Why is it not valid to follow the Imam via radio?

It is not permissible to pray behind a radio due to the absence of the condition of congregation. Distance is forgiven as long as it does not exceed three hundred cubits, according to many scholars. As for the Malikis, they permit prayer if the congregant knows the movements of the Imam by hearing his voice or the voice of someone conveying it from him, or by seeing him or seeing those who see him among the congregants, even if the distance between them is great, even if the congregant is in a house and the Imam is outside of it.