In the USA, max EIRP is
"Maximum transmitter output power, fed into the antenna, is 30 dBm (1 watt).
Maximum Effective Isotropic Radiated Power (EIRP) is 36 dBm (4 watt).
You can obtain the EIRP by simply adding the transmit output power, in dBm, to the antenna gain in dBi (if there is loss in the cable feeding the antenna you may subtract that loss).
If your equipment is used in a fixed point-to-point link, there are two exceptions to the maximum EIRP rule above:
In the 5.8 GHz band the rule is less restrictive. The maximum EIRP allowed is 53 dBm (30 dBm plus 23 dBi of antenna gain).
In the 2.4 GHz band you can increase the antenna gain to get an EIRP above 36 dBm but for every 3dBi increase of antenna gain you must reduce the transmit power by 1 dBm. The table below shows the combinations of allowed transmit power / antenna gain and the resulting EIRP.
So looking at a Mant box 19s, with antenna gain of 19 dbi, and output power of 27 dbm, does it reduce total EIRP to 36 dBm in ptmp mode, and leave it alone in ptp mode?
Thanks
does it reduce total EIRP to 36 dBm in ptmp mode, and leave it alone in ptp mode?
Why should it do that in PTP ?
Either PTMP or PTP the EiRP must and will stay inside the limits of each country…
So whatever you do it will always be equal or less than 36 dbm…
This was the response I received from MT.
“To comply with the regulations by example if you have a high gain antenna you need additional to specify the antenna gain for the device and then the Tx power will be limited by the values specified.”
So to interpret this to mean that to get 53 dBm EIRP using e.g. a dynadish 5 (25 dBi antenna gain, 27 dBm power at MCS 7) I need to change the antenna gain parameter to -1 in router os.
I see the lowest setting for antenna gain is 0, so 52dBm. The other question is whether these MT devices will operate at 200 watts reliably for an extended period?
More curious issues. The Dynadish 5 has antenna gain 25dBi, and if you look at the transmitted power, its choked back to 5dBm. Thats a total EIRP of 30 dBm. But the FCC allows 36 dBm EIRP for the worst case PTMP applications, and a minimum of 53dBm for PTP. Why does Mikrotik reduce EIRP 6dBm below the allowed FCC limit?
The same true for the mantbox 19s. 19 dBi antenna gain results in 11 dBm transmission power. Again 30 dBm EIRP, 6 dBm below allowable limit.
So antenna gain is set to 19. MT reduces the output power to keep the EIRP to 30 dbm, 6 db below the FCC limit.
Additionally, changing to PTP mode still leaves it at 11 db.
It appears that to get the legal limit afforded by the FCC in the USA, you need to reduce the MT assigned antenna gain by 6. And when using PTP, just reduce the antenna gain to 0 if you are using the high end of the spectrum. If I reduce it to zero, the TX goes to 30 dbm, and you get the antenna gain on top of that.
When using the dynadish 5, it reduced the TX to 5 dBm because of the 25 db antenna gain!
So, unless I’m missing something, using the U-NII-3 bands, in PTP mode, using a 30 dbi gain antenna, with 30 dBm from a MT radio, I can get 60 dBM EIRP legally - 1 kW signal.
Response from MT support.
“If you adjust the wireless configuration thew it is possible to use max Tx power without problems.”
What a horseshit reply. When you select regulatory domain, the expectation is that you are going to get the maximum allowed EIRP, not 25% of it. IMO, this shows a very lazy slipshod company.
Indeed … and if the actual maximum allowed EIRP depends on use case (PtP versus PtMP), then it’s more to it than just to click and forget. With versatility of ROS (we all love so much) it’s hard to determine if an AP is being used by single client or multiple clients … Indeed that information can be deducted from the contents of /interface wireless registration-table print, but what does it give? AP starts at full power, one client connects and AP still transmits full power, but when the second client connects, AP drops Tx power and client #1 disconnects? And then the AP ramps up the Tx power, giving client #1 acceptable signal and client #1 connects again … and AP drops power again … see the pattern?
On the other hand, there are MT devices which are expressly intended for PtP connections (and could thus allow for full Tx power regardless antenna gain), but there are users complaining about that (“it should be possible to use this device in PtMP scenarios” …).
But then, country limitations are not really enforced, ROS admin can always use manual settings … and assume full responsibility if local RF authorities think device is breaking the rules.
3dB of difference can be easily explained: when you have an antenna with H/V polarized inputs and 2 TX chains, the total EIRP is the sum of the two chains, but the settings in the user interface are tx output for each separate chain. So when you want to be within 36dBm EIRP limit you cant transmit only up to 33dBm per chain ( = per polarization) and the setting of power of the TX would be 33 - antenna gain.
I don’t know what the other 3dB comes from, but there likely is a similar explanation.