The Short Answer
EIRP, effective isotropic radiated power, is the power a perfect isotropic antenna would have to radiate in every direction to match the strongest signal your real, directional antenna sends along its main beam. It is the single figure that describes how loud a transmitter is in the direction it points, and it is the figure your licence conditions cap.
You build it from three numbers already on the datasheet:
EIRP (dBm) = Transmit power + Antenna gain − Feeder loss
Add the radio output power and the antenna gain, then subtract whatever the signal loses in the feeder, jumpers and connectors on the way to the antenna. A simple way to picture the chain:
Transmit power Feeder loss Antenna gain EIRP
25 dBm --> -1.5 dB --> +16 dBi = 39.5 dBm
EIRP = Ptx + Gtx − Ltx
- Ptx is the transmitter output power in dBm
- Gtx is the antenna gain in dBi, referenced to an isotropic source
- Ltx is the total feeder loss in dB
The reference matters. Gain quoted in dBi gives you EIRP. Gain quoted in dBd, referenced to a half wave dipole, gives you ERP instead, and the two are not interchangeable.
EIRP vs ERP
EIRP and ERP measure the same thing, the radiated power in the main beam, but against different reference antennas:
| Reference antenna | Gain unit |
|---|
| EIRP | Isotropic radiator | dBi |
| ERP | Half wave dipole | dBd |
A half wave dipole already has 2.15 dBi of gain over an isotropic source, so the two figures sit a fixed distance apart:
EIRP = ERP + 2.15 dB
EIRP is the larger number. Read an ERP figure as if it were EIRP and you overstate the radiated power by 2.15 dB; read it the other way and you understate it. If the units on a datasheet leave you guessing, dBm, dBi, dBd and dBc explained sorts them out.
Power units add a second trap. dBm is referenced to a milliwatt and dBW to a watt, and Australian licence paperwork often uses dBW while datasheets use dBm. They differ by exactly 30 dB:
0 dBW = 30 dBm
So a 36 dBm limit is the same as a 6 dBW limit. Confuse the two and the error is a factor of a thousand in power.
A Worked Example
A 25 dBm radio feeds a 16 dBi sector antenna through a coaxial run that, with jumpers and connectors, loses 1.5 dB. The EIRP is just the sum:
EIRP = 25.0 + 16.0 − 1.5 = 39.5 dBm
To put that in watts, convert out of decibels:
P (W) = 10^((39.5 − 30) / 10) ≈ 8.9 W
The same site is 9.5 dBW, or 37.35 dBm ERP against a dipole. Three numbers for one antenna, so the job is to make sure the figure you check and the limit you check it against use the same reference.
Rule of thumb
- EIRP is always 2.15 dB above the matching ERP.
- dBm is always 30 dB above the matching dBW.
- Every 3 dB of EIRP doubles the radiated power.
- Swapping 3 dB of antenna gain for 3 dB less radio power leaves the EIRP unchanged.
Why EIRP Is the Number That Gets Regulated
Licence conditions cap EIRP rather than radio power, because radio power alone says nothing about how strong the signal is in the field. A 1 W transmitter on a high gain dish can radiate a far stronger beam than a 10 W transmitter on a whip. EIRP folds the antenna into the figure, so it is what actually predicts interference into neighbouring services.
In Australia the ACMA sets these limits two ways. Apparatus licences carry per band EIRP conditions in their assignment instructions, and licence exempt devices are capped by the Low Interference Potential Devices class licence. Those limits change as the instruments are updated, so always confirm the current figure for your exact band rather than relying on a number from a previous job. The practical point is that compliance is decided after the antenna, not at the radio: a legal radio power can still breach a licence once antenna gain pushes the EIRP over the cap.
From EIRP to Reach and Safety
EIRP also answers the next two questions. For reach, it is the transmit side of the link budget combined into one term, since the received power is the EIRP minus the path loss, plus the receive antenna gain, minus the receive feeder loss. Every additional decibel of EIRP becomes an additional decibel of received signal power, and therefore an additional decibel of link margin if the receiver sensitivity remains unchanged, right up to the licence cap.
For safety, the same EIRP sets the power density near the antenna. In the far field it falls off with the square of distance:
S = EIRP / (4 · π · r²)
with EIRP in watts and r in metres. That relationship fixes the exposure safe distance, the radius at which the power density drops to the ARPANSA reference level, and the antenna power density tool works it through for a given EIRP, frequency and geometry.
Common EIRP Mistakes
- Confusing EIRP and ERP. They sit 2.15 dB apart, so comparing one against a limit written in the other mis-assesses the site at the transmitter.
- Mixing dBm and dBW. A 30 dB gap, a factor of a thousand in power.
- Forgetting feeder loss. A long coaxial run with its jumpers and connectors can cost several decibels. Leave it out and the EIRP is overstated.
- Treating radio power as EIRP. That ignores the antenna, which is usually the largest term in the sum.
- Assuming EIRP is the same in every direction. It is the peak of the main beam and falls away off boresight, which is what makes downtilt and azimuth planning a usable way to stay within a limit toward a given neighbour.
Frequently Asked Questions
What is EIRP? EIRP, effective isotropic radiated power, is the power a theoretical isotropic antenna would have to radiate equally in all directions to match the strongest signal a real directional antenna produces along its main beam. It combines transmitter power, antenna gain and feeder loss into one figure, and it is the value most licence conditions cap.
How do you calculate EIRP? Add the transmitter output power to the antenna gain and subtract the feeder loss, all in decibels. With the antenna gain in dBi the result is EIRP in dBm. A 25 dBm radio on a 16 dBi antenna through 1.5 dB of feeder gives 25 + 16 − 1.5 = 39.5 dBm, about 8.9 W.
What is the difference between EIRP and ERP? Both describe the radiated power in the main beam, but against different references. EIRP uses gain in dBi against an isotropic radiator, while ERP uses gain in dBd against a half wave dipole. Because a dipole has 2.15 dBi of gain, EIRP is always 2.15 dB higher than the matching ERP.
Is EIRP the same as transmitter power? No. Transmitter power is the power leaving the radio. EIRP adds the antenna gain and subtracts the feeder loss, so it describes the radiated beam rather than the radio output. The antenna gain is usually the largest term, so the two can differ by a wide margin.
What is the EIRP limit in Australia? It depends on the band and the licence. Apparatus licences carry per band EIRP conditions in their assignment instructions, and licence exempt devices are capped by the Low Interference Potential Devices class licence. The figures are updated over time, so confirm the current limit for your exact band against the relevant instrument.
How do I convert EIRP from dBm to watts? Use P (W) = 10 raised to the power of (EIRP in dBm minus 30, divided by 10). So 36 dBm is about 4 W and 30 dBm is exactly 1 W. To go from dBm to dBW instead, simply subtract 30.
Build it in noIM₃
The EIRP calculator runs this sum both ways, in dBm, dBW and watts, with the isotropic and dipole references handled for you. From there the link budget calculator carries the EIRP through path loss to a received power and a margin, and the antenna power density tool turns the same EIRP into an exposure safe distance.
Key Takeaway
EIRP is one short sum, transmit power plus antenna gain minus feeder loss, that tells you how loud the transmitter is, how far it reaches, and how close it is safe to stand. Keep the references straight, dBi for EIRP and dBd for ERP, dBm thirty decibels above dBW, and the figure does its job.
