ARRL Technician Practice Exam

If the averaging time for exposure is 6 minutes, how much power density is allowed if the signal is present for 3 minutes and absent for 3 minutes?

1. 1.5 times as much

2. 2 times as much

3. 3 times as much

4. 4 times as much

The correct answer is: 2 times as much

To understand why a power density of 2 times as much is allowed when the signal is present for 3 minutes and absent for 3 minutes, it's important to consider how averaging time and power density relate to exposure limits. The exposure limit is typically based on the average power density over a certain amount of time—in this case, 6 minutes. If the signal is present for half of that time (3 minutes) and absent for the other half (3 minutes), the average power density must be calculated over the entire averaging period. If we denote the allowed power density as P, during the 3 minutes the signal is present, the power density can be at its maximum allowed level, which is represented as P. When the signal is absent for the other 3 minutes, the power density is essentially 0. To find the average power density over the full 6 minutes, we use the formula: Average Power Density = (Power Density when present * Time Present + Power Density when absent * Time Absent) / Total Time From this, we can calculate: Average Power Density = (P * 3 minutes + 0 * 3 minutes) / 6 minutes = P/2. This shows that, to maintain