddy current losses.  I^2 R losses contribute 
>just a small amount to heating.  The core stays cool until heated by the 
>wires.  The wires get hot as that is where the loss is.  Core saturation is 
>another of the misinformations commonly expressed about baluns.

Saturation isn't so much the problem as loss tangent. If you cause 
heating in the core, likely due to eddy current losses, the losses 
increase as the core gets hotter. The increase in loss leads to more 
heating, which leads to more loss. Eventually, the core will reach a 
point where the increase in loss causing enough heating that it will heat 
right up to the curie temperature, at which point it ceases to act as a 
permeable core. Do that enough times, and you can destroy the core.

W8JI has done a lot of testing. There are some "2 kW" commercial baluns 
that fail in 5 minutes under his test conditions.

>  Yes, you 
>can cause core saturation, but the other losses will probably destroy the 
>balun first.  One phenomena I have noticed during loss testing is the run 
>away condition.  If you apply high power to a balun working into a reactive 
>load, the balun wire will get hot.  That increases the losses, causing even 
>more heating.  That condition continues until the wires in the balun burn 
>up. 

This is Loss Tangent, discussed above.

Perhaps I should clarify. Eddy current losses are lower when the core is 
far from saturation. Large cores can deal easily with the heat. As the 
core gets closer to the saturation point, the losses increase, and the 
core can't disappate the heat. 

Most likely, a core in this service reaches the loss tangent point long 
before it goes into full saturation.

> Another factor to watch out for is excessive voltage in the 
>balun.  Many low power baluns use
>only enamel insulated wire wound on the core.  The enamel insulation has a 
>break down voltage of about 500 volts, and worse case with a 5:1 SWR you 
>can get nearly 500 volts in the balun.  Most core materials have a varnish 
>insulation which also has a breakdown voltage of about 500 volts.  However, 
>the wire can have nick in the insulation where it passes over the 
>core.  Type 77 material has no varnish insulation and you need to be 
>careful when using it for a balun including winding a coax like RG-316 
>around it.  A higher power core can be wound with teflon tubing over the 
>wires, or wrap the core in glass tape.  I have also successfully used 
>teflon insulated, silver plated stranded wire for baluns.

Voltage-type balun kits using T200-2 cores often came with fiberglass 
tape and teflon-insulated wire for just this reason. 

High voltages are less of a concern for current baluns, since you aren't 
transforming the voltages of the signal. But, with reactive loads, larger 
voltages (and hence, much higher currents in the antenna half to ground 
circuit, and thus higher core flux) are present.

>>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.
>
>Sevich devotes an entire chapter in his book "Building and Using Baluns and 
>Ununs."  to baluns for antenna tuners and recommends voltage baluns, 
>sorta.  He goes to great pains to flatten response of the balun used by 
>McCoy in his original transmatch.    However, there are 2 problems with the 
>voltage balun used in the antenna tuner.  As the as the frequency is 
>increased, the phase shift in the transmission coiled transmission line is 
>no longer able to sustain a 4:1 ratio and the high frequency performance 
>falls off.  The second problem is a result of the first.  In an attempt to 
>extend the high frequency range, turns are removed from the balun until at 
>the lower frequency there is barely enough inductance to prevent the 
>primary current from flowing in the balun, decreasing the current to the 
>load and increasing losses.  This condition becomes worse when working into 
>a high impedance load.  As the load impedance increases, more primary 
>current flows in the balun as the impedance of the balun to choke the 
>primary current doesn't change and becomes a smaller ratio of the total 
>impedance.  I made a bunch of voltage baluns, but finally gave up on 
>them.  All the baluns I made would cover 160 through 15 or 80 through 10, 
>but not both.  BTW the T184 size is a good compact powdered iron core to 
>use if you are into voltage baluns, as it has an inductive index higher 
>than any core smaller than 4 inches.

Most voltage baluns I've seen use T200-2 cores. In fact, I have at least 
three 15-year old balun kits using these devices. They are meant as "2 
kW" baluns, but would be reasonably efficient in QRP or Low Power service.

I think I'll stick to current baluns, and use the balun kits as a source 
of teflon wire for my FT240-77 Guanella baluns....



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