Another weekend project that I made is this SWR (Standing Wave Ratio) meter which rf pick-up is a directional coupler fabricated from microstrip. FR4 double sided pcb was chosen with thickness of about 1/16 of an inch on a 2mil copper thickness. Processing the microstrip starts from applying a masking tape on both sides of the PCB and manually drawn the three main important traces on the top side of the PCB. The masking tape on the bottom side was retained so that at the end after etching, it will serve as a ground plane of the microstrip. After etching the board, please verify that the dimension of L1, L2 and L3 are closely followed so that our strip line impedance will be close at 50 ohm, the thickness, width and the material of the PCB has also a direct influence on the impedance.
On my prototype I intentionally separate the directional coupler from the meter unit so that when time comes, I can easily swap between other types of directional coupler design. During the initial test, I found that the prototype respond well to VHF while it requires greater amount of rf energy to produce the same full scale deflection of the 100uA meter at HF. Germanium diode used in this project is not linear specially at different power levels therefore this project is best suited for QRP application only.
