From: Winfield Hill
Subject: Re: low loss transformer availability?
Date: 6 Jan 2003 09:16:23 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> 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. 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.