GPS-antenna - 2016-02-13 19:12:42
Started - 28th of July 2013 Ongoing - todo - Design of fixtures (PCB)
The following description is not intended as a complete guideline on how create a GPS antenna. It describes an attempt to implement a classical antenna with good coverage of the sky. If someone finds it useful, then all is well. Use the information at your own risk.
In applications where GPS is used in a fixed location as a source for timing or frequency it is important to see as many satellites as possible that are available over the horizon. Patch antennas are practical in many ways, but lack the ability to see satellites close to the horizon.
A classical antenna often used to see all satellites in view is the RHCP backfired quadrifilar helical antenna. Several sites on the net (see references) discuss the dimensions and various design methods for such antennas. Here is yet an other approach to the practical implementation.
Design of the antenna
In this version of the antenna, small printed circuit boards are used in each end of the helix. I ended up using this solution because it allows adjustment of the center frequency without changing wire length and it made it easier to solder the parts together.
Figure 1: PCB fixtures to terminate the wires.
This design is specifically intended for GPS usage and will probably not scale well to lower frequencies. It is based on a brass tube with an outer diameter of 4 millimeters and with an inner diameter of 3 millimeters. The signal feed is a length of RG-58U/C cable which have been stripped of its jacket and its braided shield. It fits nicely into the tube, which now acts as the shield. Tube now becomes a rigid coaxial feed. Actually, the cable is mounted when much of the other parts are fitted to the tube as we will see later.
The circuit boards (top and bottom) are soldered to the tube using the PCB copper that surrounds the hole.
Figure 2: PCB fixtures fitted to the tube.