Question
What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?
Answer Options
- A) One or more of the broadcast stations is transmitting an over-modulated signal
- B) Nearby corroded metal connections are mixing and reradiating the broadcast signals
- C) You are receiving skywave signals from a distant station
- D) Your station receiver IF amplifier stage is overloaded
Correct Answer: B
Explanation
The AM broadcast band (MF) contains extremely strong, local signals. When two or more of these powerful signals mix in a non-linear junction, they can generate new, spurious intermodulation (IMD) products that fall on the amateur MF or HF bands, causing persistent interference. This process requires a non-linear element acting as an unintended mixer.
The most common cause of this phenomenon is nearby corroded metal connections mixing and reradiating the broadcast signals. This is known as ‘rusty bolt effect’ or passive intermodulation (PIM). The rusted joint acts as a poor semiconductor diode, mixing the strong AM broadcast signals and generating IMD products that are then radiated by the metal structure (fence, gutter, etc.) acting as an antenna.
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