Are you using a voltage divider to reduce the supply voltage to a transmitter?
Its output voltage will change if the circuit changes its supply current, like with modulation.
The voltage divider will also waste a lot of current.
Why not use a voltage regulator IC? Most "waste" only 5mA as their operating current.
Reducing or increasing the voltage to and RF amplifier is one way of adjusting the output power. Doing this to other stages could create some problems. Some commercially available tranceivers actually work well with various applied voltages from battery packs. The higher voltage packs result in higher output power - just keep in mind that the radios were designed to operate that way. Not every circuit will work well if you adjust the voltages. It would be one thing to be inconvenienced by things - quite another to be off frequency or create distortion that goes outside the band. That could get you in trouble.
It is not clear what you want to do. If it is to reduce or increase the output RF, use a transformer. There is no good reason to change the supply voltage unless you want to modulate the RF output.