From: Winfield Hill
Subject: Re: low loss transformer availability?
Date: 6 Jan 2003 16:04:27 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
John Woodgate wrote...
> Winfield Hill wrote
>>> John wrote...
>>>> Winfield Hill wrote
>>>>> For example, learning from Jerry Sevick, I designed a wideband
>>>>> 16x transformer (RIS-294) that worked in two stages. First 50
>>>>> to 12.5 ohms with 25-ohm coax, finishing with 12.5 ohms to 3.2
>>>>> ohms using 6-ohm transmission line, made from two wide copper
>>>>> strips insulated with a thin layer of Kapton tape. It handled
>>>>> over 100W, was flat from 500kHz to well over 50MHz, and could
>>>>> deliver over 10A peak into a 3-ohm load.
>>>> What I had in mind was how long the cables would have to be to
>>>> work at 3 MHz. '500 kHz' makes the answer even more interesting.
>>> Not so very long, if one uses a large high-permeability core.
>>> 'Course that can hurt on the high end. But I have found that
>>> a 100:1 frequency range usually isn't too hard to achieve.
>> John, I checked in our records.
> Well, thank you for doing that.
You're very welcome, nice to see someone interested! :>)
>> ... I used an unknown 1.4-inch dia toroid
>> made with low-permeability material, and A_L = 155nH/T^2.
>> My home-made coax (#14 magnet wire with a Scotch adhesive-backed
>> copper tape shield) made 4 turns around the toroid, so L = 2.5uH.
>> Being it was a 25-ohm coax, that should have given trouble near
>> 1MHz, but in fact we had to go down to 500kHz to load the 50-ohm
>> drive to the complete two-stage transformer. Despite the -3dB
>> input, the 3-ohm terminated output was only down 20% at 500kHz.
>> My data shows an overall flat response from 1MHz up, then +20%
>> at 50MHz, and some kind of resonance near 80MHz.
> I'm afraid you've lost me. This #14 wire with the foil shield
> was the 25 ohm line?
Right, after wrapping it with the foil. Big thick center
wire with a thin dielectric to the shield.
> I don't understand what the core does. Did you have any of the
> 50 ohm cable wound on the core?
No 50-ohm coax at all. In a transmission-line xfmr the coax
has to be the geometric mean of the Zin and Zout impedances.
> How was the 6 ohm 'cable' arranged? Wound on the core?
Yes, that was on a second core; basically I made two transformers.
Jerry shows use only one core in one of his books, but I used two.