The RF input to Q2 operating as an active mixer arrives at the collector from the antenna through an RF low pass filter (LPF) consisting of C7, C8, C9 and L4 via coupling capacitor C6. During transmit, the RF power at the collector of Q2 is coupled in reverse via the same path to the antenna port J3. ** C6 ** C6 serves as a DC block between Q2's collector and the antenna to prevent a DC short at the antenna port from shorting out the power supply and/or burning out L3. Within an order of magnitude in either direction, it's value is not critical. ** C7, C8, C9 and L4 ** C7, C8 and C9 along with L4 serve as a low pass filter (LPF) to reduce the output harmonics of the transmitter. This filter, also in the receive path, provides suppression of interference from strong out of band transmitters operating at frequencies above about 5 MHz. C9 was chosen to resonate with L4 at 7 MHz (the second harmonic of the fundamental transmit frequency). This maximizes second harmonic suppression as well as sceptibility to 40 meter foreign broadcast band interference (BCI).
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