# transmission line transformer

• posted

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

• posted

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.

• posted

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:

Ed

• posted

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.

• posted

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

John

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