This is the first of several simple PCB design projects aimed at exploring the structures commonly used in RF and in particular, microwave frequency PCB design. The center frequency for this and other related designs will be 2.3GHz, used for Bluetooth and close also to WiFi.
This board is simply two transmission lines formed as coplanar waveguides (CPWG) on a single layer FR-1 (paper phenolic) substrate. This is used because we can mill these boards easily using the Bantam Tools Deskopt PCB Mill, which is accurate enough to do RF/µWave prototyping. The point of this board is to create accurate waveguide designs which can be measured with a USB-connect VNA (Vector Network Analyzer) which will allow us to tweak the mill settings for predictable, repeatable results. We will be able to measure impedance, VSWR, bandwidth and loss of the FR-1 material.
FR-1 is not terriffic for RF - this is a well known fact - but it can be used for experimentation in spite of it's higher losses. Moreover, it's not much lossier than FR-4, which is still commonly used in the 2.3-2.5 GHz band (typically for Bluetooth and Wifi antennas).
To calculate the dimensions of the CPWG I used a trusted, free industry download from Avago Technologies: AppCAD. Download is linked to this project.
The actual copper width is meant to be 91.5 but I rounded up to compensate for the milling. Even with innacurate milling we should be within 5% of the target Zo anyway.
To calculate the optimum miter dimensions for the zig-zaggy transmission line, I used a free download also - an Excel spreadsheet calculator from Microwaves101.com - also a trusted source of information and calculators used by many PCB/RF engineers in the industry. Also linked to this project.
Soon to come - photos and embedded youtube video of the milling and measurements with the TinyVNA USB VNA tool.