What condition may be indicated if a VHF radio cannot contact nearby stations on any channel, an inline wattmeter indicates 25 watts output (HIGH power mode), and no modulation is heard when the transmitted signal is monitored with an external receiver?
ā¢ VHF radio transmission chain: microphone ā audio (AF) stages ā modulator ā RF power amplifier ā antenna ā¢ Difference between RF power output (watts) and modulation/audio on the signal ā¢ Typical symptoms of a bad microphone / mic cable / PTT versus internal AGC or feedback issues
ā¢ If the inline wattmeter shows 25 watts on HIGH power, what does that tell you about the RF power amplifier and antenna connection? ā¢ If an external receiver hears a strong unmodulated carrier but no voice, which part of the chain is most likely failing: RF output section or audio/microphone path? ā¢ Which of the listed faults would completely remove voice audio, yet still allow a strong carrier to be transmitted on all channels?
ā¢ Confirm that RF carrier power is present and at normal level (25 W) even though no one can hear your voice. ā¢ Decide whether a malfunction in AGC or feedback circuits would normally kill all audio on every channel or just distort/affect its level. ā¢ Think about how an open circuit in the mic/PTT line would change what the transmitter sends compared to a properly working microphone.
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