Looking at the S11 and S21 parameters of dual band ANTs shows acceptable performance up to 6.7GHz. In the photos below 4 markers on the S21 parameter graph can be seen. MK1 was set to the beginning of the 2.4GHz band, MK2 was set at the beginning of the 5GHz band, MK3 was set at the last frequency in the 6GHz where the dual ANTs still gave acceptable levels of performance and finally MK4 was set at the end of the 6GHz band (7.2GHz) where the performance degraded by a factor of 6dB below the performance measured in the 5GHz band.
Note: For more info on the S-Parameters and what do they mean you. this page https://www.antenna-theory.com/definitions/sparameters.php gives a simple and clear explanation of the concept.