Re: Maximum Power using FR4 board ?

Chris Bartram G4DGU

My, perhaps slightly jaundiced, view is that although fully characterised FR4 from a decent manufacturer with 1oz copper is capable of handing a couple of hundred watts in amateur applications at 1.3GHz when used for microstrip circuitry, there are better ways. John, 'XDY, gets part of the way to the solution I'd use.

That is to forget microstrip! It is a useful transmission line medium for applications which are intended for some form of production, and with care and understanding, can be made to work adequately for many low-power applications - even using FR4 - up to 10GHz. However, as a way making a one-off power hybrid, it's a poor use of the available technologies.

If I wanted to make a combiner for a couple of power amplifiers for 23cm, I'd use air stripline. In fact I've done it, and they are sitting on a shelf waiting for me to find time to get around to putting a decent 1.3GHz system on the air. Stripline, where a conductor is suspended between two ground planes, is capable of very much lower losses than any comparable microstrip design.  My 90degree hybrids use a pair of lines parallel to each other along their broad faces, and each each offset from the centre line of the sandwich. The design equations are relatively simple, and unlike microstrip are not approximations. Unfortunately, I don't think any of this has been published recently in the amateur literature. The classic book on the subject is Harlan Howe's 'Stripline Circuit Design'. That should be available in most decent engineering libraries. The maths which Howe uses isn't intimidating, and as the book was written before the age of the PC, there are a lot of design nomographs. I have a copy, but I don't currently have time to write anything further, however I'm sure that there are others here who could produce an article for 'Scatterpoint' summarising the design method.

73 and enjoy MMRT!




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