The most cited version of this standard is SAE ARP958B , published in November 1996. However, standards are periodically reviewed and updated. If you download a "free" PDF from an unofficial aggregator, you run the risk of using an obsolete version that has been superseded or withdrawn. Professional engineering relies on using the current, relevant version of the standard.
There are several sources where you can find a free PDF version of SAE ARP 958: sae arp 958pdf free
The standard primarily uses a "two-antenna method" where two similar or identical antennas are placed exactly one meter apart. By measuring the insertion loss between them, engineers can calculate the gain and the Antenna Factor. Antenna Factor (AF): The most cited version of this standard is