I built a Biquad based on the dimensions of the article featured below, and indeed I can get 4 – 5 bars strength from a base station over a hill and 8 km away.
I found this article somewhere on the internet:
Some photos of my Bi-Quad:
A friend who works with metal cut this sheet to size for me.
I cut a spare 3g modem antenna in half. Careful not to break the thin shielded wire inside.
I then soldered the wire and the shielding to the respective sides of the BiQuad.
– Securing the soldering with a blob of expoxy resin on top, and then screwed the antenna into low loss cable.
And Pesto: plug unto a 3G modem , job done!