It is shown in three different ways that the wave equation varying with time and the Helmholtz equation are equally effective for analysing the transmission of electromagnetic waves from the weakly ionized plasma in the plane geometry. Eve n when the scale length of plasma approaches the waves length, the electromagnet ic waves are still simple harmonic waves. The properties of transmission of elec tromagnetic waves from the weakly ionized and highly collisional plasma are stud ied numerically. The results show that the highly collisional plasma is a good m icrowave absorber.