The MikroTik’s RouterOS Lora settings include an antenna gain setting. The manual describes its unit as dBi while the web UI labels it with dB. So this mismatch is a first source of confusion.
Reading some old posts, the European regulatory requirements seem to allow a maximum antenna gain of 2.15 dBi.
Thus, a naive calculation would arrive at a value of 4 for the antenna-gain RouterOS setting. But - what if the RouterOS software already subtracts 2.15 dBi from the specified value? So one would then really need to specify 6 (or even 7 when rounding up) for that setting …
And what about some loss of gain because of the included SMA/SMA cable? Can that be subtracted as well?
Perhaps somebody already has some experience with that gateway/antenna combination or perhaps even did some measurements …
In EU you are allowed to use maximum 14dBm e.r.p. on most of the frequencies we use.
If the output of your transmitter is 14 dBm and your antenna is a dipole with 2.1 dBi (=0dBd) and you have no additional losses (plug, cabel) your e.r.p. (effective radiated power) is 14 dBm.
If your antenna has a gain of 6.5 dBi (=4.4 dBd) your e.r.p. is 18.4 dBm. 4.4 dBm to much.
You have to reduce the power of your transmitter by 4.4 dB to 9.6 dBm. But you can also use this 4.4 dB to compensate the loss of the antenna-cable and the plugs.
BTW: imho nobody will care about 0.xx dB.
I believe the erp is actually set at 16.1dbm, and is set assuming use of a 2.1dbi nominal ant (typically the closest off the shelf option to an ideal isotropic radiator), hence we typically arrive at a max conductive tx power of 14dbm at the output of the device into any feeder/ant., but otherwise you explaination is correct IIRC.
Practically, therefore, a higher gain ant is then only valid if being used to compensate for any long feeder or associated connector losses, unless as stated you reduce Tx power at the device. That is why we recommend use of shortest practical ultra-low loss cables and quality connectors that are tool tightened vs just ‘finger’ tight to minimise losses, with many units then shipping with a 3dbi ant as standard vs the ‘nominal’ 2.1dbi as there is some ‘assumed’ loss. Higher gain antennas then bring with them additional problems as discussed on other threads (forum search) wrt notching, directionality and possibly tilt, plus ofcourse that ant gain isnt just being applied to the desired signal it is also being applied to any received noise helping increase the noise floor and potentially reducing the SNR, thereby potentially impeding the ability to hear remote or weak signal nodes…there is no such thing as a free lunch in RF as in life!
ETSI EN 300 220-2 Annex B says 25mW (14 dBm) maximum effective radiated power e.r.p.
This are 16.1 dBm into an isotropic antenna with 0dBi or -2.1 dBd.
I hope the confusion is perfect now.
I understand that too high-gain antennas are of limited usefulness if you can’t ‘exploit’ that gain and that there are some related caveats, in general.
However, it’s still unclear to me how ‘clever’ the MikroTik wAP LoRa8 device is when it comes to these transmit power calculations.
I mean, what does its antenna gain default setting of 0 actually mean?
a) transmit power equals legally allowed limit when a 2.1 (or 2.15 ?) dBi nominal antenna is connected, or,
b) transmit power equals legally allowed limit when a zero gain antenna is connected?
IOW, when I connect a 2.1 dBi nominal antenna, do I have to
i) change the device’s antenna gain setting from 0 to 2, or
ii) keep it on 0, since the 2.1 dBi is already subtracted from the base transmit power?
An antenna gain-setting of “0” doesn’t mean anything. Do they mean 0 bananas, 0 apples, 0 dB% or maybe 0dBi or 0 dBd.
The manufacturer of the gateway should explain that.
I guess, that a value of “0” means output power 14 dBm. But I’m not sure.
Yes, this is the confusing thing. In the Web-UI it’s annotated with ‘db’ while in the online manual they document it as ‘dBi’ … I’m writing to MikroTik’s support, perhaps they can clarify this.
So I explicitly asked them what the antenna gain setting should be configured to when attaching a 2.1 dBi nominal gain antenna and a MikroTik 868 6.5 dBi gain Omni antenna in order to maximize transmission power while staying in the legal limits (when using a short/the included short cable). The answer was to set it to 2 and to 6, respectively.
They also clarified that the unit of this setting should read dBi (as documented in the online manual) and they want to fix the UI label with some future system update.
