Example 1: Is My Antenna Safe?

Problem: You're running 400W PEP into a 3-element Yagi with 7 dBi gain. Your neighbour's fence is 8 metres from the antenna. The exposure limit for your frequency is 2 W/m². Is it safe?

Step 1: Convert gain from dBi to linear: 7 dBi = 10^(7/10) = 5.01

Step 2: Calculate power density: S = PG / (4 x pi x d²) = (400 x 5.01) / (4 x 3.14159 x 64) = 2004 / 804.2 = 2.49 W/m²

Step 3: Compare to limit: 2.49 > 2.0 W/m²

Answer: It exceeds the limit! You need to either reduce power, raise the antenna higher, or ensure people can't access that area during transmission.

Example 2: What's the Safe Distance?

Problem: Same setup — 400W, 7 dBi gain (5.01 linear). What's the minimum safe distance for a 2 W/m² limit?

Step 1: Use the rearranged formula: d = sqrt(PG / (4 x pi x S_limit))

Step 2: d = sqrt((400 x 5.01) / (4 x 3.14159 x 2)) = sqrt(2004 / 25.13) = sqrt(79.7) = 8.93 metres

Answer: Keep people at least 9 metres away (rounding up for safety).

Example 3: How Much Harmonic Suppression Do I Need?

Problem: Your 7 MHz transmitter produces a 2nd harmonic at 14 MHz that's 30 dB below the fundamental. The legal limit requires harmonics to be at least 50 dB below. How much additional filtering do you need?

Answer: You need an additional 50 − 30 = 20 dB of suppression. A simple 3-element low-pass filter (3 poles × −20 dB/decade) would be marginal at one octave above cutoff. A 5-element filter would give comfortable margin.