System(1) My Ras Pi and Filter+Orange ProStick are connected to FA 26" antenna (gain 6 dBi) by 4 ft RG6 coax, and doing excellent. When keeping every thing else same, I replaced Orange ProStick+Filter combo by Blue ProStic Plus, my message rate & aircraft count went down to 40%. After adding external filter, it went to level of Orange ProStic+Filter combo.
System (2) My Orange Pi PC and Filter+Orange ProStick are connected to Cantenna (gain 1.5 ~ 2 dBi) by 12 ft RG6 coax, and doing excellent. When keeping every thing else same, I replaced Orange ProStick+Filter combo by Blue ProStic Plus, it performed slightly better than Orange ProStic+Filter combo.
From above I concluded that high gain antenna results in picking lot of cell phone signal, short cable results in negligible attenuation, and a very strong signal is fed to rf preamplifier. Since the built-in filter is after the rf preamplifier, the cell phone signal is not removed at input, and overloads the front end RF pre amplifier. Adding filter before preamplifier chops off cell phone signals and overload of front end preamplifier is no more there.
I have now ordered two attenuators (3dB and 6 dB). Waiting for delivery from China, not less than a month . Will try these to find if my opinion is right or wrong.