“Transmitter power tells you what the radio generates. EIRP tells you what the antenna actually puts into the air. In RF engineering, that distinction is everything.” – MJ Martin
The Wrong Number
When comparing the RF performance of electric meters, quoting only the transmitter amplifier output is technically incomplete. A transmitter may produce 500 milliwatts, 750 milliwatts, or 1 watt at its output stage, but that number does not tell us how much useful electromagnetic energy is actually launched into space. For that comparison, Equivalent Isotropically Radiated Power, or EIRP, is the meaningful measurement.
There is one important qualification. Canadian regulations control conducted transmitter power as well as radiated power. ISED RSS-247, for example, limits many digital transmission systems operating between 902 and 928 MHz to 1 watt of maximum conducted output power and 4 watts EIRP. The regulations therefore explicitly recognize that transmitter output and radiated power are different quantities. (ised-isde.canada.ca)
Canada and the ISM Spectrum
Industrial, Scientific, and Medical, or ISM, spectrum consists of internationally designated frequency bands intended to accommodate equipment that generates RF energy for industrial, scientific, medical, and related purposes. In Canada, the designated ISM bands include 13.553 to 13.567 MHz, 26.957 to 27.283 MHz, 40.66 to 40.70 MHz, 902 to 928 MHz, 2.400 to 2.500 GHz, 5.725 to 5.875 GHz, and 24.00 to 24.25 GHz. (ISED)
Licence-exempt communications systems frequently operate within portions of these bands under specific ISED Radio Standards Specifications. For utility metering, 902 to 928 MHz is particularly important because it provides an excellent compromise among propagation distance, building penetration, antenna dimensions, and bandwidth.
RSS-247 establishes technical requirements for many frequency-hopping and digital transmission systems operating at 902 to 928 MHz, 2400 to 2483.5 MHz, and other specified bands. (ised-isde.canada.ca)
What EIRP Actually Means
ISED defines EIRP as the power supplied to an antenna multiplied by the antenna gain in a particular direction relative to an ideal isotropic antenna. (ISED)
In practical engineering terms:
EIRP (dBm) = Transmitter Power (dBm) – RF Losses (dB) + Antenna Gain (dBi)
Think of the RF amplifier as an automobile engine. Knowing engine horsepower tells us something, but it does not tell us how effectively that power reaches the pavement. The transmission, drivetrain, and tires matter. In radio, the amplifier produces RF power, while feedline losses, impedance matching, connectors, antenna efficiency, enclosure effects, and antenna gain determine how effectively that power reaches the air.
Consider two electric meters. Meter A produces 30 dBm, or 1 watt, but experiences 2 dB of RF loss and has 1 dBi of antenna gain. Its EIRP is approximately 29 dBm.
Meter B produces only 27 dBm, or 500 milliwatts, but experiences 1 dB of loss and has 6 dBi antenna gain. Its EIRP is approximately 32 dBm.
The second meter has half the transmitter amplifier power, yet approximately twice the EIRP.
Declaring Meter A superior because it has a 1-watt amplifier would therefore produce exactly the wrong conclusion.
The Measurement That Matters
For comparing the transmitted RF strength of competing meters, EIRP is the defensible measurement because it considers the complete RF transmission chain rather than one component inside the radio.
That is also why regulatory specifications distinguish conducted output power from EIRP. In the 902 to 928 MHz band, RSS-247 permits many digital transmission systems to operate at up to 1 watt conducted output while limiting EIRP to 4 watts. (ised-isde.canada.ca)
A manufacturer cannot simply install a larger amplifier and declare that it has created a superior radio. Antenna gain, RF losses, spectral characteristics, bandwidth, emissions, and ultimately the amount of permitted energy radiated into space must all be considered.
EIRP Is Not the Entire Link Budget
EIRP should not, however, be confused with total communications performance. Receiver sensitivity, antenna radiation pattern, polarization, modulation, forward error correction, interference rejection, noise floor, duty cycle, installation environment, and link margin all affect communications reliability.
EIRP tells us how effectively the transmitter puts RF energy into the air. The link budget tells us whether that energy can successfully carry information to the receiver.
So, when someone claims that one electric meter has a “more powerful radio,” the technically appropriate response is not simply, “What is its amplifier output?”
The better questions are:
- What is its certified EIRP?
- What is its antenna pattern?
- What is its receiver sensitivity?
- What is the complete link budget?
- And, most importantly, does that engineering translate into measurably better communications reliability in the field?
A particularly strong takeaway from this discussion is: “Amplifier power tells you what the radio produces. EIRP tells you what the world actually sees.”
About the Author:
Michael Martin is the Vice President of Technology with Metercor Inc., a Smart Meter, IoT, and Smart City systems integrator based in Canada. He has more than 40 years of experience in systems design for applications that use broadband networks, optical fibre, wireless, and digital communications technologies. He is a business and technology consultant. He was a senior executive consultant for 15 years with IBM, where he worked in the GBS Global Center of Competency for Energy and Utilities and the GTS Global Center of Excellence for Energy and Utilities. He is a founding partner and President of MICAN Communications and before that was President of Comlink Systems Limited and Ensat Broadcast Services, Inc., both divisions of Cygnal Technologies Corporation (CYN: TSX).
Martin served on the Board of Directors for TeraGo Inc (TGO: TSX) and on the Board of Directors for Avante Logixx Inc. (XX: TSX.V). He has served as a Member, SCC ISO-IEC JTC 1/SC-41 – Internet of Things and related technologies, ISO – International Organization for Standardization, and as a member of the NIST SP 500-325 Fog Computing Conceptual Model, National Institute of Standards and Technology. He served on the Board of Governors of the University of Ontario Institute of Technology (UOIT) [now Ontario Tech University] and on the Board of Advisers of five different Colleges in Ontario – Centennial College, Humber College, George Brown College, Durham College, Ryerson Polytechnic University [now Toronto Metropolitan University]. For 16 years he served on the Board of the Society of Motion Picture and Television Engineers (SMPTE), Toronto Section.
He holds three master’s degrees – in business (MBA), communication (MA), and education (MEd). As well, he has three undergraduate diplomas and seven major certifications in business, computer programming, internetworking, project management, media, photography, and communication technology. He has completed over 80 next generation MOOC (Massive Open Online Courses) [aka Micro Learning] continuous education programs in a wide variety of topics, including: Economics, Python Programming, Internet of Things, Cloud, Artificial Intelligence and Cognitive systems, Blockchain, Agile, Power BI, Big Data, Design Thinking, Security, Indigenous Canada awareness, and more.
Martin is a volunteer, a photographer, a learner, a technologist, a philosophizer, and a romantic optimist.