The manufacturer should have specific requirements.
Garmin for example:
• Be attached away from major protrusions, such as engines, nacelles, propellers, and antenna masts.
• Be as far as practical from landing gear doors, access doors, or other openings that could affect its
radiation pattern.
• Be vertically attached on the bottom of the aircraft.
• Not be attached within three feet of the ADF sense antenna or any other communication antenna.
• Not be attached within six feet of a DME antenna.
• Be attached to a minimum of three feet of cable from the GTX 35R/45R to prevent RF interference