What are B5c EIRP, antenna limits in USA?


A post in 2016 noted that the EIRP limit for the B5c in the US was still +36 dBm. But that’s not the actual rule. EIRP limits for PtP are in practice largely based on two things. One is the out-of-band emissions (OOBE), which are pretty strict, especially on U-NII-1. The other is the way it is type approved. Which leads to my question…

A certain other well-known radio vendor came out with a new point to point model last year that looks at first glance like it’s aimed squarely at the B5 line. It is 802.11ac wave 2, two frequencies, priced not unlike Mimosa. BUT that vendor also sells FPGA PtP radios for about ten times that price. So when they did the US type approval, they specified a +23 dBm antenna gain. An obscure feature of US rules is that you can use a Part 15 radio with the specified antenna or one of similar type (panel, dish, etc.) and the same or lower gain. Test with a 30 dB dish and the user can pick any other 30 dB or 25 dB or whatever dish, but not a 34 dB dish. Said vendor type approved the new PtP radio with only a 23 dB antenna, and is quite smug about its not being allowed with a higher gain antenna. Nice for short hops, I suppose.

Since I’m currently looking to push a lot of bits for a lot of distance (up to 18 km) in a rural mountain area, I need lots of EIRP. So [deleted ;-)] that other new radio, and back to Mimosa. The B5c with a 2’ dish looks like a nice option. The rules do not specify any EIRP limit in the 5725-5850 range; it’s just limited by OOBE or the type approval.

So what would be the highest EIRP, and biggest dish, I’d be allowed to use on U-NII-3 with a B5c? It wasn’t totally obvious in the FCC test reports. Also, what would it be on U-NII-1 (where the nominal limit of +53 is hard to achieve due to OOBE)? Thanks.



From playing with the Mimosa Design tool it looks like they have modeled the B5c links with a 32 dBi antenna. I know that the B5 line will automatically adjust their maximum TX power so that when you set the correct antenna gain the system will not go over EIRP limits.

Though, I could not find an exact answer for your question I bet @DustinS could tell you how big of an antenna you can put on your B5c.


Though now that I am spending some time thinking about it, I do not know of any radio manufacurer who has a “maximum antenna gain” as long as you are not going over EIRP limits I don’t think the FCC will care either…


Use of the radio in the US violates FCC Rules if it uses an antenna with higher gain than allowed in its type approval. Yes, you can lower the power in order to stay within the EIRP, but the actual rules are actually a bit stricter than that, and you could risk an enforcement action. In this case I’m dealing with a CAF-funded project so its technical specs are going to be scrutinized by the FCC beforehand, so it has to be fully compliant!
I know the design tool allows for a 32 dB antenna, but conceivably that doesn’t apply in the US. Hence the question. Just trying to be 100% legal. Also, quite frankly, the OOBE in the FCC test report’s spectrum analyzer shots didn’t look all that good. Not awful, but the Wi-Fi chips are built with an assumption that you won’t exceed Wi-Fi PtMP power levels (+36 EIRP). The U-NII-3 rules were actually loosened up a bit since the B5c came out, though, which may raise the allowable EIRP. My current B5 seems to allow +24 dBm/channel (2 chains) on U-NII-3, which is +49 EIRP, and even +20 into a 29 dB antenna would be good (4 dB more receive gain than the B5).


I don’t have an exact answer on this… I’ll let @David answer this when he gets a moment.


@Fred Here is how our B5c works (PTP FCC rules, PTMP are different)

Max B5c System power is 30 dBm. The end user needs to correctly specify the antenna gain in the B5c GUI. The B5c will regulate EIRP based on Antenna Gain + System Power. The radio will not let you overdrive the EIRP based on the correct antenna settings. Here’s our logic:

U-NNI-1 Max EIRP is 40 dBm, max allowed system power is 30 dBm
U-NNI-2 Max EIRP is 30 dBm, max allowed system power is 24 dBm
U-NNI-3 Max EIRP is 53 dBm, max allowed system power is 30 dBm

The B5 has the antenna gain pre-configured and is not changeable. The B5 GUI will limit the max system power based on the band you using to keep EIRP legal. If you have the latest code on the B5 is should take into account the newer U-NNI-3 EIRP max.


Okay, thanks. Then it seems that you did type approve it for a high gain antenna, which is normal practice for connectorized PtP radios. Just being sure, given how I “discovered” that rule when The Other Guys did it “wrong” (their forum showing great results in other parts of the world). I just have to keep the long links up in U-NII-3, and probably keep the PtMP access links down in U-NII-1 where the PtMP limit of +36 is harmless. Which is sort of normal anyway.