Why A small loop responds to the magnetic field and has a figure-8 (cosine) pattern with two deep nulls exactly 180 degrees apart. Because the nulls are sharp, they give a precise bearing line, but that line points two ways, so you cannot tell whether the signal came from ahead or behind. The usual cure is to add a sense antenna (a vertical whip) whose omnidirectional response combines with the loop to make a cardioid pattern with a single null, resolving the ambiguity.
Watch out The choice saying there is no clearly defined null gets it backwards: the small loop's nulls are deep and sharp, which is exactly why it is favored for direction finding. Loops are also very useful on MF and HF, not just VHF and up.
Loop nulls are sharp but come in pairs. Add a sense antenna to turn the figure-8 into a cardioid with one null.