Yes, it would be legal to have 128 radials in the ground, as long as your antenna plus ground wire is 3 meters or less.
The FCC has issues with radiating grounds, and considers them part of the antenna system. But ground radials, either lying on top of the ground or buried, do not radiate.
Yes. Buried wires do not radiate.
Imagine someone using a quadcopter as a platform for a mobile Part 15.219 antenna. One could even run radials along the arms of the quadcopter. These craft are constantly becoming more powerful and capable of staying aloft longer per charge. Stabilizing a 10' whip antenna on such an aerial platform and wirelessly communicating with it might be challenge, but just imagine having a mobile Part 15.219 platform capable of taking the broadcast to different areas of town... In fact, a broadcaster could even land the damn thing on a high platform for a rest as needed ... Just beware of the wind. LoL
Actually being buried horizontally in the earth is not all that is necessary to keep the radials from radiating.
The best example is a description of the minimum possible ground radial: two wires.
Why not one wire? We're getting closer to the answer.
If you bury a single wire attached to the transmitter ground it will radiate and the length will count toward the length of the antenna and ground lead.
But two wires of equal length makes a proper minimal ground radial setup, but only under certain conditions. The wires must be laid in exactly opposite directions and be exactly the same length as each other.
Another way to look at it is one very long wire buried horizontally in the ground, tapped in the exact middle and attached to transmitter ground.
Why is that the right way to do it? Because the two sides of the wire on either side of transmitter ground are in opposite phase and therefore added radiation is canceled out.
I call this one radial wire tapped in the center a "shunted dipole."
Every additional set of ground radials that are added must follow the same rule: one long wire of equal length transmitter grounded to the center.
What if a single radial set is of opposite but UNEQUAL lengths? The answer to this came from a veteren engineer who told me that the transmitter's harmonics can go out of FCC compliance by radiating with more energy than the supression requirements allow!
In the end, no matter how many radials there are, the total needs to be an EVEN number!
Buried wires which carry an unbalanced RF signal do radiate. This was observed at my station using a loop antenna (click) with a meter to detect RF. This detected RF coming from each underground radial as the signal strength would peak over each radial.
As Carl pointed out, the net radiation from underground radials will cancel if viewed from a distance if the radials are symmetrically placed around the antenna. Viewed from a distance, the RF currents are equal and opposite and the fields produced by each pair cancel to zero.
As an example, underground plastic gas lines are installed with a metal conductor to which a transmitter can be attached and the line located with a receiver.
Neil
With properly balanced ground radials not being radiators, they still are of value to the antenna system, correct? Return path making the intentional radiator more effecient?
There was local lore of an AM station with an owner who would water the lawn around the tower during dry summer months to help his station get range. Ground conductivity from the water making the ground radial network more efficient?