A new formulation for implementing T-junction matching networks used in diplexer design is proposed in this paper. The investigation exploits the electrical length of quarter-wavelength microstrip transmission lines, and the guided-wavelength of the lines at one giga-Hertz frequency, in achieving the T-junction design formulation. This novel design would eradicate the uncertainties associated with frequent tuning and optimisation of T-junctions to achieve the desired energy distribution in diplexer designs. A prototype diplexer for separating the transmit from the receive frequencies within the front end of a wireless cellular base station is investigated and used to demonstrate the new design method. The prototype microwave diplexer with Tx and Rx centre frequencies of 2680 MHz and 3000 MHz, respectively, have been designed, implemented using microstrip, simulated, and presented. The simulation results of the circuit model and that of the microstrip layout prototype diplexer, demonstrate decent agreement with a high isolation of better than 50 dB between the transmit (Tx) and the receive (Rx) channels. The in-band lowest insertion loss is located at 1.1 dB, with a better than 20 dB in-band return loss across both the Tx and Rx bands.