Positive/Negative interference in a carrier signal
For exact cancellation, you need 180 degrees or 1/2 wavelength. 900 MHz has 1/3 meter wavelength, thus 1/6 meter delay is needed. For 800 MHz,
you need 1/6 * 900/800 = 9/48 meters.

This assumes the delayed path provides exactly the same energy (no attenuation) as the direct path.