What condition may be indicated if a VHF radio cannot contact nearby stations, an inline wattmeter indicates less than 1 watt output (HIGH power mode), and nominal voltage (e.g., 12.8 vdc) is applied to the VHF radio power input line?
• VHF transmitter power stages: driver stage vs Power Amplifier (PA) and then the antenna connection • How an inline wattmeter measures RF power (it must see RF energy flowing toward the antenna, not DC voltage) • What an open circuit or improper bias in the PA stage would do to the RF output power reading
• If the radio is in HIGH power mode and has normal DC supply voltage, which part of the transmit chain could still cause very low RF power at the output? • Would an open between the PA and antenna show as low power on the wattmeter, or would it still see the transmitter’s attempted output? Think about where the wattmeter is in the circuit. • Does the wattmeter care about DC voltage at the antenna connector, or only about RF power at the transmit frequency?
• Confirm where the wattmeter is installed in the system (relative to PA output and antenna connector). • Decide which options describe a condition that would reduce RF output power from the PA itself (not just affect the antenna). • Eliminate any option that focuses on DC at the antenna connector, since RF power measurement is not based on DC.
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