Hi mates, I've a friend which has different wifi routers, I'm moving him to capsman, outdoor he has a ubiquiti airMAX Bullet so I move to a RBGrooveGA-52HPacn for a minimal installation time.
My question is, this antenna is created for 2.4GHz, will it work also with 5GHz? Maybe at reduced power? Does RBGrooveGA-52HPacn work at same time with 2.4 and 5GHz or I need to select which one?
Any suggestion is appreciated, thank you!
Yes, it has two radios.
Oops 
, wrong, it has one radio that can work on the two bands and you need to select which frequency to use between 2.4 and 5 GHz,
WHICH antenna?
The Groove comes with a dual band antenna, and AFAIK most N connector antennas are dual band.
EDIT: Corrected fundamental mistake.
The TP-link antenna mentioned in topic title is designed for 2.4GHz band. What happens when used in 5GHz band is completely undefined. In best case it'll work somehow with really shitty gain (e.g. with -15dBi gain it would completely attenuate 5GHz signal). In worst case it would destroy 5GHz radio due to mismatched impedance.
So I'd say: don't be cheap, get a proper dual-band antenna for the new AP. Alternatively forget about 5GHz band (disable it altogether in Groove's configuration).
I see now, you have an old antenna and you would like to re-use it. 
Even if - generally speaking - I am cheap, in this case you should really get a new, proper antenna (or use the standard one that comes with the groove).
That TP-Link one should be 12 db gain, the one that comes with the Groove is 6/8 db, if you respect the maximum transmit limits (without license) I don't think that a 12 db antenna will have so much advantage.
Anyway an antenna like (example):
https://store.rokland.com/en-de/collections/wi-fi-antennas/products/12-dbi-gain-dual-band-5ghz-2-4-ghz-omni-directional-802-11ac-outdoor-antenna
costs below 50 €.
The risk is that - even if now you disable the 5 GHz antenna, when/if you need to reset or netinstall the device or possibly fiddling with it after an upgrade, etc. you can accidentally re-enable it, and as mkx stated there is the possibillity that the radio could be damaged.
Thank you mates, yes I'm aware of SWR and all those conseguences, this is why I'm asking you here, based on your experience, possible somebody who tested directly this antenna may say that.
Yes you got me, in this case I'm cheap, there are no resons to spend more money in such situation, this is an outside hotspot and useful for external cameras only, which are 2.4GHz only, and it may serve occasionally my friend when moving outside, he hasn't demands to be that fast&efficent as the 5GHz assures.
I've bought this AP second hand, and it will come without antenna, I can say more, I don't want to change the actual situation reducing antenna's dB, risking that may the farest camera wil not more stream as it does before. This place is far to me, so I don't want to go back there after the work is finished.
My goal is only to have this AP working with Capsman in harmony with the ones inside, without all the inconvenience of having 2 different SSID near as it is now, myfriend can live without 5GHz if complicated, as he does now 
In case I enable 5GHz, there is any way to check if something goes wrong with the signal? Any parameter can tell TX is suffering? My idea is to set a very low power at the beginning, put my mobile close to antenna and connect it to 5GHz SSID only, then check on my phone how it receives signal, then do traffic and check on router the CCQ, if the values are optimal as they should, chances are antenna is compatible, if not, better to turn that radio off and forget.
Could I easily reduce the 5GHz power to a sefe level by one parameter, or should I specify TX dB for each speed? Which value is the lower and you recomend me?
As RBGrooveGA-52HPacn doesn't now if an antenna is effectlively connected at its first boot I supppose it starts with the both radios off as default, isn't that?
If not, after the first boot I need quickly to turn 5GHz off.. anyway I think if it stays on for some seconds it will not be that dangerous for the TX radio
The recommendation, for ANY device with external antenna connection Is to ALWAYS connect a suitable antenna BEFORE switching It on, the alternative Is placing a terminator (essentially a 50 Ohm resistance soldered inside a connector).
This Is to prevent the TX radio to fry ( not necessarily It will, but It may).
You won't know if the device will activate the radio or not until you will check its configuration (and by that time the final stage of the TX radio may already be gone poof).
I don't think this Is a risk in your case, if you connect the 2.4 GHz antenna, It will anyway work to dissipate the power emitted by the TX radio, but don't take my opinion as a guarantee of any kind.
The question is now;
Will a 5GHz radio shouting on the same 2.4 GHz antenna compromise 2.4 GHz reception?
it can be, tests need to be done to now that.
Unfortunately router will come without antenna and I don't have an N connector to create a dummy load, I can go buy one eventually.
Chances are, suddenly turn off 5GHz after the router boots may be not that dangerous for some seconds, I hope.
This is the antenna unmounted, maybe I can find sombebody who knows if those inductors may leave antenna operate at its double frequency (5GHz)
https://www.youtube.com/watch?v=1HbBTY4kuEI
As I already explained, antennas are properly designed (and hopefully tested) only for certain frequency range. In principle they don't have same characteristics outside that frequency range. And when it comes to safety of connected radios, antenna gain is not the most important, it's impedance (and consequently VSWR). And no, operating at integer multiples of designed frequency doesn't help in this case.
And no, operating Tx for "only a few seconds" doesn't guarantee that transmitter won't fry. But: you're saying that 5GHz is not requirement in this particular use case. So if transmitter fries, in best case it'll only affect 5GHz transmitter (and not, e.g., combiner which combines both radios to single antenna connector).
False assumptions...
It's time to fix everything:
It only has one radio module dual band, single chain.
It either works on 2.4GHz or 5GHz, it doesn't work on both at the same time.
Just check, on default start at 5GHz.
And writing of whether or not to turn it on with the antenna inserted:
It's been around since 2007, and I don't care, because even if it doesn't have a link to other devices, as soon as you turn it on,
the beacon has never burned out any radio module, not even a competitor's.
Aside from the fact that it doesn't transmit continuously, it certainly doesn't transmit to anyone, and it's never been configured...
Thank you for your report, actually Mikrotk loves by default to boot wth the WiFi on and without password, in this way the admin can easly connect and program it.
In this case the router transmit regulary its SSID, I believe they are bursts of few bytes on air, something it sends. Glad to now the router won't damage without an antenna connected, anyway better not risking.
Also precius to know it works only on one mode, that closes the topic I guess..
Definitely my bad , I mis-read the device specs.
Sorry for the confusion.