It is not "juice" related. You said your bike has no battery. Is there a rectifier either directly in the wires to the horn or in the entire electrical system? If not, you have AC going to the horn & there aren't many that work well on AC.
If you do have a rectifier somewhere in the horn circuit, does the horn work better when you put some extra load - eg headlight - on the electrical system? If so, with no load the regulator works to short out the stator most of the time, leaving only a series of short duration pulses of power to run the electrics. While the peak of each pulse might be 15 or so volts, the average voltage over time is probably only a few volts. Revving the engine shortens the duration of the pulses and reduces the average voltage, making the horn work worse as the engine speeds up.
Putting the electrical system under more load makes the pulses wider and increases the average voltage.
The best way to fix it is to put a capacitor across the electrical system - after a rectifier of course! It effectively works as a small battery and smooths the DC supply voltage to somewhere up near the peak voltage of the pulses.
If you already have a small rectifier fitted into the wires going to the horn, just a capacitor directly across the horn shoud help.
Check the threads about putting a capacitor across the electrical system. There have been lots of them.