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Posted by Bert (192.100.124.219) on December 12, 2003 at 20:33:58:
In Reply to: transformer design posted by Tomas on December 12, 2003 at 12:39:06:
: Greetings!!
: Im designing a RF transformer. I know thats not an easy thing to explain in few lines, but anyone has any idea to help me? for example: should I use GAP?
As you might have guessed: it depends. A gap increases the saturation treshold, so you can zap more power through it, but it lowers the inductance. In RF applications, a gap is almost never used.
:The CORE most have great quality to prevent power loss or could be air?
That depends. If it is for power purposes (TX antenna balun, TX ouput stage...) you need a core. No question about it. An air wound coil is ok for very low inductances, but a real transformer is practically out of the question. Air would coils on the other hands go into the gigahertz region, ferrite is limited to a few 10s of MHz.
Rule of thumb: yes you need a core, and most likely ferrite is your material.
: And it should be EE, EI or toroidal? The wire most be a Cu or other material?
The core shape is almost always a toroid in RF apps.
The wire is often just copper, but at the higher frequencies (tens of MHz), when the skin effect starts to come into play, silver plated or pure silver conductors are used.
: How can I prevent parasite capacitances?
It is usually not an issue since you often have few turns (in MHz region) and the turns are spaced far apart so that paracitic capacitance is usually very low.
: Anyone has any link?
Not really, try google.
: thanks
: best regards
: Tomas
A good start is an ARRL book, or most good RF books have information too.
If you have more specific questions, post them on the board (use the new one) and I will try to answer.
Bert
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