For at project at the university I needed an antenna signal splitter to share the signal from a single GPS antenna between up to three GPS receivers. I found a few commercial products available but decided it would be much more fun and learning to build it myself.
My design goals were:
There are some challenges building an antenna signal splitter for GPS receivers:
To block the DC voltage to and from all but one of the GPS receiver outputs, capacitors are inserted in series with the center conductor of the transmission line for each of those outputs. Ideally the capacitor shall block any DC voltage and behave as a short at the L1 and L2 frequencies.
The reactance 5) of a capacitor is given by:
Where f is the frequency in Hz and C is the capacitance in Farad. Below is a plot of the reactance at the L1 and L2 frequencies for capacitance between 1 and 150 pF (Python code):
In the real world capacitors are not ideal however, the construction and external leads add inductive and resistive components. This causes a number of complications, one of them being that capacitors have a series self-resonant frequency (SRF) due to parasitic inductance6).
In theory the lowest resistance at a working frequency would be obtained by selecting a capacitor with a SRF at this frequency. But since capacitor values are not accurate, SRF should be higher than the working frequency to avoid inductive properties.
SRF depends on the capacitor size and type and is stated in the capacitor data sheet. So selecting the capacitor for this project comes down to making sure that SRF is higher than the L1 frequency and the reactance is as low as possible. Anything below 4 Ohm should be acceptable. Looking at some data sheets the reactance should ideally be between 50 and 75 pF.
The construction of the housing is pretty much like 7) but since I chose to use TNC sockets I had to add washers to allow room for all three sockets. the small screw on the fourth side only serves the purpose to close a hole I accidently drilled.