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| Filter layout and dimensions in mm | Filter assembled with connector and GND rods |
The use of a band pass filter at the input of receiver is strongly recommended.
His function is to attenuate interference on adiacent frequencies (such as GPS satellite signals on 1575 MHz and 1227MHz, telemetry downlink from other satellites, spurious emissions from cellular phone base stations ...) and also to limitate bandwidth of the LNA.
Higher performances, on this band, are guarantee by coaxial cavity filters but the microstrip filter described below give good results and is easy to study at PC simulator; moreover its great advantage is that it is easy and cheap to build.
It is an edge coupled microstrip filter, derived from the project shown on Seti League web site, just with few tuning more.
It is realized on a common double side PCB, with FR4 dielectric of 1.6mm thikness.
The topology with full measure is shown in picture below:
After some tests, the filter realized has shown an in band attenuation of 7dB and a -6dB bandwith of 150MHz, centered at 1420MHz.