# transmission line transformer

Discussion in 'Electronic Basics' started by Charles de Smurf, Jun 5, 2007.

1. ### Charles de SmurfGuest

Hello,

Winding a transformer with a transmission line is a technique often used
to improve the bandwidth of a transformer. The transmission line is said
to "absorb" the parasitics of the transformer. This part I nearly
understand. Some explanation might still be helpful.

However, I'm wondering how winding a transformer with a coaxial cable
might improve the bandwidth of a transformer. I know a coaxial cable is
a transmission line, but because of the shielding I thought no EM-field
(or only very little) could leak out to absorb those parasitics ...

Can someone explain this to me please?

Thanks.

Charles

2. ### John PopelishGuest

I'm not an expert on transmission line transformers, but
I'll share my thoughts, so someone more knowledgeable can
correct me.

In an ordinary two winding transformer, the only energy
coupling mechanism between the two windings is the common
magnetic field they share, in the core. But transmission
lines couple two conductors together with a traveling wave
that is guided by the pair of conductors. At frequencies
where little field escapes from the pair, there is no
external field to be coupled by the transformer core, and
the transformer would work just as well if the pair were
just jumbled in space. But, based on the thickness and
conductivity of the shield conductor, there is some low
frequency, below which, the net magnetic field (representing
the current imbalance in the two conductors) leaks out
through the shield and a transformer core has something to
work with. It is this low frequency end of the signal
spectrum that shows improved coupling when you wrap the coax
around a transformer core.

3. ### ehsjrGuest

I may be misunderstanding you, but I get the impression
that you think that because it is called "transmission line"
it must be coax. That is not what is meant in the context
of transmission line transformers. Sevick wrote a fascinating
book on the subject, and a paper on it can be found here:
http://www.highfrequencyelectronics.com/Archives/Feb04/HFE0204_Sevick.pdf

Ed

4. ### Charles de SmurfGuest

I know it can be any transmission line, but didn't understand how it may
work with a coaxial cable, which in theory would behave the same whether
or not it is winded around a core. John Popelish told me that's because
of the leaking fields in the low-frequent area.
Seams interesting, thank you.

5. ### John LarkinGuest

I don't trust Sevick's book. Every single transformer has the same
high-frequency rolloff, and he built his own test equipment.

John