If the umbrella pole is metal, it will reflect signals. If the antenna is located in a spot where the reflected signal is in phase with the original, you'll get a signal boost. If the reflected signal is out of phase with the original, it will decrease the signal. If the pole is between the transmitter and receiver, it will block the signal.
The effect you get will be random, depending on the wavelength of the signal you're monitoring, the exact distanct between the antenna and the pole, and the location of the transmitter antenna, receiver antenna, and pole.