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13.3 T
RANSMITTING
Do not transmit without a matched 40M antenna or 50 dummy load connected.
To transmit press the PTT button on your microphone and talk. The LED on the front panel will
flash as you speak. This provides an indication of the power output, as the current through the
LED, and therefore its brightness, is dependent on the peak RF output. If the LED is illuminated
at a constant level when talking, it indicates that you are driving the transmitter too hard and
causing clipping of the RF signal.
Overdriving will create distortion and excessive harmonic generation and must be
avoided.
To check your signal you can either, connect the MDT to a dummy load and monitor yourself
with headphones on a nearby receiver, or have a friend that lives close by listen to your signal.
The idea is to increase the Mic gain progressively while the receiving station sweeps across your
transmission looking for distortion and unwanted spurious byproducts. Set the Mic gain control
just below the point where these are noticeable. A screenshot from an oscilloscope placed
across a dummy load to monitor the output waveform is shown in Figure 23. The audio input to
the MDT mic socket was a 1KHz sine wave.
Congratulations your new DSB QRP transceiver is ready to put on the air.
Place the lid on the case, do up the two screws and have fun!
MDT Construction Manual – Issue 2
Figure 23 DSB transmit waveform
Page 42
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