3.6GHz PMP450 OEM Antenna

I know this has been talked about a few times on various lists. We have several 4x90 clusters of 3.6 450 all using the OEM Laird sector. Very consistently now, we're seeing short-range SMs (about 1/4 to 1 mile) with lower than expected downlink SNR. This obviously hurts overall capacity. I understand from several sources that this is the opposite sector on the same frequency interfering. Many of the SMs are showing only 16-20dB SNR and the expected 4X downlink rate. These are bare SMs, no reflectors.

So my question is, has Cambium truly verified the F/B ratio of this antenna? The spec sheet says 32 or 35dB, I forget which. That should be more than enough. I really don't understand it. And 18dB SNR doesn't cut it.

We have the measured pattern data for this antenna, and it meets the F/B specification (which is 35 dB).

Not sure why the radios might be seeing interference from the back lobe.  Are these anomalies only on the really close-in SMs?  Is ATPC working on those units?  

George, 

Does this problem go away when the back sector is switched to a different frequency? How far off the ground is the sector and the SM? Yes, we have verified the F/B ratio on these antennas. 

Thanks,

Sriram

On both 3.65 and 5 GHz, I've had trouble with close links shooting up at the tower.  Not getting the signal or modulation expected, poor signal strength ratio, wanting to run MIMO-A.  I suspect either the null fill not really doing the job, or the sector antenna not having the expected performance when you are shooting up at it.  I'm not convinced it's a F/B problem.

Mark Radabaugh recommends the MTI sectors and thinks it's due to their high F/B ratio, but I notice that sector also has spectacular null fill if you believe the published patterns.

George's SMs are 1/4 to 1 mile. At that distance, I'm not entirely sure how much the SM is looking up at the sector.  It would be good to see a problematic link plotted in LINKPlanner. That will help visualize if this is a null fill issue. 

Thanks,

Sriram

OK, I've been watching a site for a couple days. This particular site is about a 160 foot grain elevator in a very small town, like 20 houses and businesses. Most of the customers are well outside of town.

One of the APs watchdog reset itself a few days ago, so I found some SMs registered on secondary. I dropped sessions to get things back to normal. Uplink and downlink SNRs and rates all looked good. So I was scratching my head because a few days before that is when I was seeing the bad downlinks to near SMs.

Fast forward to today, I looked things over this morning and all looked fine. Now this afternoon I'm seeing some short range SMs with 15-20dB downlink SNR. One SM in particular is about 500 feet from the tower. The other day his SNR was solid at >32dB. Today it's sitting at <20dB and downlink rate is 4X as expected.

Watching it continuously, I see that it jumps back up to 35/35dB once in a while. The customer is definitely moving traffic, so the data can be trusted.

So now I'm wondering if I'm seeing the drifting timing issue that Mark @ Amplex has talked about quite a few times. I bet that's what's going on here!

Here's that SMs stats..