Is Delta Loop Antenna a Multi-Band Antenna?

Yesterday while I was contacting a friend at about 2500 km from here on 21 MHz, he told me that he was using a 20 m delta loop antenna and it works on multiple bands even without a tuner and I was quite surprised. Another friend on the other side of the globe told me online that his 40 m delta loop antenna works on higher non-WARC bands with reasonable SWR. Wanted to know more about the multi-band capability of delta loop antenna. My Google search immediately brought me to a 40-10m multiband delta loop antenna!

General information is that delta loop antenna is a full wave length antenna in the shape of the Greek alphabet Delta (Δ). It will need a 4:1 balun at the feed point for impedance matching. If the delta loop is mounted vertically, the low take off angle of electromagnetic waves is good for working distant stations. If mounted horizontally, it is useful as Near Vertical Incidence Skywave (NVIS) Propagation for medium range communications in amateur radio especially in hilly terrain, forest region and even in cities with high rise buildings.

Another type of delta loop antenna is a multi-element Delta loop antenna which when compared to a Yagi antenna of same gain, has a wider beam width. This could be an important advantage for working DX as well as contests. Multi-element delta loop antenna has directivity like the Yagi antenna, though it is practically difficult to contruct for HF bands due to the dimensions. Usual single element delta loop antenna mouted in the vertical orientation has bidirectional radiation pattern along the broadside of the antenna. For example, if the base wire of antenna is North-South, direction of gain will be towards East and West, which can be up to 3dB over a center fed dipole antenna. Like other loop antennas, it is not only a good transmitting antenna, but also a low noise receiving antenna.

Supplier of one commercially available multi-band 40-10m delta loop antenna has mentioned that it cannot be operated by a coaxial cable. Any parallel wire feeder can be used to operate on 7 bands using a tuner. They have also mentioned that best results are achieved with a symmetric tuner, but any other tuner with a balun also works well. Others have used coaxial cable with 4:1 balun for multiband operation. There are also descriptions of shortened delta loop antennas for those with space constraints. Commercial options for 3.5-54 MHz and kW range rotatable ones could be seen as mentioned online.