how do i make sure crystal oscillator module only produces radio waves when an antenna is attached?

i am planning to build a radio transmitter. i connect a 100 mhz crystal oscillator to a battery. if i connect the 100 mhz crystal oscillator module on a breadboard to a battery and there is no antenna, it will still produce radio waves. if i use a perfboard instead of a breadboard, will the crystal oscillator module only produce radio waves when i attach an antenna?

David,
If you have a 100 MHz oscillator, it will produce localized radio waves at 100 MHz when it oscillates. When you connect the oscillator on a breadboard, you are using relatively long wires. These wires are acting as an antenna. I assume that you have a radio receiver nearby that you are using to detect the radio waves.
You can't really prevent the circuit from producing radio waves but what you can and should do is to reduce the size of the radio waves to where they can't be picked up unless you attach an antenna.
The best way to start with that is to make the wires in your oscillator circuit as short as possible. If that doesn't work you may have to put a shield around the circuit.
It's likely that by building the circuit on a perfboard using the shortest connections you can, you will reduce the radio waves to not be detectable without an antenna.

are you an engineer, elizabeth simon?

I'm an engineer as well as a ham radio operator. In my day job I try to reduce the radio waves generated by my circuits. In my spare time, I generate radio waves for fun.

Hi David,You can check out people at their profile page, e.g.:
https://www.eeweb.com/profile/elizabeth-simon

Hi.Even without antenna, the circuit produces oscillations and emits a carrier signal. If the power of circuit is high, the signal could return back and destroy the transistor. You can check, in this case, the SWR ratio.Giovanni

The radio waves carry the information to the receiver location. At the receiver, the oscillating electric and magnetic fields of the incoming radio wave push the electrons in the receiving antenna back and forth, creating a tiny oscillating voltage which is a weaker replica of the current in the transmitting antenna.