Yes, that’s true, but a higher gain antenna at the SM end helps rcv but not xmt. And SM-->AP is the direction you may actually need a better signal because the AP likely has a sector antenna and is mounted higher so it sees more interference.
It would not be unusual to have a 16 dBi antenna at the AP but a 25 dBi antenna at the SM. The antenna gain would help the rcv signal at the SM, but it would probably have to lower its conducted power by 9 dB to stay within the regulatory EIRP limit. In contrast, in U-NII-3 the CPE end is treated as point-to-point and can use antenna gain to exceed the AP limit of 36 dBm EIRP (subject to OOBE limits). From: AF <[email protected]> On Behalf Of Adam Moffett Sent: Thursday, November 21, 2019 11:27 AM To: [email protected] Subject: Re: [AFMUG] dumb DFS questions There might be something I don't understand, but I thought you had flat EIRP limit of +30dbm whether it's an SM or an AP. On 11/21/2019 12:11 PM, castarritt . wrote: 6 dBm loss for the AP transmit isn't the end of the world. It's the up to 23 dBm loss on the SM transmit power that destroys the usefulness of DFS for PTMP past a couple miles. The ~16 dBi gain 90° sectors 2-300' up in the air just can't hear those SMs over all the noise they are picking up. What we need is the ability to run downlink on DFS and uplink on 5.2 or 5.8. On Thu, Nov 21, 2019 at 10:56 AM Adam Moffett <[email protected] <mailto:[email protected]> > wrote: Yeah I think on most equipment you can set alternate channels that are just shifted over 5mhz from where you were. And yeah I think the channel needs to be clear for a few minutes before you can go back to it. Assuming you don't really have a TDWR near you, I don't think DFS events are that big of a deal. My understanding is that DFS events are more likely if you lie to the software about antenna gain to cheat the EIRP limit. False detects happen, but I don't think it's a daily event. Disclaimer: I've mostly used it on Point to point with dishes. I'm not sure if you'd pick up more anomolies on a sector antenna. The biggest bummer is the EIRP limit. When you're trying to get that 32 SNR for the 256QAM then losing 6db kind of hurts. Or when you've already got someone hooked up 10 miles away and lowering the power ruins them. Where you really want to use DFS (In my opinion) is at a site where you have a bunch of customers within 1-2 miles. Unfortunately I don't have sites like that. -Adam On 11/21/2019 11:31 AM, Ken Hohhof wrote: We mostly avoid DFS frequencies on APs because of the impact if we get false radar detects. Also we are mostly a Cambium shop. So I’m a bit confused about DFS on other vendor equipment like Ubiquiti as well as home routers. Question 1 – what happens when there’s a DFS detection? On the Cambium gear, we have to select 1 or 2 alternate frequencies. But on other gear, I don’t see this. When there’s a DFS hit, does it jump to another random frequency? Does it rescan the current frequency until it tests clear and only then resume transmission? Is the answer right in front of me and I’m being stupid? Maybe in the case of routers they are exempt because of low EIRP? Question 2 – what about 40/80/160 MHz channels? We have a competitor using Ubiquiti gear and advertising residential subscriber speed plans up to 100x100. Clearly they must be using at least 40 MHz channels if not 80 MHz, or else their marketing people have burning pants and long noses. And I don’t see how a WISP, especially one surrounded by other WISPs, could use wide channels other than in DFS bands. We have some PTP links using 40 MHz but only 10 and 20 MHz channels on our APs. So assuming you are using 40 or 80 MHz in DFS, what happens when there’s a DFS detect? Does the whole 40 or 80 MHz have to find a new home? Can it slide over 2.5 or 5 MHz and substantially overlap the previous occupied spectrum? DFS bands come with enough spectrum to use wide channels, but is there enough to jump around when you take a DFS hit? -- AF mailing list [email protected] <mailto:[email protected]> http://af.afmug.com/mailman/listinfo/af_af.afmug.com
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