How Antennas Work at 915 MHz

How Antennas Work at 915 MHz

An antenna is a transducer that converts electrical energy (RF current on a transmission line) into electromagnetic waves and vice versa. Understanding the physics of this conversion is essential for making informed antenna choices in LoRa mesh deployments at 915 MHz.

The Electromagnetic Wave

When alternating current flows in a conductor, it creates an oscillating electromagnetic field that detaches from the wire and propagates through space as a wave. At 915 MHz, the wavelength in free space is approximately 32.7 cm (about 13 inches), calculated by:

λ = c / f
λ = 300,000,000 m/s ÷ 915,000,000 Hz
λ ≈ 0.328 m (32.8 cm)

This wavelength determines the physical dimensions of resonant antenna elements. A half-wave dipole at 915 MHz is about 16.4 cm long; a quarter-wave monopole is about 8.2 cm. These are the building blocks of virtually all practical antennas.

Radiation Patterns

The radiation pattern describes how an antenna distributes power in three-dimensional space. It is typically depicted as a polar plot showing relative power density in different directions from the antenna.

• Omnidirectional: Radiates equally in all azimuthal (horizontal) directions, forming a donut-shaped pattern around a vertical axis. Most LoRa node antennas are omnidirectional.
• Directional: Concentrates energy in one or more preferred directions. Used for long-range point-to-point links or sector coverage.
• Main lobe: The primary direction of maximum radiation.
• Side lobes: Minor lobes at other angles, generally undesirable and wasting power.
• Null: Directions where radiated power drops to near zero. High-gain vertical antennas often have a null straight up and straight down.

Antenna Gain: dBi vs dBd

Gain is the most frequently misunderstood antenna specification. Antenna gain does not mean the antenna amplifies power - it cannot; antennas are passive devices. Gain describes how effectively an antenna concentrates available power in a specific direction compared to a reference antenna.

A manufacturer claiming "5 dBd gain" actually means approximately 7.15 dBi. Always convert to dBi before doing link budget calculations. Be cautious of inflated gain claims on inexpensive antennas - a vertical omni physically cannot achieve more than about 8 - 9 dBi without becoming so tall that its elevation angle rises and its coverage pattern degrades.

Isotropic vs Real Antennas

The isotropic radiator is a mathematical construct - a perfect point source that radiates uniformly in all directions. No real antenna achieves this. The simplest real antenna, the half-wave dipole, already has 2.15 dBi of gain because it concentrates radiation into its broadside plane rather than wasting energy off the ends.

Real antennas introduce additional losses: conductor resistance (ohmic loss), dielectric loss in radomes or matching components, and impedance mismatch. The efficiency of a real antenna is:

Gain (dBi) = Directivity (dBi) + Efficiency (dB)

A well-made antenna will have efficiency above 90%; cheap antennas can fall to 50% or lower, turning claimed gain into a fiction.

Near Field vs Far Field

The space around an antenna is divided into regions based on the character of the electromagnetic field:

For practical LoRa mesh purposes, you are always operating in the far field - links are meters to kilometers long. The near field is only relevant when mounting antennas close to metal objects, where reactive fields can detune the antenna and alter its pattern significantly.

A key takeaway: keep antenna elements at least λ/4 (about 8 cm at 915 MHz) away from metal surfaces, and preferably λ/2 or more. Even a metal enclosure lid placed too close to an antenna can shift its resonant frequency and reduce efficiency by several dB.