Hi Rick,
Thanks for taking the time to provide such a thoughtful response! Much
appreciated.
So, let me clarify a couple of points:
1) The use case for a universal choke is at the output of the amplifiers and at
the base of my tower. I run a SO2R switch and all bands go via one of the two
feed lines from the shack to the SO2R switch at the base of the tower. While
using different materials for different bands certainly makes sense (and is
practical), it won't work for a true honest to God SO2R setup.
2) The reality of commercial availability did hit me fairly quickly, so I
decided to limit my choices to materials that are readily available- both 31
and 52 meet the requirement. You can buy large OD mix 52 toroids from the usual
distributors.
What I am wondering is if using different mixes in the same choke would
"flatten" the curve and extend it across the entire 160 to 10 m range. The
second question I wonder is if I can stack the two different mixes or that
would be undesirable and they should be in series.
Rudy N2WQ From: Richard (Rick) Karlquist <richard@karlquist.com>
To: Rudy Bakalov <r_bakalov@yahoo.com>; Reflector -tower
<towertalk@contesting.com>
Sent: Sunday, July 3, 2016 12:36 PM
Subject: Re: [TowerTalk] Fair rite materials for choke baluns
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
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