TS832 audio-video transmitter lowers its input voltage
A quick search for TS832 specs (example here) shows its expected current draw is 600 mA. If you are using a &quot;transistor radio&quot; rectangular 9V battery like this

it isn't up to the task. Its total capacity of ~500 mAh would be drained in less than an hour. And its internal resistance will cause the output voltage to be significantly less than 9V, as you are measuring, and the circuit to not work.
You could use 6 AA cells in series. Those cells have 2400 mAh capacity, so might last 4 hours. Or you could use 2 LiPo cells in series, but you'll have to deal with recharging circuitry. As a beginner, I'd recommend sticking to the AA cells. You can get rechargeable AA cells and a commercial recharger. Good luck!