From: Winfield Hill
Subject: Re: low loss transformer availability?
Date: 6 Jan 2003 12:10:27 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
John Woodgate wrote...
> Winfield Hill wrote
>> John wrote...
>>> Winfield 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.
> CORE? You must mean something very different from what is in my mind. If
> you wound the cables on to a toroid, how did you do that with the 6 ohm
> line? "With great difficulty" is not an acceptable answer. (;-)
As I mentioned (later post in this thread) I used two 1.4" diameter
toroid cores, one for 50 to 12.5 ohms, the second for 12.5 to 3 ohms.
The second transformer stage required 6-ohm transmission line, which
I made by cutting 0.35"-wide strips of 0.003" copper shim stock, and
taping the strips together with Kapton tape, with one layer of 0.003"
tape as the dielectric. It wasn't difficult to wind this strip-line
for four turns around the perimeter of the toroid.
Jerry Sevick would test his transformer design and construction by
finding its optimum operating impedance, defined as the impedance
which had the widest operating bandwidth. I didn't do that test.