Mains powered very low power transmitter - need help please

I need to build a mains powered FM transmitter, for relative short
distances, about 100 - 200 meter. Going to use them in a kindergarten
and they have to be very audio sensible, and at the same time to work
7/24... Frequency - about 100MHz..
I'm aware of the safetly rules, they will be placed near the ceiling;
I've seen several devices as "surveillance or spy devices", and I mean
exactly the same devices, but for other purposes. The devices - as I've
understood are powered by a capacitor (250V), a diode bridge, a
regulator and an electr. cap. for filtering.
One of the most important things - they use no ANTENA, but how this can
be made is "misterious" for me. They claim they use the MAINS wires as
antena....
I'll be very thankful for any help, idea or circuit :)
TIA,
Regards, Nina

.

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