That's a collinear array, of a type sometimes called a Franklin antenna.

It's very hard to estimate the gain, because even when constructed
perfectly, the velocity factor of the coax prevents the inside and
outside from both being the ideal length. And at that frequency, very
small imperfections in building technique can have a major impact on the
gain. But it should be a fun project and it might give noticeable gain
over a quarter wave monopole.

Roy Lewallen, W7EL

KG0WX wrote:
http://wireless.gumph.org/content/4/...s-antenna.html

The author calls it a sector antenna but I'm sure I've seen it in ham
applications but with another name.

Also, can anyone quote me a dbi gain figure ) for this?

Thanks for the help!

Ken KG0WX