The DRA818U radio module is known to produce harmonics at higher frequencies. To avoid problems with other radio equipment a low pass filter was installed in the latest version of the Talky GPS transponder.
To check the efficiency of the filter, a spectrum analysis was performed. Tx power at the ground frequency was unaffected (1 Watt) while the harmonic was reduced from 2.2 milliWatt to 13 microWatt.
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