Operating at 16 MHz clock it is in the "high" radio frequency range (3 MHz to 30 MHz).
Can you explain what your setup and measurement have to do with decoupling capacitors.
It seems what you're doing here is just building an 8-Mhz RF generator. What size R across the scope probe?
I was hoping no-one would notice that. It is probably more of a demonstration that signals are likely to stray from one part of a circuit to another.
One way to keep RF emissions lower is to keep the area between the wires low.
What value do you want? I presume over the scope probes? Or across the output?