Basically ancient Austronesian & Austro-asiatic backstrap weaving loin looms are equipped with a circular warp. Later on by the impact of other cultures some regional models took over flat warp-methods. Most American loin looms seem to be equipped traditionally with a flat warp. There is a third possible method of a so-called false circular warp, what means to produce a flat warp, that endings have been connected to a circular warp; but it's pretty seldom in use!
Circular warps are great for the production of Ikat textiles. The decision to use a circular warp or a flat warp-method on most models of loin looms is a question of tradition; in your own practise you may be able to run both techniques with slight modification on nearby all models of loin looms, no matter it's a foot loom or a station loom. Just a foot loom like the Paiwan loom isn't a good idea to equip with a flat warp. But try, if you need to!
Leadings of Circular Warps
Circular warps and even false circular warps can differ the way they are led. While many warps are led back in standard horizontal manner, some leadings are triangular-shaped (Phang Thak, Atayal box loom). Few of the warps are L-shaped (Gorontalo & Arunachal). Of course, there is no vertical leading of loin looms, but a range of diagonal and horizontal warp leadings.
Examples of loin looms with a circular warp