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Modulation Monitor is a simple and useful circuit for hams. You can listen to on air transmission from your rig and be sure about the quality of the transmission. It requires no direct connection with the transmitter.

In the prototype, headphones of 1K impedance are used. If you want to use ordinary low impedance head phones, connect a suitable length of wire from the junction between the diode and the coil through a 1000pf capacitor. L1 is easily available from junk box of hams since it the same as that used by hams in their QRP Transmitter.

The entire setup can be housed inside a plastic box and placed near your transmitter adjust gang condenser C1 for maximum sound. The circuit is for 7 MHz ham band. Suitably changing the values of L1 and C1, it can be used for other ham bands also.

Modulation Monitor

L1         - 23 Turns 24 SWG
             Wound over 1.5Cm
             dia PVC tube
C1         - 2J PVC Gang Capacitor
D1         - IN34 or IN4148
C2         - 0.01 ceramic disc
EP         - 1 K OHMS Headphone

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