large difference in AP vs. SM rssi

I’ve had a Canopy 900 AP up and running fine for some time.
It’s about 140ft up with an omni antenna.

The other day I noticed that there was a huge difference in the Power Levels at the AP compared to the SM. This is a new development, so I’m wondering what has changed. For example, one customer is -59 at the SM and -79 at the tower. Another customer has degraded to -84 (RSSI 1103) at the AP while -70 (1469) at his house. The RSSI used to be very close at each end.

What can I do to correct this?

What about Jitter?

jitter is usually 1 or 2 at the SM, 3 or 4 at the tower

This sounds indicative of interference at the AP - the AP is having difficulty hearing the SMs. The AP Tx power is the same, so the SMs still see the same amount of power.

Try flipping the AP into SM mode, and look at the spectrum analyzer. You may find that you need to move frequencies.

Also, take a screen shot for future reference.

Due to Lightning struck, I had a similar kind or situations several times using 5.2 APs with an omni antenna BUT the other way round.
SM having difficulty to hear AP therefore, some SM little bit far not even able to register. But those which are registered, AP can hear SM loud and clear.

Which means AP looses it TX power or transmitting very low, therefore we had to replace the AP. We have almost replaced 4 AP’s at different places having omni antenna so far.

But in your case as Jerry mentioned could be just Interference, if I were in your place,
1. I will put on SM below AP and run Spectrum analyzer.
2. Change the Frequency of AP if not.
3. Replace the AP.

Other possibilities could be multi-path or ducting (environmental). Every spring we have a problem with RF fade in this area. It lasts about a month, things are slowly getting back to normal. It affects a lot of links (mostly the borderline ones) - some one way, some both ways. Not much we can do about it - find new locations for antennas, space diversity on towers for backbone links.


Great information, thanks.

I suspected the AP just because once we have a link up and running for a while and a problem occurs, it has always been the AP (we have about 25 towers). I’ve just never seen this specific thing happen.

It seems we’ve had a lot of problems with AP’s loosing their output strength (they are all on omni’s). We ground, sometimes grounding twice.
What helps is a short whip of LMR 400 (say 3’) from the antenna to a ground then about 10’ of LMR 400 that is coiled at the end, another ground, then the radio. It’s like some towers are just in the wrong spot for static charge.

Here is what canopy support said:

The power level shown on the AP is refering a different signal then the power level shown on the SMs. We would expect them to be different. You could attempt fine tuning the alignments of the SMs to try to obtain a higher power level at the AP.

25 SM’s didn’t get out of alignment. That tower has people from 1000ft to 10 miles away. Throughput is not affected.

I like the idea of waiting it out and seeing if it resolves itself.
Next I’ll try an analysis/different channel, and lastly replace the AP.

25 SM's didn't get out of alignment

So, it is every SM on that AP being affected? Different story - it is likely either the AP itself or it's antenna system.


yes, every SM…

I’m thinking AP.

Thanks again for the insight.

Has anybody ran their 900 AP on the ground using LMR 600 or something?

I just hate climbing this particular tower.

Canopy support is correct, generally there will be a difference between AP and SM. I have found it’s rarely more than 3dB unless there are LOS issues. You might get larger variances from some SM’s, but not all.

You may have been fighting interference from the beginning! Have you done the spectrum analysis? It could be as simple as moving to another frequency - no climbing needed.

As far as the AP on the ground, you just need to decide how much loss you can live with. LMR600 is about 3dB/100 with connectors. That’s HALF your power…

I agree with Jerry. At this point, start with the spectrum analysis. As he said, it could be as simple as changing frequencies.

I do run one AP on the ground - we have Andrew LDF5-50A (7/8") running to it as well as an SRL-420 at the top, 300’. And as mentioned, it depends how much loss you can afford in your cable system.


The difference for the last year and before last week was around 3db.
I know that it will most likely always be different on each end, but it went from a 3db difference to 20db practically overnight. One interesting observation: the futher out the SM, the less of the difference. I might be
-57/-80 across the street and -72/-85 10 miles away.

I don’t think it could be a physical antenna problem because the SM’s are hearing the AP loud and clear, in every direction.

If it was interference at the AP, wouldn’t the SM’s also be affected?

I’ll definitely try the spectrum analyzer before I climb, but I have a feeling a new AP will fix the problem.

Can you use Waveguide with 900?

If the interference is near the AP, then the SMs will not really be affected.

I’d start looking for another source of 900MHz.

We have all of our APs in the buildings, running 7/8 Andrews to omnis. Depending on how much loss you have in your cable run, you could install an amp to recover cable loss keeping in mind the legal ERP reqs.

Just to follow up…
We replaced the AP and everything is as it should be.
We run that AP with a big omni antenna. I think it just took a hit in a storm or something which made it weak.