Hi @paulb, the IEEE norm is to use a 1/2 wave centre-fed straight dipole as a baseline antenna for RF testing in UHF. This is also the most common external antenna. Antenna gain is normalised at 2.15dBi in the perpendicular axis. For EU 868mHz each side of the dipole is 1/4 wave or about 8.5cm so typical length is about 18cm.
The transmission line is also very important and any serious comparison needs to standardise the type and length of coax used, e.g. RG58, 2m long.