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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Binocular choke extras
Date: Thu, 20 Mar 2025 16:24:26 -0700
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On Fri, 21 Mar 2025 00:08:53 +0100, Jeroen Belleman
<jeroen@nospam.please> wrote:

>On 3/20/25 16:43, john larkin wrote:
>> On Thu, 20 Mar 2025 10:49:08 -0400, Joe Gwinn <joegwinn@comcast.net>
>> wrote:
>> 
>>> 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
>> 
>> I have the Sevick book but it's not very useful.
>> 
>> We make super wideband tline transformers from micro-coax and pot
>> cores.
>> 
>> https://www.dropbox.com/scl/fi/32s2rfcu4q4iq6l6v1eb4/Pot_Core_TXline.JPG?rlkey=6k7xusurck0jf1ky9n6ja2ebz&raw=1
>> 
>> https://www.dropbox.com/scl/fi/adcocf1rb7lnanj7zo9xp/TX_1.jpg?rlkey=m7prsxj94fa57ynqoep0ydgnl&raw=1
>> 
>> Or toroids, which are harder to make.
>> 
>> https://www.dropbox.com/scl/fi/xuqjzt3h1oq7uexwiu6c8/T750_1.JPG?rlkey=si165mntuu0h40zgsbi0qzxj7&raw=1
>> 
>> 
>
>Those are 1:1 baluns. It's not too hard to get stupendous bandwidths
>with those. Six decades of frequency should be quite easy. It gets
>harder when you want different impedances at the ends.
>
>Jeroen Belleman

I call them transformers. We use them to isolate pulse generator
outputs, and sometimes to get a voltage step-up.