E9B07: Antenna gain: Difference in radiated power between a

Q: What is the difference in radiated power between a lossless antenna with gain and an isotropic radiator driven by the same power?

The correct answer is C: They are the same. The correct answer is C: They are the same. The difference in radiated power between a lossless antenna with gain and an isotropic radiator driven by the same power is that they are the same. Both antennas radiate the same total power; the gain antenna just concentrates it in certain directions. Antenna gain doesn't create power - it redistributes it. A lossless antenna with gain takes the same input power and focuses it in certain directions, making those directions stronger. But the total radiated power equals the input power (for a lossless antenna). An isotropic radiator radiates equally in all directions. Both radiate the same total power; the gain antenna just has more power in some directions and less in others. Gain is about directionality, not total power.

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E9B

E9B07E9B

What is the difference in radiated power between a lossless antenna with gain and an isotropic radiator driven by the same power?

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Deep Dive: E9B07

The correct answer is C: They are the same. The difference in radiated power between a lossless antenna with gain and an isotropic radiator driven by the same power is that they are the same. Both antennas radiate the same total power; the gain antenna just concentrates it in certain directions. Antenna gain doesn't create power - it redistributes it. A lossless antenna with gain takes the same input power and focuses it in certain directions, making those directions stronger. But the total radiated power equals the input power (for a lossless antenna). An isotropic radiator radiates equally in all directions. Both radiate the same total power; the gain antenna just has more power in some directions and less in others. Gain is about directionality, not total power.

Why Other Answers Are Wrong

Option A: Incorrect. The power radiated isn't increased by gain. Gain redistributes power directionally, but total radiated power equals input power for lossless antennas. Option B: Incorrect. Front-to-back ratio is about directionality, not total radiated power. Both antennas radiate the same total power. Option D: Incorrect. The isotropic radiator doesn't radiate more power. Both antennas radiate the same total power when driven by the same input power.

Exam Tip

Gain antenna vs isotropic = Same total power. Remember: A lossless antenna with gain and an isotropic radiator driven by the same power radiate the same total power. Gain redistributes power directionally but doesn't create it.

Memory Aid

**G**ain **A**ntenna **P**ower = **I**sotropic **P**ower (think 'GAP = IP', same total power)

Real-World Example

You drive a lossless Yagi (with gain) and an isotropic radiator with 100 watts each. Both radiate 100 watts total. The Yagi concentrates power in its forward direction (maybe 10× stronger forward), but the total radiated power is still 100 watts. The isotropic radiates equally in all directions. Gain is about directionality, not total power.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E9B

Reference: FCC Part 97.3

Key Concepts

Antenna gain Radiated power Isotropic radiator Power conservation

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E9B topic.