Introduction
Most engineers understand how to impedance match an antenna to 50 ohms. But what actually makes an antenna radiate? And why do some antennas radiate efficiently while others barely get the job done?
In this blog, we explore the physics behind RF radiation, and how radiation resistance, antenna geometry, and loss mechanisms determine performance in Siretta’s industrial antenna range.
What Makes an Antenna Radiate?
At the most fundamental level, an antenna radiates because of accelerated charges. When RF current (oscillating current) flows through a conductor, it produces time-varying electric and magnetic fields. According to Maxwell’s equations, these fields generate self-sustaining electromagnetic (EM) waves that detach from the antenna and propagate through space.
This process isn’t unique to antennas. Any conductor carrying high-frequency current will radiate to some extent — this is why EMI (electromagnetic interference) exists. What makes an antenna special is its ability to radiate efficiently at the desired frequencies, and in a controlled direction.
Impedance Matching vs Radiation Efficiency
Impedance matching an antenna to 50 Ω ensures that as much RF power as possible is delivered from the source into the antenna. But matching does not guarantee efficient radiation.
If the antenna has high losses (resistive heating, ground loss, etc.) or low radiation resistance, much of that delivered power will be wasted.
An antenna can be perfectly impedance matched and still make a terrible EM radiator.
Figure 1: Matching (VSWR / Return Loss) tells you how much power enters the antenna. Radiation Efficiency tells you how much of that power is radiated. Both must be considered when assessing antenna performance.
Understanding Radiation Resistance
Radiation resistance is the portion of an antenna’s input resistance that actually radiates EM waves, not heat.
Where:
R_rad: Radiation resistance
R_loss: Loss resistance (ohmic, dielectric, etc.)
A high-efficiency antenna has a high radiation resistance and low loss resistance. For example, a quarter-wave monopole may have R_rad≈36Ω, while ground losses might add 20Ω . This results in ~64% efficiency.
Why Small Antennas Are Inefficient
As antennas become electrically small (much smaller than a wavelength), their radiation resistance drops dramatically, while losses remain relatively constant.
Small loop antenna: R_rad ~ 0.1 Ω
Loss resistance: 10 Ω or more
Efficiency: Less than 1%
This is a well-known limitation captured by the Chu-Harrington limit, which states that small antennas can’t be both efficient and broadband.
Common Loss Mechanisms in Antennas
1. Ohmic losses — due to resistive heating in conductors
2. Skin effect — RF current flows on surface, increasing effective resistance
3. Dielectric losses — lossy PCB substrates or nearby materials
4. Ground losses — monopoles over poor ground conductivity
Especially in compact or embedded antennas, these losses can dominate.
How Siretta Designs for Radiation Efficiency
Siretta’s antennas are engineered to maximize radiation efficiency without compromising on size or ruggedness. This includes:
• Optimizing geometry for resonant modes
• Carefully selecting materials with low loss tangent
• Integrating ground planes and matching networks in PCB antennas
Whether it’s a rugged LTE whip antenna, a GPS patch, or a multi-element puck, Siretta ensures that your RF power gets put to work — radiating effectively.
Figure 2: Siretta antenna undergoing radiation pattern and efficiency testing in a calibrated anechoic chamber.
Impedance matching is important, but it’s not the whole story. To design truly effective wireless systems, you need antennas that efficiently convert input power into radiated EM waves.
Siretta’s antennas are built with this physics in mind, ensuring performance across LTE, 5G, GNSS, LoRa, Wi-Fi, and more.
Figure 3: Example of radiation efficiency (%) across frequency from a Siretta antenna datasheet. Efficient radiation performance is maintained across key operating bands.
FAQ
What is radiation resistance in an antenna? It’s the part of input resistance responsible for radiating power into free space.
Can an antenna be impedance matched but still inefficient? Yes. Impedance matching means no reflected power if done perfectly, but that power may be lost as heat rather than radiated.
Why are small antennas less efficient? They have low radiation resistance and higher relative losses.
What causes antenna loss? Ohmic heating, poor dielectrics, skin effect, and ground loss.
Want help selecting a high-efficiency antenna for your IoT or industrial project?
Contact Siretta’s sales team or explore our antenna portfolio today.
