Hi, I had build a transmitter. I check its transmission frequency at antenna using my smartphone's radio and it shows 97MHz. However, when I test again using oscilloscope, it shows 50MHz and another 107MHz in spectrum analyser. Using the same song as input, but why 3 different results?

 

The scope and analyser, or the inductance and capacitance of their test leads, are presumably loading the circuit too much and affecting the tuning.

 

Alec_t has probably guessed correctly.

For deeper insight, post a schematic of your transmitter.

 

I see from another forum that your circuit is on a breadboard with straggly wiring. Yet another reason why the frequency is unstable.

 

@Alec_t @DickCappels Thanks for your reply. Just like what you said, too much wiring... I will try to make it a PCB and try again.

 

Assembling it on a PCB will help- but not cure the problem.

A stable transmitter needs multiple stages of buffering and isolation to prevent the load from pulling the output frequency.
Good designs typically use crystal oscillators as a frequency standard, either as a direct source, or via a phase-locked-loop frequency synthesizer.