We are a small wisp and currently only utilizing 5Ghz equipment for our backhauls. So far everything is fantastic other than having to move our equipment to 20Mhz Channels. When moving to 40/80 our throughput goes out the window. We are considering migrating some of our backhauls to 24 but due to where we live (very rural), short links according to specs just aren’t happening. It appears almost all our backhaul links are right around 5k or just under. Has anyone tested the B24 for distances further than the recommended 3k? We’d love a solution other than having having to pay for licensed links (We just don’t have that capital yet).
The 3k limit is pretty hard, you could probably get links to come up at 5k, but they will have significantly degraded performance and be very susceptible to weather.
If your trying to reach further, I would look at licensed links or the Ubiquiti AirFiber24 or the 24HD. I have used the 24 pretty extensively and it has worked very well for us at distances of 2-3 miles.
We have customers that have gone well over the 3km recommended distance. They understand that in doing they will have issues with rain, etc.
I have a 4km b24 link that has been working very well. It does degrade under heavy rain, and disconnects when wet snow accumulates on one of the radios. Otherwise, it’s been working great for what it is.
The 24 GHz band is unlicensed because it’s a serious water absorption frequency. So rain fade tend to limit distance. Also, the US power limit is usually pretty low. Racom has a 24 GHz unlicensed radio approved for +10 dBm conducted power in the US (they read the rules cleverly and use an external antenna), vs. 0 dBm allowed on B24, AF24, or any other integrated-radio antenna. But 5 km is a lot for that band. Unless perhaps you’re in the desert.
I’m using B5c’s for long backhauls in a rural area. They can shoot quite far.
@Fred, that would be a hard no there buddy.
FCC Hard limit for the 24 GHz band is 33 dB EIRP (total TX Power, radio output + dB Gain of antenna) So if you run your radio above -3 dB, say with a 33 dB antenna, you are breaking FCC rules.
Now, with the Racom radio, if you were to use a antenna that is only 23 dB so that you could use your full TX power you would also be illegal because FCC requires a minimum of 33 dB antenna gain. So Racom will have no advantage over anyone else as far as total EIRP in FCC Land.
Well, no, that’s not what the rule specifically says! The rule specifies a maximum field strength at 3 meters, which equates to 33 dBm EIRP in the far field. But the near field is different, and the size of the near field is a function of antenna size. So Racom only tested the radio with a 4 foot dish! That has a really big near field, and thus it was able to run 10 mW with that dish before it hit the field strength limit. (The EIRP was really high.) The FCC then as a general rule allows you to run the tested antenna or one of the same type (dish) and smaller size. Thus any smaller dish is okay. That would exceed the near field strength, but they didn’t test that, so, via the smaller-dish loophole, it’s legal with a 1’ or 2’ dish too. Sneaky of those Czechs to figure out American rules, eh?
The rules look pretty clear to me, if you want to try playing around with loopholes go for it, but I wouldn’t risk playing games to achieve links that wouldn’t be possible without them. If the FCC comes along and decides that you are breaking the rules they get to shut you down and fine you, Racom probably won’t help you out with lawyer fees to tell the FCC that they are wrong. Good luck explaining to a judge that you know the FCC’s rules better then they do.
No American (or as far as I know, any other) company decided to play those games, they have as good of engineers and lawyers as anyone else in the world, there is probably a pretty good reason for them to not duplicate what Racom is doing.
William, you do not seem to have all that much experience with the FCC, where the letter of the rules outweighs any assumed spirit of the rules. The power limit on 24 GHz is in 15.249(b)(1):
(1) The field strength of emissions in this band shall not exceed 2500 millivolts/meter.
(3) Antenna gain must be at least 33 dBi. Alternatively, the main lobe beamwidth must not exceed 3.5 degrees. The beamwidth limit shall apply to both the azimuth and elevation planes. At antenna gains over 33 dBi or beamwidths narrower than 3.5 degrees, power must be reduced to ensure that the field strength does not exceed 2500 millivolts/meter.
© Field strength limits are specified at a distance of 3 meters.
Nowhere is EIRP specified. That was computed as +33 based on the far field. Also note 15.204( c):
(c ) An intentional radiator may be operated only with the antenna with which it is authorized. If an antenna is marketed with the intentional radiator, it shall be of a type which is authorized with the intentional radiator. An intentional radiator may be authorized with multiple antenna types. Exceptions to the following provisions, if any, are noted in the rule section under which the transmitter operates, e.g., §15.255(b)(1)(ii) of this part…
(4) Any antenna that is of the same type and of equal or less directional gain as an antenna that is authorized with the intentional radiator may be marketed with, and used with, that intentional radiator. No retesting of this system configuration is required.
Those are “black letter” rules, not interpretations. Perhaps the FCC meant for Racom to be approved. After all, big antennas have weaker near-field field strength, and are thus less likely to interfere with field disturbance sensors or short-range radar.
A user of such approved equipment is not subject to sanction. If it really were questionable, they’d get a warning first. Quite frankly I see more “funny stuff” on 5 GHz, where some vendors’ radios (not Mimosa) don’t actually enforce band-edge emission limits.
Yes, I do admit you probably would get a warning before being fined, but that is only the normal FCC practice, there is not hard and fast rule that prevents the FCC from dropping a hammer on your head out of the blue. So if you want to build links off of stuff that might piss off the FCC, you should keep in mind the FCC can come knocking at any time, and you WILL lose in any court case against them.
The Field strength is a measurement of power at a certain point. To get a field strength of 2500 mV at 3 meters from the required 33 dBi antenna you cannot put out more then 0.001 Watts from your radio, (which just so happens to be just under 0 dBm) so when you account for losses you get 33 dBi antenna (required minimum) + 0 dBm coming out of your radio = 33 dB EIRP. Plug the numbers into any online calculator and you will get the same results.
As you copied the text of where I linked to I will point out section (3) where it clearly says 33 dBi is the minimum antenna gain, if you use a larger antenna you cannot exceed 2500 mV/m which means you can’t use a bigger antenna and not drop your power or drop antenna gain below the 33 dBi mark, Even if your radio is certified for a larger antenna
So, for one final time, don’t just trust me. I am not an RF Engineer, I don’t get paid to know and give advice about FCC rules, but there are tons of other people who are. Hundreds of engineers and lawyers who work for other companies none of whom are trying to exploit this supposed “loophole”. Don’t trust the random internet guys, look at the law abiding companies that work their asses off to follow the rules and provide products that will both perform and be legal and see what they did.
Or ignore me, it’s fine, I won’t be the one getting a letter about how I got someone killed because I thought I, or I believed a company that is trying to sell me something, was smarter then the FCC.
William, it’s not really a loophole. It’s the metric in the rule. The 33 dBi gain is the minimum, but there is no maximum. It just says that the field strength can’t exceed 2.5v/m at 3 meters.
Now for some RF engineering. (I don’t know this math myself; I look it up.) An antenna has three fields, effectively. There’s the reactive near field, the radiative near field (Fresnel field), and the far field. The radiative near field gets bigger as the antenna does, and is essentially (in relatively plain English) the area in which the waves have not been fully collimated yet. The bigger the antenna, the more dispersed the power is at any point on the antenna surface (think radome). That should be intuitive. A 4’ antenna thus has a lower field strength than a 1’ antenna within the reactive field. Its signal only fully responds to the inverse square law beyond the radiative near field.
Basically, the signal from the 4’ dish is spread over a 4’ diameter at the antenna, and spreads more gradually (tighter focus) than the one from a 1’ dish. So the field strength detector at 3m (9.8’) will properly notice less power from a 4’ antenna than from a 1’ antenna, as it was dispersed by the antenna. Beyond the radiative near field, the field from the big antenna is stronger; that’s what gain is about. The FCC chose to measure close in, not far out, so an antenna with a big near field can overcome the 3m spacing.
A handy-dandy web near field calculator says that at 24 GHz, a 1.2m antenna will have a reactive near field of 7.3m and a radiating near field of 230m! Contrast with a .3m antenna where it is 0.9m reactive and 14.4m radiative. So even a B24 (which btw I LIKE A LOT) has a radiative far field beyond the test distance, but not dramatically so.
BTW antenna patterns are really applicable to the far field, and can be relatively random as you get into the near fields. This sometimes matters for interference rejection.