  An isotropic antenna radiates and receives electromagnetic wave uniformly in
magnitude in 3D space. A multi-frequency quasi-isotropic antenna can serve as a
practically feasible solution to emulate an ideal multi-frequency isotropic
radiator. It is also an essential technology for mobile smart devices for
massive IoT in the upcoming 6G. However, ever since the quasi-isotropic antenna
was proposed and achieved more than half a century ago, at most two discrete
narrow frequency bands can be achieved, because of the significantly increased
structural complexity from multi-frequency isotropic radiation. This limitation
impedes numerous related electromagnetic experiments and the advances in
wireless communication. Here, for the first time, a design method for
multi-band (>2) quasi-isotropic antennas is proposed. An exemplified
quasi-isotropic antenna with the desired four frequency bands is also presented
for demonstration. The measured results validate excellent performance on both
electromagnetics and wireless communications for this antenna.
