It is some while since I used a magnetic loop antenna on HF TX, but these antennas work well, if you can accept their very narrow bandwidth. In fact the sharper the tuning, the more efficient it is likely to be. I used a small magnetic loop made with small-bore central heating copper pipe and was amazed how well it worked with WSPR spots from Japan on 20m with the loop in the shack, which was an upstairs bedroom at the time. Ideal applications are where frequent retuning is not needed such as PSK31, JT65 or WSPR. On 10m they can be very efficient. Even on 20m I was very impressed. Bandwidths are usually measured in kilohertz. Losses have to be kept low if they are to be efficient. Even thin wire will work but the antenna will not be as efficient. As a minimum coax cable or copper pipe should be used for the loop. At QRP powers ordinary air-spaced capacitors work fine to bring the loop to resonance. At 100W these may well arc over.