[TowerTalk] Fair rite materials for choke baluns

[Earl Morse]

Not a ham application but I had to make a balun for work to test a wide band antenna design for EMC immunity testing.  It covered 150 kHz to over 20 MHz at 15 kW.

Sevick's book will help you a lot in doing these designs (K9YC's writings too).  You will discover about all the limitations and mutually exclusive characteristics of ferrite transformers that will limit your design.  Figuring out the desired amount of inductance at the low frequency in order to have enough core and turns to achieve it without saturation and then finding out the number of turns limits your high frequency response when the winding inductance resonates with the interturn capacitance.

Any core can be the right core at least for a little while.  I cranked out 4 of the baluns of two different designs (Ruthroff and Guanella) using junk I had around the lab within a day.  The parallel transmission line tranformer worked better than the coaxial one for bandwidth.  Only one caught fire during testing but that was because it arced over since I didn't have nice teflon sleeving and Thermaleze wire to make them with.  

Wide band, high power transmission line transformers are difficult to make.  I would highly suggest limiting the design to the antenna at hand, I mean how often are you going to move your 1.8 MHz transformer over to your 28 MHz yagi?  It will certainly make the design a lot easier to realize.

Earl
N8SS

  
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You seem to be seeking the "holy grail" of a universal
choke covering 160 to 10 meters.  I don't know of
any antenna that covers 160 to 10 meters, so why
do we need a choke that does that?  So my opinion is
that is doesn't make sense to do that.  It would
be better to simply use the optimum ferrite material
for the particular band(s) that the choke is to be
used for.

A place where there actually is a need for a 160 to
10 meters (or even 6 meters) choke is in solid
state amplifier design, for the DC feed choke.  I
studied this problem extensively a few years ago
looking at all Fair-Rite materials, pretending I
could get any shape, etc.  I even made some measurements
to fill in gaps in the Fair-Rite data sheets.  After
all that, the winner was ... boring old 43 material.
Luckily, this is probably the most available material.
Even a lot of competitors make their own version.
However, I found that the imitations were markedly
inferior to Fair-Rite 43 in this particular application.
They would probably be fine for general purpose RFI
fixing.

Rick N6RK


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