Thanks Stefan VE4NSA for the web address. This appears to be for a 2.4 GHz antenna and I have found many sites with such designs. The author explains how to scale the antenna for other frequencies. It's a good site, but not quite what I was looking for and that's probably my fault for not making myself clear enough.

I have been looking for more generalised information explaining the principles and operation - something explaining how to calculate size and spacing and explaining why the calculations are made this way.

Thanks John AA1YE for the pointer to the book by Robert A Sainati. I know my local library doesn't have a copy as I have borrowed the few antenna design books they have several times. Technical books are, alas, disappearing from library shelves. However, with the information supplied I can ask them to try and find me a copy using their "interloan" system. If it's what I want, I'd happily buy a copy, but like to know that first (hi hi).

Thanks for the help guys.

Sil