Antennas & Link Budgets for Wi-Fi HaLow (920MHz)

February 2026

Radio frequency (RF) is fundamentally an energy-transfer problem, governed far more by geometry and physics than by schematics.

In Wi-Fi, smartphones, radar, or satellite links, the goal is to move information through air and space. A radio encodes data onto alternating electrical energy at a chosen frequency, guides that energy through conductors, and uses an antenna to launch it into free space as an electromagnetic wave. A receiving antenna captures a fraction of that traveling energy and converts it back into electrical form, where the original information is recovered. Information rides on that energy.

How effectively energy is transferred from transmitter to space, through space, and back into a receiver determines range, data rate, reliability, and noise tolerance. Antenna design choices are about controlling how energy couples between guided structures and the electromagnetic field.

Frequency and wavelength define scale

The governing constraint in RF is the relationship between frequency and wavelength:

$$ \lambda = \frac{c}{f} $$

where $c$ is the speed of light and $f$ is frequency.

At 920MHz (Wi-Fi HaLow in Japan), the wavelength is approximately 32.6 cm. All antenna behavior is referenced to this length. Antennas are resonant structures^[1]; their electrical properties depend on their physical size relative to wavelength. A quarter wavelength at 920MHz is about 8 cm. A half wavelength is about 16 cm.

A structure is resonant when it naturally oscillates and exchanges stored electric and magnetic energy most efficiently at a specific frequency.

If a device is only a few centimeters long, the antenna is considered “electrically small”^[2]. Electrically small antennas store far more energy than they radiate. Radiation efficiency^[3] drops, which means bandwidth narrows, and sensitivity to the environment increases. At 920MHz, millimeter-scale geometry changes are electrically significant.

“Electrically small” means the antenna’s physical size is much smaller than the wavelength, typically less than about one-tenth of it.

Radiation efficiency is the fraction of input power that is actually radiated rather than stored or dissipated as heat.

Radiation efficiency can be expressed as:

$$ \eta = \frac{R_\text{rad}} {R_\text{rad} + R_\text{loss}} $$

where $R_\text{rad}$ is radiation resistance and $R_\text{loss}$ represents ohmic and dielectric losses.

Radiation resistance corresponds to power that becomes electromagnetic waves. Loss resistance corresponds to power dissipated as heat. Electrically small antennas exhibit low radiation resistance, so even modest conductor or dielectric losses significantly reduce efficiency.

What an antenna does

We should clarify that an antenna does not amplify power. It “couples” guided electrical energy into free space. That coupling efficiency depends on resonance, impedance match, and the surrounding environment. When matched and undisturbed, most delivered power becomes a propagating electromagnetic wave^[4]. When detuned or mismatched, power is reflected or dissipated.

A propagating wave is energy that has detached from the antenna and travels outward through space.

Range is therefore dominated by coupling efficiency, not marginal transmit power increases.

Gain is spatial redistribution

Antenna gain (dBi)^[5] is referenced to an ideal isotropic radiator^[6]. It measures directional concentration of radiated energy.

dBi expresses gain relative to an isotropic radiator on a logarithmic decibel scale.

An isotropic radiator is a theoretical point source that radiates equally in all directions.

Higher gain does not increase total energy but redistributes radiation spatially, increasing intensity in some directions while reducing it in others. “Omnidirectional” in this context typically means uniform in the horizontal plane, not uniform in three dimensions.

At 920MHz, an external omnidirectional antenna may provide around +2 to +4 dBi. A compact internal antenna can exhibit negative effective gain once enclosure detuning and losses are included. The difference often exceeds practical transmit power adjustments.

Polarization and orientation

Polarization^[7] defines the direction of the electric field. Most sub-GHz systems use vertical polarization.

Polarization describes the orientation of the electric field vector of the wave.

Polarization mismatch between antennas can introduce 10–30 dB of loss. Orientation therefore directly affects link margin^[8]. Again, omnidirectional does not imply orientation independence; it describes azimuthal symmetry only.

Link margin is the excess signal strength above the minimum required for reliable decoding.

Impedance as energy acceptance

Impedance describes how a structure responds to alternating energy. It includes both dissipative and reactive components^[9] and is expressed in ohms. Only the resistive portion of impedance represents radiated or dissipated power; the reactive portion stores and returns energy each cycle.

Reactive components store energy temporarily in electric or magnetic fields rather than dissipating it.

Most RF systems standardize on 50Ω as a compromise among power handling, loss, and noise. Transmitters, cables, and antennas are designed to present 50Ω at the operating frequency. If an antenna impedance deviates from 50Ω, energy transfer is reduced.

Reflections as transfer failure

Impedance mismatch produces reflections, quantified by the reflection coefficient^[10]:

The reflection coefficient is the fraction of voltage reflected due to impedance mismatch.

$$ \Gamma = \frac{Z_L - Z_0}{Z_L + Z_0} $$

where $Z_L$ is antenna impedance and $Z_0$ is 50Ω.

The reflection coefficient $\Gamma$ is a voltage ratio: it expresses the ratio of reflected voltage to incident voltage at the impedance boundary.

Reflected power is proportional to the square of the magnitude:

$$ P_\text{reflected} = |\Gamma|^2 $$

Thus a reflection coefficient magnitude of 0.5 corresponds to 25% reflected power.

When $Z_L = Z_0$, reflection is zero and power transfer is maximal. Increasing mismatch increases reflected energy and reduces radiation.

VSWR^[11] expresses the same condition differently.

VSWR (Voltage Standing Wave Ratio) expresses mismatch by comparing maximum and minimum voltages along a transmission line.

1:1 indicates ideal match.
2:1 reflects ~11% of power.
6:1 reflects ~50%.

Mismatch affects both paths

Impedance mismatch reduces radiated power at the transmitter and reduces captured power at the receiver.

If mismatch causes a 3 dB loss at the transmitting antenna, only half of the intended power is radiated. If the receiving antenna exhibits the same 3 dB loss, only half of the arriving power is converted into useful signal voltage.

These losses are additive in the link budget. A 3 dB loss at each end reduces total link margin by 6 dB.

Because link margin determines maximum range, small impedance or efficiency losses at each antenna reduce link margin symmetrically and therefore scale directly into reduced range.

Environmental interaction

Within roughly one-tenth wavelength, fields are predominantly reactive rather than radiative^[12]. Objects in this region alter stored energy distribution.

In the reactive near field, energy oscillates around the antenna but does not yet propagate outward as a traveling wave.

At 920MHz, one-tenth wavelength is approximately 30–35 mm.

Nearby metal changes effective electrical length, shifts resonance, alters radiation resistance^[13], and absorbs energy. Internal antennas therefore vary strongly with enclosure geometry, PCB layout, battery placement, and user interaction. External antennas operate in more stable boundary conditions^[14].

Radiation resistance is the portion of an antenna’s impedance that represents power actually radiated as electromagnetic waves.

Boundary conditions are the physical constraints—nearby conductors, dielectrics, and geometry—that shape the electromagnetic fields.

Predictable impedance and repeatability

Predictable impedance means stable electrical behavior across units and environments.

External SMA antennas fix geometry, ground reference, and spacing from conductive structures during manufacture. Internal antennas depend on host device configuration and vary accordingly.

Repeatability is often more important than peak efficiency.

Multiple antennas and coupling

Closely spaced antennas couple through near fields. Mutual coupling^[15] modifies impedance and radiation patterns and can reduce receiver sensitivity.

Mutual coupling is interaction between nearby antennas where each alters the current distribution of the other.

A separation of at least quarter wavelength (~8 cm at 920MHz) is generally acceptable. Half wavelength is preferable. Polarization diversity mitigates coupling but does not replace physical spacing.

Antenna efficiency vs transmit power

A 3 dB transmit power increase doubles power.
A 10 dB antenna improvement increases effective radiated power tenfold.

Because antennas affect both transmit and receive paths, efficiency improvements often exceed practical transmit power adjustments in system impact.

Link budget as system model

System performance reduces to the link budget:

$$ \text{Link Margin} = P_\text{TX} + G_\text{TX} + G_\text{RX} - \text{Path Loss} - \text{Losses} $$

Path loss is the reduction in signal strength as energy spreads and attenuates over distance.

Antenna characteristics directly influence gain and losses. In low-rate systems such as Wi-Fi HaLow, receiver sensitivity is typically strong; antenna efficiency and placement become limiting factors.

Free-space path loss

As energy propagates, it spreads geometrically. In ideal free space, the power density decreases with the square of distance. This spreading loss is captured by the free-space path loss (FSPL):

$$ \text{FSPL (dB)} = 20 \log_{10}(d) + 20 \log_{10}(f) + 32.44 $$

where $d$ is distance in kilometers and $f$ is frequency in MHz.

At 920MHz and a distance of 1 km:

$$ \text{FSPL} \approx 20 \log_{10}(1) + 20 \log_{10}(920) + 32.44 \approx 91.7\ \text{dB} $$

This establishes the baseline loss imposed purely by geometric spreading. A system must overcome roughly 92 dB of propagation loss over 1 km before accounting for antenna mismatch, polarization loss, enclosure absorption, or multipath effects.

Every dB of antenna inefficiency directly subtracts from the margin available to overcome this spreading loss.

For example, a 14 dBm transmitter with +3 dBi transmit and receive antennas yields the following received power at 1 km:

$$ 14 + 3 + 3 − 91.7 ≈ −71.7 \text{ dBm} $$

This value represents the received signal power at the receiver input under ideal free-space conditions. Whether the link succeeds depends on how far this level sits above the receiver’s sensitivity threshold. The result, −71.7 dBm, is an absolute power level. The unit dBm expresses power relative to 1 milliwatt:

$$ P(\text{dBm}) = 10 \log_{10}\left(\frac{P}{1\text{ mW}}\right) $$

A negative dBm value does not mean “negative power” but that the received power is less than 1 milliwatt. Because dBm is a logarithmic scale referenced to 1 mW, values below 1 mW produce negative numbers. For example, −70 dBm corresponds to 0.0000001 mW (100 picowatts).

Link margin, by contrast, is expressed in plain dB because it represents a difference between two power levels. If the receiver sensitivity is −105 dBm, then:

$$ -71.7 - (-105) = 33.3\ \text{dB} $$

The 33 dB is not an absolute power. It is the excess signal strength above the minimum required for reliable decoding.

Core model

At 920MHz, wavelength sets physical scale. Antennas are resonant structures constrained by geometry. Impedance mismatch reflects energy. Polarization mismatch reduces coupling. Nearby conductive material detunes resonance. Small geometric variations produce measurable electrical effects.

An antenna is an energy-coupling structure defined by wavelength and boundary conditions. Once viewed through that model, its behavior becomes deterministic.