NBS (now NIST) published the dimensions of a reference dipole for such uses: 2 1¦Ë dipoles above a 1.6¦Ë x 2¦Ë ground plane. Built properly it should have gain accurate to a few tenths dB or less at the design frequency.
The EIA published a similar standard gain antenna, shown in 5.2.5 of this document: The paper also has good information on antenna gain testing.
73, Don N2VGU
Donald S Brant Jr