Antenna Gain vs. Near-Field Measurements: Understanding the Difference

In the world of amateur radio and RF engineering, few concepts are as misunderstood as antenna gain. One of the most common misconceptions is the belief that measuring the field strength close to an antenna (the near field) can tell you something meaningful about how well that antenna radiates into the far field. This is simply not true. Let’s break it down.

What is Antenna Gain?

Antenna gain refers to the ability of an antenna to concentrate radiated power in a specific direction, compared to an ideal isotropic radiator. It is measured in the far field, which is the region where electromagnetic waves have stabilized into their final radiated form, and the angular distribution of power becomes meaningful.

High antenna gain means more power is focused in a specific direction. This is what matters for long-distance communication, contesting, or DXing: how much signal gets from point A to point B.

Near Field vs. Far Field

The near field is the area close to the antenna, typically within about λ / (2π) (where λ is the wavelength). In this region:

  • The electric and magnetic fields are not fully formed radiating waves.
  • The impedance is not stable and does not match free space.
  • Field strength varies erratically with distance and direction.
  • The behavior is dominated by reactive energy, not radiated energy.

In contrast, the far field is where:

  • Fields behave like traveling waves.
  • The ratio of E-field to H-field becomes constant (about 377 Ω).
  • Angular pattern and gain become meaningful.
  • Antenna performance is measured in terms of how much energy escapes into space.

Why Near-Field Meters Mislead

Some operators try to evaluate antennas by walking around with field strength meters, holding them 1 to 2 meters away from the antenna and comparing numbers. This is flawed because:

  • Near-field readings reflect reactive coupling, not radiated power.
  • Small changes in position or body proximity can skew results.
  • Two antennas can have similar near-field strengths but vastly different far-field gain.
  • Even a dummy load can produce a strong near-field signal while radiating poorly.

Important Note:

In some cases, a higher near-field measurement may actually indicate increased ground losses, not better performance.
For example, a poorly grounded vertical with just one or two radials may dump more RF into the surrounding earth.
This leads to a stronger local field, but less power reaching the ionosphere or distant stations.
More field strength nearby? It might just mean your signal is going bananas into the ground instead of being radiated efficiently.

In other words: just because something "looks hot" in the near field doesn’t mean it goes far.

Real Ways to Measure Gain

If you really want to compare antennas, do it properly:

  1. Use a calibrated test range with known distances (preferably in the far field).
  2. Compare received signal strength from a known source over a known path.
  3. Log S-meter or SDR readings under controlled conditions.
  4. Use NEC or full-wave simulations to model expected gain and pattern.

Better yet, test in real-world conditions. Swap antennas on the air, log reports from distant stations, and average the results over time and angle.

The POTA Performer Fallacy

This confusion often arises in the portable operating community (e.g., POTA), where operators measure the near field of their small verticals and conclude they have high gain. The reality? Cutting corners on radial systems or misconfiguring loading can dramatically reduce far-field efficiency and gain, even while near-field readings look fine.

Conclusion

Near-field strength tells you nothing about far-field performance. Measuring close to the antenna is useful for EMC testing or debugging leakage, but not for evaluating radiation pattern or gain.

To understand how well your antenna really performs, you must measure or simulate what it does in the far field. Because that’s where the signals travel, and where it actually counts.

Bottom line: If you're judging your antenna based on what your near-field meter says at 1.5 meters... you're measuring the wrong thing.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.