[HCARC] Baluns - Lots of Questions

[Kerry Sandstrom]

Hi Gary,

       Several years ago the ARRL published a book called "Transmission Line 
Transformers" by Jerry Sevick.  I think it was published in 1987 since that 
is the copyright date on mine.  The ARRL Antenna Book, any edition, also 
talks about baluns and transmission line transformers.

    They are called transmission line transformers because that is what they 
are:  transformers made from lengths of transmission line that are coupled 
together.  A 1:1 balun is basically a 1:1 transformer that is unbalanced on 
one end and balanced on the other.  A 1:1 balun can be made from two quarter 
wave sections of coaxial cable.  Chapter 18, figure 6 of the 18th edition of 
the ARRL Antenna Book shows this kind of a 1:1 balun.  Figure 5 of the same 
chapter shows a 4:1 balun made with a half wavelength section of coax.  VHF 
guys have been using these baluns for decades.

    HF baluns and transformers haven't been as popular.  You can imagine 
some of the reasons, quarter and half wave sections of coax for 80 meters 
aren't exactly small and light!  Many years ago, the 1950's, Heathkit sold 
an HF balun that was reasonably broadband that consisted of a two large air 
wound bifalar coils.  The ARRL Antenna Book 9th edition, Figure 3-41 shows 
this kind of a balun and how to connect it either for 1:1 or 4:1 operation. 
It worked from 80 through 10 meters.  They solved the 'you need a different 
one for each band' problem but they still weren't small.

    Several years ago ferrite material became available at reasonable prices 
with good RF performance.  Ferrite is a non-linear material.  I don't 
believe there is any linear ferrite!  When you apply one or more RF signals 
to a nonlinear device, the device wiil generate harmonics and 
intermodulation products from the applied RF signals.  Ferrites also have 
some losses.  When you wind a coil on a ferrite, you will get both harmonics 
and intermodulation products.  There is no way of avoiding it.  By the way, 
even air is nonlinear if you are running enough power.  I think the power 
density is Megawatts per centimeter squared.  I only wish I could run power 
like that.

    "Ham Radio", an old ham magazine that has not been published for ten 
plus years and was fondly (and not so fondly) called the Proceedings of 
Amateur Radio  because of its technical content, had an article about 
saturation in ferrites.  I'll see if I can find it.  I think the article was 
primarily concerned with ferrites in a receiver front end limiting the 
intermodulation performance of the receiver.  That should give you an idea 
of the problem.  Several years ago one of the ferrite balun manufacturers 
had a lot of trouble with their transmitting baluns generating harmonics and 
causing TVI.  It is a real problem.

    There are some ferrites that will operate at a higher power before they 
cause a problem.  They are still generating harmonics, they are just below 
the problem level at moderate power levels.  Baluns with more ferrite 
material, larger and heavier, will handle more power generally before they 
cause a problem.  I believe ferrite baluns that are used in high SWR 
situations have more of a problem.  Ferrite really came into their own in 
wideband solid state amplifiers.  Interestingly, they were running at 
impedance levels below 50 Ohm.  The input impedance of high power 
transistors used in wideband power amplifiers, your 100 W 160 to 10 meter 
XCVR for instance, is very low, often as low as 5-10 Ohm.  A couple 4:1 RF 
transformers to go from 50 Ohm to 12.5 Ohm and then to 3 Ohm were ideal. 
4:1 transformers to go from 75 Ohm to 300 Ohm are about the highest 
practical impedance levels.  You can build transmission line transformers 
that are 1:1, 4:1, 9:1, 16:1, etc.  I don't believe that there is a 6:1 
possibility.  You can build RF transformers for any ratio you want, but they 
aren't transmission line transformers.

    You will have to find someone else to tell you aboput voltage baluns and 
current baluns and choke baluns.  I personally think they are just a lot of 
mumbo jumbo.  I don't use ferrite baluns much myself.  I have an old Hy-Gain 
15 meter yagi that uses a balun and hairpin match.  I laso have a Heathkit 
antenna tuner that has a balun for balanced output, but I've never used it. 
I have built some solid state transmitters that use ferrite transmission 
line transformers to match the driver to the final stage.

    I'm sure thast this is more than you wanted to hear, but I don't believe 
you can understand the issues with any less.

Kerry