Consider a circular current carrying conducting coil.

If the radius of a current carrying coil is halved keeping the current unchanged, what happens to the magnetic field directed along its axis at far off points?

becomes one-fourth

Magnetic field due to a current carrying loop at a point far-off : B = $\frac{\mu_{o}}{4\pi}$ $\frac{2nIA}{x^{3}}$

n : no. of turns in the coil

I : current through the coil

A : Area of the coil

As radius increases to double the value, the area of coil quadruples.