Round vs Oval coils?

It's not the shape as much as the design of the internal windings. The round coils are usually (not always) of a concentric design, whilst the oval coils are usually of a "DD" design.

For similar sized coils, the concentric coil will go a little deeper at its center (conical detection pattern) and has better immunity to radio interference. The "DD" coil has an elongated detection pattern and performs better than the concentric design in heavily mineralized ground.

Hope this helped a little.

DD coils have the entire center strip active, and provide a wider sweep. They are not as tight on pinpointing. The frequency of the pulse is what affects depth. Lower frequencies tend to go deeper but miss smaller targets. Higher frequencies are shallower but are better for smaller stuff, thus the multi frequency and usually higher end machines. Concentric coils project their signal as a conical shape that comes to a point as depth increases, thus a very small search area as the depth increases.

All my concentrics are round but I have both round and oval DD's. Just using shapes my oval coil can fit into smaller spaces than a round coil. Rob