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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Binocular choke extras
Date: Thu, 20 Mar 2025 17:42:47 +0100
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On 3/20/25 16:30, Don wrote:
> Joe Gwinn wrote:
>> Liz Tuddenham wrote:
>>> Jeroen Belleman wrote:
>>>> 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.
> 
> Thank you for all your interesting threads as of late, Liz.
> 
> It's difficult for me to follow this conversation. Maybe my followup
> simply repeats what you guys already said.
>      The tubes are shields designed to fit inside the binocular core
> holes. You then soldier them together with a PCB at each end like so:
> 
>      <https://www.qsl.net/g3oou/BBTrans_4278a.jpg>
> 
> It becomes more interesting with coax:
> 
>      <https://www.qsl.net/kf8od/ldmos53.jpg>
> 
> Danke,
> 

Not at both ends! That is not going to work!

Jeroen Belleman