Near Field and Far Field
If you stand directly under a large broadcast tower, the electromagnetic environment is extraordinarily complex. The fields swirl and pulse in a way that has little resemblance to the clean, orderly radio wave arriving at your receiver twenty miles away. These two situations — right next to the antenna and far from it — are so different from each other that engineers have given them different names and different mathematical treatments. Understanding near field and far field is essential for interpreting antenna measurements, understanding RF safety calculations, and knowing why obstructions close to your antenna hurt performance.
Three distinct regions surround every antenna. The field structure is very different in each zone, and the boundaries depend on wavelength and antenna size.
View LargerWhy Different Regions Exist
When an antenna radiates, it does not simply launch a clean plane wave into space the moment you apply RF power. The electromagnetic field structure immediately surrounding the antenna is extremely complex. At and very near the antenna, the electric and magnetic fields are largely in the form of stored energy — the same kind of energy storage that occurs in a capacitor or inductor. These fields are reactive; they store energy during one part of the cycle and return it during another part, much like a spring stores and releases mechanical energy.
As you move further away from the antenna, the balance gradually shifts. The stored reactive fields fall off very rapidly with distance (as 1/r² or 1/r³), while the radiated fields fall off more slowly (as 1/r). At some distance, the radiated fields dominate completely, and you have the clean, propagating electromagnetic wave that your receiver eventually detects. But between "right next to the antenna" and "far away," there is a complex transition zone where neither the reactive nor the radiated fields dominate clearly.
This is why engineers define three distinct regions: the reactive near field (dominated by stored energy), the radiating near field or Fresnel region (transition zone where both are significant), and the far field or Fraunhofer region (dominated by propagating radiation). The boundaries are not sharp — they are approximate and depend on frequency and antenna dimensions — but the regions are physically meaningful and practically important.
Reactive Near Field
The reactive near field is the region immediately surrounding the antenna, where the stored electromagnetic energy dominates over the radiated energy. In this region, the electric and magnetic fields do not have the fixed ratio of 377 ohms (the wave impedance of free space) that characterizes far-field radiation. Instead, the field impedance varies strongly with distance and direction, and the fields have significant phase differences between them.
Think of this region as the antenna's "electromagnetic environment" — the space the antenna is actively interacting with, almost as if it were a large lumped component. The energy stored in this region does not radiate away; it oscillates in and out of the antenna with each RF cycle, contributing to the antenna's reactive impedance (the inductance and capacitance effects at the antenna feedpoint). Anything you place in this region — a building, a metal object, another wire, even your own body — will interact with the stored fields and change the antenna's impedance and radiation pattern.
The reactive near field extends from the surface of the antenna to a distance of roughly λ/(2π), which is approximately 0.159 wavelengths. For a 14 MHz HF antenna (wavelength ≈ 21 meters), this reactive near field extends about 3.4 meters (11 feet) from the antenna. For a 146 MHz VHF antenna (wavelength ≈ 2 meters), it extends only about 0.32 meters (about 1 foot). At microwave frequencies (2.4 GHz, wavelength ≈ 12 cm), the reactive near field is only about 2 cm from the antenna.
RF safety standards use the reactive near field boundary in their calculations. FCC maximum permissible exposure (MPE) standards recognize that the field structure close to a transmitting antenna is fundamentally different from the far field, and that simple plane-wave power density calculations do not apply in the near field. If you are operating at high power and are concerned about RF exposure for people (yourself, family members, neighbours), you need to be aware of this region and ensure that people are not present within it during transmitting.
Radiating Near Field (Fresnel Region)
Beyond the reactive near field lies the radiating near field, also called the Fresnel region. In this zone, the radiated energy dominates over the stored reactive energy — the antenna is clearly radiating. However, the radiation pattern has not yet stabilized. If you were to measure the field strength at different angles around the antenna in this region, you would get a different answer depending on how far from the antenna you were. The pattern changes as you move outward.
This happens because the wave fronts from different parts of the antenna are still reaching the measurement point at significantly different phases. For a small antenna — one much smaller than a wavelength — this is not a major issue. But for a large antenna or antenna array (like a multi-element Yagi or a phased array), different elements of the antenna are spatially separated. Their contributions add up differently at different distances in the Fresnel region, causing the pattern to vary with distance until you are far enough away for the geometry to "sort itself out."
For antennas where the largest dimension (D) is comparable to or larger than a wavelength, the Fresnel region extends from the reactive near field boundary out to approximately 2D²/λ, where D is the largest dimension of the antenna and λ is the wavelength. For a small antenna like a simple dipole (D ≈ λ/2), this boundary is at roughly 2 × (λ/2)² / λ = λ/2, which is just half a wavelength from the antenna. In practice, the Fresnel region for simple dipoles is quite small.
For larger antennas, the Fresnel region can be surprisingly large. Consider a 10-element Yagi for 144 MHz. At 144 MHz, λ ≈ 2.08 meters. A 10-element Yagi might have a boom length of 5 meters (D ≈ 5 m). The far-field boundary is approximately 2 × 5² / 2.08 = 24 meters. This means you need to be at least 24 meters (nearly 80 feet) from such a Yagi to be in the true far field where the pattern is stable. Antenna measurements made closer than this would give misleading results.
Far Field (Fraunhofer Region)
The far field — also called the Fraunhofer region, borrowing terminology from optics where the same mathematics applies — is the region where the radiation pattern has stabilized and no longer changes with distance. In this region, the wave fronts from all parts of the antenna are essentially parallel to each other when they arrive at any distant point, so the interference pattern (the radiation pattern) is fixed. Moving twice as far away does not change the pattern — it only reduces the field strength.
In the far field, the electromagnetic wave has a specific character: it is a transverse electromagnetic (TEM) plane wave, with E and H fields perpendicular to each other and to the direction of propagation, and with the ratio E/H = 377 ohms (the impedance of free space). The power density (watts per square meter) falls off exactly as 1/r² — the inverse-square law. Double the distance, quarter the power density. This is the familiar behavior of all far-field electromagnetic radiation.
The far field is where antenna gain measurements are taken. It is where link budget calculations apply. When your receiver is picking up signals from a transmitting station, both stations are in each other's far field (unless you are in very close proximity), so the simple inverse-square law applies and gain and path loss calculations work as expected.
For most practical HF antennas (simple dipoles and verticals), you are in the far field of your own antenna well before you reach the ionosphere, and certainly before you reach any other station. Even for large antenna arrays, the far field starts within a few tens of meters at HF frequencies. For most ham radio purposes, the far field effectively begins anywhere beyond a few wavelengths from the antenna — perhaps 30 to 60 meters for HF antennas, a few meters for VHF antennas.
Boundaries Between Regions
The exact boundaries between the three regions depend on the antenna's largest physical dimension (D) and the operating wavelength (λ). The following table summarizes the boundaries and worked examples for common ham radio antennas.
| Region | Boundary Criterion | 7 MHz Dipole (D = 20 m, λ = 43 m) | 146 MHz Yagi (D = 1.5 m, λ = 2.05 m) |
|---|---|---|---|
| Reactive Near Field | r < λ/(2π) ≈ 0.159λ | r < 6.8 m | r < 0.33 m |
| Radiating Near Field (Fresnel) | 0.159λ < r < 2D²/λ | 6.8 m to 18.6 m | 0.33 m to 2.2 m |
| Far Field (Fraunhofer) | r > 2D²/λ | r > 18.6 m | r > 2.2 m |
These boundaries are approximations, not hard physical walls. The physics changes gradually, not abruptly. For antennas where D is much smaller than λ (like a short mobile whip), the 2D²/λ criterion gives a far-field boundary that may actually be smaller than the reactive near-field boundary. In that case, the commonly used rule of thumb — be at least a few wavelengths away for a valid far-field measurement — is more appropriate.
Frequency: 14 MHz. Wavelength λ = 300/14 = 21.4 meters. Dipole length D ≈ 10 meters (half wavelength).
Reactive near field extends to: λ/(2π) = 21.4/6.28 = 3.4 meters.
Far field begins at: 2D²/λ = 2 × 10² / 21.4 = 200/21.4 = 9.3 meters.
Result: For a simple 14 MHz dipole, you are in the far field at any point more than about 10 meters from the antenna. Even a nearby receiving antenna at 20 meters distance is solidly in the far field. This is typical for simple wire antennas — the far field starts very close to the antenna in terms of wavelengths.
Practical Implications for Ham Radio
Objects Close to Your Antenna Affect Its Performance
Any object within or near the reactive near field of your antenna will interact with the stored electromagnetic energy and alter the antenna's impedance and radiation pattern. Metal rooflines, gutters, power lines, the mast itself, supporting ropes if they contain metal strands, nearby trees with wet leaves — all of these can affect antenna performance when they are within a few meters of an HF antenna or within about half a meter of a VHF antenna.
This is why antenna modeling software like EZNEC always asks you to specify nearby objects (ground, mast, support wires) — those objects are part of the electromagnetic environment the antenna exists in. Two identical dipoles installed in different locations — one clear of obstructions, one near a metal building — will have different impedances, different radiation patterns, and different efficiencies. The antenna itself is identical; the environment has changed it.
Antenna Measurements Must Be Taken in the Far Field
When you use an antenna analyzer to sweep your antenna, you are measuring the feedpoint impedance, which is determined by the near field environment of the antenna. That measurement is valid regardless of distance. But if you want to measure the actual radiation pattern of your antenna — how much power goes in each direction — you must make those measurements in the far field. Radiation pattern measurements made in the Fresnel region will give incorrect results because the pattern has not yet stabilized.
Professional antenna test ranges are designed to ensure the measurement antenna is always in the far field of the antenna under test. Range lengths of many wavelengths are typical. Compact antenna test ranges use mirrors and lens techniques borrowed from optics to synthesize far-field conditions in a smaller space. Amateur antenna measurements are often compromised by nearby objects (houses, trees, ground irregularities) and by measurement distances that may not be strictly in the far field — this is why published antenna gain figures often differ from what you actually observe in practice.
RF Safety Calculations
The FCC requires ham operators running more than a certain power level to perform a routine RF evaluation and ensure that their stations comply with MPE (Maximum Permissible Exposure) limits. The MPE limits are specified in terms of power density (mW/cm²) for the far field. In the near field, the actual exposure can be higher or lower than a simple far-field calculation suggests, which is why the FCC guidelines include near-field correction factors and why staying out of the reactive near field of a transmitting antenna is particularly important.
For a 100-watt HF station with a dipole at reasonable height, the near-field boundaries are usually manageable — you need to stay a few meters from the antenna during transmitting, which is easily accomplished. For VHF and UHF operators running high power into Yagi arrays mounted close to the ground or at head height, the near-field region is much smaller but also much closer to where people stand. These operators need to be more careful about ensuring adequate separation.
Interaction Between Multiple Antennas
If you have more than one antenna — for example, a 40-meter dipole and a 15-meter beam — and they are close to each other, the near field of one antenna may interact with the other. The affected antenna acts as a parasitic element, absorbing and re-radiating some of the power from the driven antenna and altering both antennas' patterns and impedances. This interaction is exploited deliberately in Yagi-Uda beam antenna design (covered in Lesson M14I), but when it happens accidentally between independent antennas it is a problem. The general rule is to separate co-located antennas by at least a few wavelengths of the lower-frequency antenna for minimal interaction.
- The reactive near field (within ~0.159λ) is dominated by stored energy, not radiation. Objects here affect antenna impedance and pattern.
- The radiating near field (Fresnel region) is the transition zone where the radiation pattern is still changing with distance.
- The far field (Fraunhofer region, beyond 2D²/λ) is where the pattern is stable and 1/r² propagation applies. All antenna gain specs and link budget calculations assume far-field conditions.
- For simple wire antennas at HF, the far field starts within a few meters to a few tens of meters. For large arrays, it can be much further.
- RF safety MPE limits apply in the far field. In the reactive near field, actual exposure may be higher.
Frequently Asked Questions
If the far field starts just 10 meters from my HF dipole, why does tree-cover close to the antenna affect my signal?
Trees within the reactive near field (within a few meters) interact directly with the stored electromagnetic energy and change the antenna's impedance. Trees further away — in the Fresnel region or far field — can still absorb or scatter the radiated wave. Wet foliage at HF frequencies is a moderate absorber. Trees blocking the path in a specific direction will attenuate the signal in that direction. The effects are different in the near field (impedance change) vs. the far field (absorption/scattering), but both matter. General advice: clear your antenna of close obstructions and get height above surrounding foliage.
My antenna analyzer shows SWR changing when I stand near the antenna. Is that the near field?
Yes, almost certainly. Your body is a significant conductor at HF frequencies. When you stand in the reactive near field of the antenna, your presence loads the antenna — you become a parasitic element. This changes the impedance seen at the feedpoint, which is what your antenna analyzer measures. The effect is most pronounced at HF where antenna near fields extend several meters. When taking SWR readings, stand as far from the antenna as possible (away from the far end of the feedline), or use a remote-controlled antenna analyzer or one with a long display cable. The SWR reading when standing several meters away is the accurate one.
Does near-field energy pose more of a health risk than far-field radiation?
It can. The near field has both electric and magnetic field components that do not have the fixed 377-ohm ratio of a plane wave. The E-field and H-field peaks do not coincide in time or space the same way they do in the far field. Regulatory MPE limits are specified for plane-wave (far-field) conditions. In the reactive near field, the actual field strengths can be much higher than a simple power-density calculation would suggest, which is why the FCC recommends staying out of the reactive near field during transmitting. For most amateur installations at reasonable power levels, this means staying about 1–5 meters from HF antennas during transmission.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.