[TowerTalk] Fair rite materials for choke baluns

[Richard (Rick) Karlquist]

On 7/3/2016 7:55 AM, Rudy Bakalov via TowerTalk wrote:
> I have been thinking of building a few chokes for my station and long feedline runs. As I started looking into the different material mixes that Fair Rite offers I came across some that have not received any attention by the ham community, at least judging by the lack of written materials. Mixes 31 and 43 seem to get all the attention.
> So, I was wondering why no references to mix 52 for the higher ham bands and mix 71 for 160m? Looking at the Fair Rite online catalog it seems that a 3 toroid design, using cores of materials 71 (160m), 31 (80-40m), and 52 (20-10m).
> I also looked at this table, which also seems to indicate that 52 is better than 43
> http://karinya.net/g3txq/chokes/
>
> So, what am I missing? Why is 52 rarely mentioned and the debate is usually 31 vs 43? Does it make sense to build a "universal" choke using 2 or 3 different cores to cover the entire 160 to 10 m range?
>
> Rudy N2WQ
> _______________________________________________

Sorry for the long answer, but this is not a
simple question to deal with.

The general answer to your question is that many
mixes are only available in a limited selection
of shapes.  So a material that might otherwise
be suitable, might not come in the shape you
need.

The other issue is that stock in "distribution"
(IE available from authorized Fair-Rite distributors)
is a subset of what is shown in the Fair-Rite
catalog.  What is not stocked, may require waiting
an indeterminate time until it happens to be in
stock.  Or it may mean having to place a large
minimum order and wait many months.

If you can come up with a compelling "value proposition"
for some specialty ferrite component, maybe you
could organize group buy to get over the threshold
of minimum buy.

It is also possible to go directly to the manufacturer
and have them make a run of custom ferrites.  I did this
for a client once, where we wanted a custom shape in
68 material, which is normally only made in toroids.
The client was building CO2 lasers excited at 81.36
MHz and 68 material was the only material that worked
at that high of a frequency, IIRC.  The client had to
pay several thousand dollars for this run.  That
wouldn't be out of the question for a ham group
purchase IF there was a real advantage to be gained.

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