What are possible causes of low SNR  on PMP450 SM even though the RSL is good?

Noise, and a lot of it specifically on the vertical chain. I’d run an SA from the SM’s side and see if it sees any interference. You might have a home WiFi router on or near the same channel as your AP/SM.

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Was there any more additional information on this? I have a similir issue. I have two towers 22 miles apart but feel they maybe interfering and not an easy fix to change my Frequencys around to test. I have some way off SNRs like you show. Where one chain is perfect and the other is not at all. It looks like its interference from the upload SM to AP.  

In that case, I would do exactly as Eric suggests, and run a spectrum scan at the SM site to see if there is interference at that site.

i have run SA on Sm side but not showing any inteference. Can suggest any other option to check.

Hi Guys , Ive been associated with Cambium since  5 years and have come across such issues. This particalarly happens mostly due to interference issue at AP or else SM side. Most cases you can also findd If your device is SU MIMO A then you will get only Vertical SNR ie One Antenna's value as in 22V/NA H , if that is MIMO B then you will get both the values. 

So summing up it is interference case (if RSSI is good) and it may be allignment issue so in some cases proper optimization is advised and we got the desired results.

Hope that helped.


Are you connecting to a Medusa AP?  I have seen where the initial SNR numbers right after you connect dont look so good until you actually pass traffic or run a Link Capacity test.  It seems that it takes a little push for the beam forming to get going (just my opinion).

What firmware are you running on the SM vs. the AP?  I have seen radios on our network report strange SNR and Link Capacity results on mixed firmware.  Seems there was a particular firmware that generally didn't report its SNR numbers correctly (13.2.1 but I am not sure)

Are you using a reflector, stinger, or bare pod and what is your distance from the AP?  A bare pod has a fairly wide eye to the world and will tend to hear more noise around it.  At the same time using a reflector too close could cause self-inflicted noise.

Does the SNR get better if you de-tune vs having it precisely in tune?  I see there is a 4dB variation between V to H, so a better tune might be possible but could make it worse if the noise is directly inline with the tower you are shooting to. (I know it sounds weird, but I have seen it happen)

Does the RSL or SNR bounce dramatically?  This will usually suggest interference of some kind whether it be RF or physical.  I have run across connections where the signal was great but the SNR was terrible due to thick trees and bushes.  In some cases, the BER results on the individual chains is a great indicator to watch as well as the errors under the Radio tab.  Substantial bounce in these number can also be due to a nearby AP that is running a different timing.

Does the Link Status tab under your AP reflect the same numbers as the SM is showing you?  This is one the most useful tabs in my opinion.  It shows some great details about the link in one spot.

I don't know that I am providing you with any solid answers, but its a few things you can check and hopefully it helps.

Good Luck