I have to make a simple transformer, that measures the output voltage from the secondary coil, with relation to the distance between the primary and the secondary coil. So, I don't have to make it a step up or step down transformer, I just have to see how the voltage changes with distance. So do you think I should use a non-iron core transformer, or do you have any other ideas, that would be suitable in any way? Please help me, I am stuck. Do you think, using a non-iron ( air core) transformer will work, or should I try something else

On my side I think it is not better to use non-iron core transformer because trnsformer usually function in the presence of iron,so I would like to advice you just use iron core transformer and you will observe that it will functin in a specific way.
On the other hand iron core is useful because it results in the magnetic flux in the iron ring changes hence it induces an alternating emf which is the application of transformer.