Question E4E11:
What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?  

Choices:  
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 is  B: Nearby corroded metal connections are mixing and reradiating the broadcast signals  

Correct answer: B — Nearby corroded metal connections are mixing and reradiating the broadcast signals

1) Why B is correct
- Corroded or loose metal-to-metal contacts behave like weak semiconductor junctions (nonlinear diodes). Nonlinearity causes mixing of strong local AM carriers at $f_1$ and $f_2$, creating intermodulation (IM) products that are reradiated and can fall anywhere on MF/HF.
- Common sources: rusty gutters, fences, tower hardware, guy wires, oxidized connectors.

Useful IM product frequencies:
- Second order: $$f_{IM2} = |f_1 \pm f_2|$$
- Third order: $$f_{IM3} = |2f_1 \pm f_2|,\ |2f_2 \pm f_1|$$

Example: If $f_1 = 1.0$ MHz and $f_2 = 1.2$ MHz:
- $f_1 - f_2 = 0.2$ MHz (200 kHz, MF)
- $f_1 + f_2 = 2.2$ MHz (HF)
- $2f_2 - f_1 = 1.4$ MHz (MF)

2) Why the other choices are incorrect
- A: Over-modulation causes spectral splatter around a single station’s carrier (tens of kHz), not sum/difference products between two different stations across MF/HF.
- C: Skywave refers to propagation of an existing signal on its own frequency, not two local stations “combining” to make new spurious frequencies.
- D: Receiver IF overload can create intermod inside your receiver, but the question points to signals “combining” and being radiated locally by something external; that’s characteristic of nonlinear junctions in corroded metal.

3) Formulas and rules of thumb
- Intermod frequency relationships:
  - $$f_{IM2} = |f_1 \pm f_2|,\quad f_{IM3} = |2f_1 \pm f_2|,\ |2f_2 \pm f_1|$$
- Growth with signal strength (two-tone):
  - 2nd-order products rise about 2 dB for each 1 dB increase in input.
  - 3rd-order products rise about 3 dB per 1 dB input.
- Mitigation tips:
  - Find/clean/bond corroded joints; tighten hardware.
  - Use broadcast-band high-pass/notch filters ahead of the receiver.
  - Apply ferrite chokes on nearby conductors to reduce common-mode pickup.