I need to boost the signal on a fm modulator

[andrromedos]

Hello all!
I want to make drive-in cinema for me and my friends and I'm planning to stream the sound over the radio, so everyone can tune in on the same frequency and listen to the movie with their car speakers. The only problem is that the FM modulator I'm planning to use is too weak. I wanted to make a bigger antenna for it, but I can't exactly find the place to solder it to, can anyone help?
(I couldn't upload the images, so I put them on imgur) https://imgur.com/a/1YQeEGG

 

[Harald Kapp]

The transmit power of the modulator you have is limited for a reason: Are you aware of the regulations applicable to using FM radio where you live? Amplifying the modulator's output so it can reach multiple receivers within a wider range than originally planned and designed for may will violate the law.

 

[Nanren888]

Many places have provision for low power FM stations. Some places have compulsory inspection of equipment before starting, others don't.
These licenes will often get you a Watt, but take some arranging.
In areas where this is possible, there are often pre-approved hardware options. Often only pore-approved hardware is permitted.
Might be worth searching this to see if it is an option in your area.
low power fm or LPFM