What is the term for a structure made from wire that is erected a short distance off the ground and insulated from it?

The term for a structure made from wire that is elevated a short distance off the ground and insulated from it is known as a counterpoise. A counterpoise essentially functions as an artificial ground system for antennas, especially in situations where there isn't a good conducting ground available. It is made of one or more wires that are typically horizontal and placed above the ground, acting as a substitute for the ground plane.

In radio wave propagation, the deployment of a counterpoise can enhance the performance of an antenna by improving its radiation characteristics and efficiency. It can effectively reduce ground losses that would occur if the antenna were in direct contact with the ground, allowing for better signal transmission and reception in certain configurations. The usage of counterpoises is particularly prevalent in vertical antenna setups where optimum performance is desired, especially on lower frequencies.

While antennas are structures designed for radiating or receiving radio waves, not every antenna is designed as a counterpoise, nor does it fit the description provided. Transformers are components that transfer electrical energy through inductively coupled conductors, and ground systems generally refer to systems connected to the earth for grounding purposes, which does not align with the insulated structure described in the question.