multiple fm transmitters

Hello

I am working on an audio installation. I am looking to build a quad channel audio system using fm transmitters instead of amplifiers. I have 4 0.5w hlly transmitters and lots of small radios. Yesterday was the first time I started to test how to connect all the equipments. There was lots of noise in the signal. But I maybe attribute this to the fact that my studio has lots of stuff plugged into to only two wall sockets (one of which has a 200w 240 to 120v step down convertor).. computer, mixboard, 100w reference amplifier plus the radios and transmitters.

How can I go about reducing the humm and how feasible is it to broadcast all four transmitters at the same time from the same room to the same room (but to different frequencies of course)?

Is it only a case of using batteries in the radios or having everything plugged into separate wall sockets?

Thanks