Country and Regulatory Domain Settings · Topic 1 of 2
Antenna Gain and EIRP
Learn how EIRP combines transmit power, antenna gain and cable loss, and how RouterOS and CAPsMAN use the regulatory domain to limit output power.
- 4 min
- EIRP is transmit power plus antenna gain minus cable loss.
- Regulators limit EIRP, so a higher gain antenna needs lower transmit power.
- All the values in the formula are decibels, and the result is in dBm.
What it is
EIRP is the effective isotropic radiated power, the strongest signal an antenna system sends in its main direction.
Regulators limit EIRP, not just the power the radio produces. A radio that is legal on its own can break the limit once a high gain antenna is fitted.
For this reason the antenna gain must be taken into account when you set the power, whether you configure one router or push a configuration from a CAPsMAN manager.
How it works
You add the antenna gain to the transmit power and subtract the losses, then compare the result with the legal limit.
The EIRP formula
EIRP (dBm) = tx power (dBm) + antenna gain (dBi) - cable and connector loss (dB)All three values are in decibels, so they add and subtract directly. The result is in dBm.
Worked examples
A radio sends 17 dBm into an antenna with 10 dBi of gain and a short cable that loses 1 dB. The EIRP is 17 plus 10 minus 1, which is 26 dBm.
If the limit for the channel is 30 dBm EIRP, the setup is legal. If you swap in a 16 dBi antenna, the EIRP rises to 32 dBm and exceeds the limit. The fix is to lower the transmit power by 2 dB or more.
A higher gain antenna means the radio must send less power to stay legal. The gain does not make the link free of limits.
Two directions of the link
EIRP applies to what you transmit. A high gain antenna also helps you receive, because it collects more of the incoming signal.
The other end must also be able to reach you. If your EIRP is high but the client is weak, the client may hear you and still be unable to answer.
How RouterOS applies the limit
On the legacy wireless package, the interface has a frequency mode that decides how strictly the country rules apply.
| Frequency mode | Effect |
|---|---|
regulatory-domain | Channels and power follow the country rules, and the antenna gain is used to hold EIRP to the limit |
manual-txpower | You choose the transmit power yourself, and the channel list still follows the country |
superchannel | The channel and power limits of the country are lifted, so the operator is fully responsible |
/interface wireless set wlan1 country=germany antenna-gain=10 frequency-mode=regulatory-domain
/interface wireless print detailcountryselects the regulatory domain.antenna-gainis the gain of the fitted antenna in dBi.frequency-modeselects the rule set.
With regulatory-domain, RouterOS reduces the usable transmit power as the antenna gain goes up, so the EIRP stays within the limit.
If you enter the wrong antenna gain, the router calculates the wrong power. A gain set too low can let the real EIRP exceed the legal limit.
In CAPsMAN
A CAPsMAN manager pushes a configuration to each CAP radio. The country and the transmit power belong to that configuration, so you set them once on the manager and every provisioned radio receives them.
On the wifi driver the configuration is a configuration profile with a country and a transmit power. CAPs with different antennas may need different profiles, because the same radio power gives a different EIRP with a different antenna.
/interface wifi configuration add name=cfg-outdoor country=Germany tx-power=17Indoor and outdoor
The legacy wireless package also has an installation setting with the values indoor, outdoor and any. In many countries the allowed channels and power differ between indoor and outdoor use. Set it to match the real installation.
What to remember for the exam
Seven facts cover this topic.
- EIRP is transmit power plus antenna gain minus cable loss.
- Regulators limit EIRP, so a higher gain antenna needs lower transmit power.
- All the values in the formula are decibels, and the result is in dBm.
antenna-gaintells RouterOS the gain of the fitted antenna in dBi.frequency-mode=regulatory-domainkeeps the channels and power within the country rules.superchannellifts the country limits and makes the operator responsible.- In CAPsMAN the country and power live in the configuration on the manager.
Common mistakes
These four errors are easy to make.
Forgetting the cable loss. Leaving out the loss makes the EIRP look higher than it really is.
Leaving antenna gain at zero. The router then thinks the antenna adds nothing and may allow too much power.
Using one profile for different antennas. The EIRP differs, so each antenna type needs its own settings.
Treating superchannel as normal. It removes the safety limits and is the operator's legal risk.
Check yourself
Work through these three questions.
How do you calculate EIRP?
Add the antenna gain in dBi to the transmit power in dBm and subtract the cable and connector loss. The result is in dBm.
Why must you lower transmit power when you fit a higher gain antenna?
The limit applies to EIRP. A higher gain raises the EIRP, so the power must come down to stay legal.
What does
frequency-mode=regulatory-domaindo?It keeps the channels and the power within the country rules. The configured antenna gain is used to hold the EIRP to the limit.
Topic 1 of 2 in Country and Regulatory Domain Settings
Test yourself on Country and Regulatory Domain Settings
The full MikroTik MTCEWE question bank has 476 questions, with timed practice exams and an explanation for every answer.
This study guide is independent and is not affiliated with, endorsed by, or sponsored by MikroTik. Vendor names, certification names and exam codes are trademarks of their respective owners and are used for identification only.