How to Fix It
1. Make sure the directional antenna is located in a spot on your roof where it faces away from the building. If it is pointed across the roof then it is probably picking up radiating signal.
2. Try to move the interior and exterior antennas further apart. Power down the amplifier before moving antennas. Ideally, you would move the exterior antenna further away and to a higher location, if it can still receive the same signal strength. Note that vertical separation is more helpful than horizontal separation, although both will benefit the problem. After you’ve adjusted the antenna(s), return power to the amplifier and check if the oscillation is still occurring. If so, you might try moving the antennas again.
3. Lastly, if you are out of options on moving antennas, you can shield the exterior antenna by moving it to a spot that aligns an object such as a chimney or air conditioning unit between the exterior antenna and the rest of your building. When the exterior antenna has been installed indoors, you could shield it with something metal like a filing cabinet or foil. Place it behind the exterior antenna to block the signal.