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.

The post How Do Antennas Actually Radiate? The Physics Behind Efficient RF Design appeared first on Siretta Limited.

Objective

In a few industrial applications, the user may require a modem to send an SMS to recipient with a cellular handset to receive an alert from a remote system. It may also be necessary for a system capable of processing SMS notification to trigger an automatic response.

This application note shows how to send and receive SMS messages using a Siretta Modem.

Solution

ZETA modem is set to send and receive SMS messages from a specified mobile number and is tested to make sure it communicates with that mobile number.

A typical arrangement is shown below:

Method

Resources used for this application note are as follows:

• ZETA-NLP-LTE1 (EU) modem (Low Power Modem with RS232 and USB serial): Ultra Low Power LTE Cat 1 (EU) – Siretta – Enabling Industrial IoT
• Modem Starter Kit Quick Start Guide: ZETA-xxP Quick Start Guide
• Initial Modem Setup Guide Application Note: Initial Modem Setup Guide App Note – LP (siretta.com)
• An SMS enabled SIM card which has the ability to send and receive SMS Messages
• A PC with an installed ‘Terminal Emulator’ program: Window 10 Laptop installed with Tera Term used for the purpose of this guide.
• AT command reference manual: ZETA-NLP-LTE1 (EU) AT Command Manual
• Cellular Handset for sending and receiving SMS messages.

Note: All Siretta Industrial ZETA modems support SMS messaging services. The SMS messaging service must be enabled by SIM card provider.

1 . Follow the Initial Modem Setup Guide Application Note

2. Check the network registration using AT+CREG?

Receive following when registered to a local network:
+CREG: 0,1
Receive following when registered to a roaming network:
+CREG: 0,5

Note: If you do n.ot receive a 1 or a 5 for the +CREG command, you are not registered on the cellular network and will not be able to send an SMS message.

3 . Issue the following AT command:
AT+CMGF=?
To check supported messaging modes

Where:
0 is PDU Mode
1 is Text Mode
4. Set Modem to ‘Text’ mode by issuing the following AT command:

AT+CMGF=1

Note: The Siretta modem will automatically set its ‘Service Centre Address’

5. You can check the ‘Service Centre Address’ by issuing the following AT command: AT+CSCA?

Where “145” indicates the international dialling code scheme which contains “+” and UK dialling code. The number “+447958879879” is the mobile phone number with the UK international dialling code for the SIM installed in the modem.

6. Enter the recipient mobile number by using the following AT command.
AT+CMGS=” Mobile Number”
For this guide the mobile number 07776327870 is used

Receive “>” when successful.
7. Type your SMS message to send after “>”
“This is a test SMS from Siretta ZETA-NLP-LTE1 (EU) reply RECEIVED”.

8. To send an SMS message press and hold “Ctrl” and press “Z” on the PC keyboard to send a special termination character to the modem.
9. When the SMS has been sent you will see the confirmation and SMS message queue location with the following message:
+CMGS: XXX

10. You have now sent an SMS to the mobile number 07776327870.
11. Below is a screenshot from the recipient’s mobile showing the SMS message that has been received.

12. Reply to the SMS with “RECEIVED” from the mobile device to the Siretta ZETA-NLP-LTE1 (EU) modem.

13. To check received the message from the mobile device, input the following AT command AT+CMGL.

14. View the time-stamped ‘received unread’ SMS message sent from the mobile user.

15. Issue the following AT command to read the new message.
AT+CMGL=”REC READ”

16. You have now received a reply from the mobile phone using the Siretta ZETA-NLP-LTE1 (EU).
17. Issue the following AT command:
AT+CMGL=?
To see the available message options from the Siretta ZETA-NLP-LTE1 (EU).

Where:

“REC UNREAD” shows received unread SMS Messages.
“REC READ” shows received read SMS Messages
“STO UNSENT” shows stored unsent Messages
“STO SENT” shows stored sent Messages
“ALL” shows all above SMS Messages

For more information about Siretta’s ZETA Modems product portfolio or to view Siretta’s selection of Industrial modems please visit Industrial Modems – Siretta – Enabling Industrial IoT or alternatively contact your Siretta representative.

Additional Reading

The post ZETA Modem SMS interactions with Cellular Handsets appeared first on Siretta Limited.

At Siretta, our expertise in solutions in Industrial IoT leveraging cellular technology has been at the forefront of our mission. We believe that by working with a global leader like Siemens, we can bring value for connectivity solutions that will shift the way businesses and industries operate.
Peter Rees Product Manager for High Accuracy Metering within Siemens comments:

“Siretta were both proactive and responsive to implement the approval enhancements we required for our strategic project. This provided us with a cost effective solution”

Siemens selected the QUARTZ-GOLD-LTE product with optional RS485 interface however, being installed in substations, required the EMC approvals to meet Heavy Industrial Requirements. Siretta worked with their test and approval partners to approve the product to meet EN61000-6-2 within the requirements of the client timescales.

With RS485 serial connectivity and 2 ethernet ports the QUARTZ-GOLD-LTE is not only small in size but also extremely versatile and powerful.

The cellular router project with Siemens is a testament to our shared vision of connectivity that empowers industries and businesses. We are excited to embark on this journey and looking forward in supporting Siemens in further projects.

To find out more about Siretta’s Industrial Router Range, click here.

The post Siretta Partners with Siemens for Metering Solutions appeared first on Siretta Limited.

With a Siretta SNYPER, you can quickly and easily measure cellular signal strength, quality, and other network parameters, helping you to identify and troubleshoot issues as they arise.
Based on the successful Graphyte platform.

The SNYPER 5G Graphyte and IoT Graphyte has vastly improved processor, graphics, memory, display and many more!

To find out more about the SNYPER-5G Graphyte click here and here for the IoT.

The post We are proud to announce our highly successful SNYPER-5G Graphyte and IoT variants. appeared first on Siretta Limited.

The SL500 modems are ready for use with the SirettaLINK management portal, making savings in development and time to market. The SL500 solution moves away from the traditional approach of setup and management using AT commands which require a high skill level to implement correctly.

So, what makes the SL500 Industrial Modems a strong contender in the modem market?

Here are the main Top 5:

Experience Seamless Connectivity
With Siretta’s SL500, connecting remote equipment to a central location has never been easier. Say goodbye to complex setups and technical headaches – our intelligent modems are designed for simplicity, ensuring you’re up and running in no time.

Effortless Setup and Configuration
Worried about the technical know-how needed for setup? The SL500 solution boasts a user friendly interface that requires very little technical knowledge, it can be configured remotely via our intuitive SirettaLINK management portal. No need for on-site configuration – you’re in control from wherever you are.

Streamline Mass Deployments
Managing multiple devices doesn’t have to be a logistical nightmare. With the SL500, you can create and test device profiles prior to mass deployment. Tailor settings to your exact needs and ensure a smooth roll-out process that’s custom to your application.

Tailored SIM Management
We understand that every project is unique. That’s why our solution allows you to assign specific SIM details and account ownership effortlessly using the SirettaLINK configuration tool.

Precision at Your Fingertips
Take control with our SirettaLINK management portal. You’re in charge of configuring the SL500 solution, with advanced validation of all configuration parameters. Minimize errors and maximize efficiency, all with the guidance and feedback you need.

Where can SL500 modems be used?
Car Parking Payment
Retail Kiosks
Energy / Water Utilities
Security / Fire Alarm Systems
Gaming Machines
Healthcare
Temperature Audit Trail
Elevator Systems
Payment Systems
Fridge / Freezer Monitoring

If you are interested in finding out more about Siretta’s SL500 modem range, please get in touch with us at sales@siretta.com or checkout the product page, where you will find more details here.

The post Seamlessly Connect and Control with Ease! appeared first on Siretta Limited.

Businesses that continue to use 3G face a number of difficulties as a result of the Sunset. Businesses will need to update to 4G or 5G compatible devices if they currently use IoT devices that rely on 3G connectivity. Particularly for smaller businesses or those with a high number of devices, this can be a major budget expense.

The solution for businesses whilst 3G sunsets, presents challenges. However, it also creates opportunities for businesses to improve their technologies and gain a competitive and future-proofing advantage. Upgrading to 4G or 5G network compatible modems or routers, will provide businesses with faster and more reliable internet connectivity. This can lead to improved productivity, enhanced communication, and the ability to leverage new technologies that require high-speed connectivity.

For Low Power usage, NB-IoT and CAT-M technology would be the preferred choice to compliment devices that transmit a small amount of data on an infrequent basis, meaning power savings can be very noticeable.
At Siretta we can help solve your business challenges by understanding your needs and advising you the best IoT connectivity solution from LPWAN, LTE and 5G NR technologies.

The post The sunset of 3G is not far away appeared first on Siretta Limited.

Keep a look out for the launch of the NEW SNYPER-AP model which will be coming very soon.

To keep up to date with the latest updates on the launch date, register your interest here.

The post Did someone say a NEW SNYPER is coming to the market? appeared first on Siretta Limited.

Let Siretta introduce to you the NEW QUARTZ-GOLD-5G Model.

The QUARTZ-GOLD-5G router is a high speed dual port gigabit ethernet industrial router designed for transferring large amounts of data via the 5G NR cellular network.

To find out more about how the QUARTZ-GOLD-5G could help with your application click here. 

The post Siretta’s 5G Router range is expanding…… appeared first on Siretta Limited.

The OSCAR 67 is a highly efficient directional antenna, with a peak gain of 9 dBi, a VSWR of 1.35 and the ability to handle up to 100 watts of power!

Keep a look out on email announcements and social media channels for the official launch date coming soon!

Find out who your local distributor is by clicking here.

The post NEW Oscar Antenna coming soon appeared first on Siretta Limited.

It is here, Siretta’s first 5G Router QUARTZ-ONYX.

The QUARTZ-ONYX router is a high-speed quad port gigabit ethernet industrial router designed for transferring large amounts of data via the 5G NR cellular network. To find out more why not take a look at the Datasheet or check out the QUARTZ-ONYX Flyer.

To find out where you can get your hands on one of these, check out our Where To Buy page.

The post NEW! Siretta’s first 5G Router has landed! appeared first on Siretta Limited.