The method of enabling Bias-T on dump1090-fa given in RTL-SDR web page (linked in opening post of this thread) is based on the method originally found by @obj, and was posted by him in Flighaware forum in May 2017 here:
Enable BiasT on RTL-SDR v3 before Dump-FA/Piaware start - #2 by obj
VARIENT #1: Enabling Bias-T on Flightradar24 image Pi24
The method to switch on biast on dump1090-fa does not work with Flightradar24’s Pi24 image which has an integral dump1090-mutability, whose start is controlled by the data feeder software “fr24feed”. Therefore I have added a line ExecStartPre= in file fr24feed.service. This line runs bias-t start and logging script before fr24feed and its associated dump1090-mutability are started.
I have posted method to turn-on bias-t on Pi24 image in my following post dated April 30, 2018 in Flightradar24 forum:
RTl_SDR v3 bias tee power on/off - Flightradar24 Forum
VARIAENT #2: Enabling Bias-T on dump1090-mutability (stand-alone, and NOT an integral part of Pi24 image)
The method to switch-on bias-t through dump1090-fa.service does not apply to dump1090-mutability, as the dump1090-mutability.service files are automatically generated by systemd-sysv-generator, and all the changes made by user are overwritten at restart / reboot.
However as the systemd-SysV-generator generates dump1090-mutability.service by reading from file /etc/init.d/dump1090-mutability, I have added Bias-T start and logging script in file `/etc/init.d/dump1090-mutability, and it works OK.
Please see my post dated May 01, 2018 in Flightaware forum:
Dump1090-mutability and RTL-SDR V3 Bias-T Auto Switch-ON