Re: Binocular choke extras

On Thu, 20 Mar 2025 07:30:58 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:

Jeroen Belleman <jeroen@nospam.please> wrote:
>

On 3/19/25 22:40, Liz Tuddenham wrote:

I've just taken delivery of a couple of ferrite 'binocular' choke cores;
each one came with two thinwalled metal tubes and some bits of printed
circuit board.  The tubes appear to go through the holes in the choke
and the holes in the boards fit over the ends of the tubes, with copper
areas that could possibly be soldered to them.
 
Does anyone know what purpose these serve?
 
 

 
This sounds very much like the transformers used in wideband RF
power amplifiers. See for example Helge Granberg's application
note 762. The tubes with a piece of circuit board form the single-
turn low impedance winding, and a number of turns of insulated
copper wire going through the tubes form the high-impedance
winding. This makes for a good coupling factor and consequently
good wideband operation.

>
That's the sort of thing I suspected.  Each square pad surrounding the
end of a tube is individually isolated but they could easily be joined
to make a loop circuit with some wire straps.
>
I intend using this as a 1:1 balun and was worried that the conventional
way of twisting the primary and secondary conductors together before
threading them through the core would create a capacitive imbalance.  If
I use the tubes as a 1-turn secondary and thread the inner of the feed
co-ax through them, this will give much lower capacitance imbalance.

This also sounds like it could be a transmission-line transformer;
these are very wideband.  The ferrite cores serve as RF chokes,
ensuring the shield and center currents are exactly equal and
opposite.  It is _not_ an ordinary RF transformer, despite the name.

"Transmission Line Transformers", Fourth Edition, Jerry Sevick, W2FMI,
2001, 289 pages, ISBN 1-884932-18-5, TK6565.T7 S48 2001,
621.384'11--dc21.

Joe