This paper presents an automatic humidity control system in Sajor-caju mushroom growing house by using the transmitted and received RF signal from rectangular monopole antennas which are fabricated on FR4 substrate printed circuit board. An analysis of power variation from a travelling wave of the humid air in the mushroom growing house is used to control a water spray system to keep the humidity for the Sajor-caju mushroom. The built-in humidity control system operates at a frequency of 915 MHz, consisting of the transmitter and receivers. Both of antennas are installed across the length of the mushroom growing house. The transmission part is a synthesizer controlled by a field programmable logic gate array. The signal is amplified by power amplifier before sending to the transmitting antenna. At the receiver, the electromagnetic wave is converted to DC voltage and then it is amplified in order to use as the utilize data of the water spray in mushroom growing house. The DC voltages from humidity level of 70% - 80% are determined as a threshold value for the decision. The experiment of the humidity control of the mushroom growing house was tested from 6 am to 6 pm. From the measured results, it is showed that the humidity has the range from 74.9% to 77.5%, which is in the range of the appropriate humidity for the mushroom growing house.