Question E0A05:
What hazard is created by operating at microwave frequencies?  

Choices:  
A: Microwaves are ionizing radiation  
B: The high gain antennas commonly used can result in high exposure levels  
C: Microwaves are in the frequency range where wave velocity is higher  
D: The extremely high frequency energy can damage the joints of antenna structures    

Correct Answer is  B: The high gain antennas commonly used can result in high exposure levels  

To calculate the ERP:  
$$ERP = P_T \times G_A$$  
where:
- $P_T$ = transmitter power in watts
- $G_A$ = antenna gain (linear)

This shows how to compute the effective radiated power for the given setup.

Why the correct answer (B) is correct:
- Microwave stations often use very high-gain antennas (dishes, horns), typically 20–40 dBi or more. High gain concentrates power into a narrow beam, creating high power density in front of the antenna.
- This can exceed maximum permissible exposure (MPE) limits and cause tissue heating (thermal hazard), especially at close distances.

Why the other choices are incorrect:
- A: Microwaves are ionizing radiation  
  Incorrect. Microwaves are non-ionizing. Photon energy is too low to ionize atoms: $E = hf$; at 10 GHz, $E \approx 4\times10^{-5}\ \text{eV}$, far below ionization energies (several eV).
- C: Wave velocity is higher at microwave frequencies  
  Incorrect. In free space, all electromagnetic waves travel at $c \approx 3\times10^8\ \text{m/s}$, independent of frequency.
- D: Extremely high frequency energy can damage antenna joints  
  Not the primary hazard. Structural joints are generally unaffected; the safety concern is human exposure (thermal effects), not mechanical damage to antenna parts.

Useful formulas and rules of thumb:
- Far-field power density from a directional antenna:
  $$S \approx \frac{P_{\text{avg}}\,G}{4\pi R^2} = \frac{\text{EIRP}}{4\pi R^2} \quad (\text{W/m}^2)$$
  where $P_{\text{avg}}$ includes duty cycle, $G$ is linear gain, and $R$ is distance.
- Convert gain: $G_{\text{linear}} = 10^{G_{\text{dBi}}/10}$.
- Safe-distance estimate (far-field):
  $$R_{\min} \approx \sqrt{\frac{\text{EIRP}}{4\pi S_{\text{limit}}}}$$
- FCC MPE reference limits (typical exam values):
  - Occupational/controlled, 1.5–100 GHz: $S_{\text{limit}} = 5\ \text{mW/cm}^2 = 50\ \text{W/m}^2$
  - General public/uncontrolled, 1.5–100 GHz: $S_{\text{limit}} = 1\ \text{mW/cm}^2 = 10\ \text{W/m}^2$
- Far-field boundary for an aperture (dish diameter $D$):
  $$R_{\text{FF}} \gtrsim \frac{2D^2}{\lambda}$$
  Inside this distance (near field), fields can be more complex and peaks higher—exercise extra caution.
- Practical rule: Never stand in the main beam of a microwave dish; aim the beam above head height and control access to the front of the antenna.