If you have a very large dish, say 7m diameter and point it at a satellite whose signals are intended for reception by small 70cm diameter antennas then the 20 dB increase in total power of the signals into the LNB may be sufficient to overload some of the transistor amplifier stages inside. This is not always obvious. Measuring the composite output power of the LNB using a power meter is suggested and comparing this with the -1 dB compression point in the manufacturer's specification. An alternative is to do an antenna pattern test on both a high power and a low power satellite. Any non linearity problem with the high power satellite is then clearly visible. Special low gain or high power output level LNBs are available for use with large dishes.