PTP820G Received Signal Differentials

While staging a hop of PTP820G (all indoor) using waveguide-to-coaxial adapters, 50 ohm coaxial cable/connectors, and attenuators, I noticed that there was a 10 dBm difference is RSL between the sites.  For example, one site had a RSL of -30 dBm and the other site had a RSL of -40 dBm (with the transmit power set to the same numeric value at both sites).  It should be noted that this is a 1+1 MHSBY path (in Cambium venacular, it is technically a 2x 1+0 (radio protection and hardware protection), but in my example, the sites were talking Primary Radio to Primary Radio.  It should also be noted that when I changed the frequencies of the path to match the exact frequencies printed on the branching drawer, the RSLs became balanced; for example, both sites showed a RSL of -30 dBm.  I chalked this up to some type of imperfection in my attenuation/path-simulation network.  

We are in the process of installing and performing path alignment on this system, and despite the fact the tower crew is having difficulty finding the main lobe, I noticed that as it stands, the sites are once again seeing a RSL differential of 10 dBm.  As I type this note, one site has a RSL of -57 dBm and the other site has a RSL of -67 dBm.  The LinkPlanner predicts that both sites should have a RSL of around -35 dBm.  

Although the path is not yet optimized as we are nowhere near the engineered RSLs, the observation I made during staging seems to have manifested itself in the field.  The tower crew has performed both waveguide and antenna sweeps at both sites, and all sweeps report well.  

The frequencies we have been allocated by the coordinator/FCC are certainly within the documented frequency range of the branching drawers.  But again, during staging, when I programmed the system to use the exact frequencies printed on the branching drawer, which I understand to be the frequencies used to test/tune the branching drawers, my RSLs became balanced.  

I am concerned that even when the tower crew finishes path alignment that I will be left with unbalanced signals with a differential of 10 dBm.    

What would cause something of this nature?  Incorrectly tuned/filtered branching drawers?  Bad radios?  Because the path is 1+1 MHSBY, I can certainly move/swap radios, but I believe I did this during staging to no avail/revelation.  

One final note regarding the branching drawers.  During staging, we determined that we were shipped incorrect branching drawers.  The branching drawers we were originally shipped were for a 1+0 path.  Once we determined we were shipped incorrect branching drawers, we were immediately shipped the correct branching drawers for a 1+1 system.  Going from memory, I believe that the imbalance in RSLs was present when the incorrect drawers were installed as well as when the correct drawers were installed, and I was able to achieve a balanced RSL while the incorrect branching drawers were installed as well as when the correct branching drawers were installed by configuring the system to use the exact frequencies printed on the branching drawers.  Again, at that time, I simply chalked up the imbalance to some type of impedance mismatch in the simulated antenna system.  

Any thoughts or suggestions are greatly appreciated.  

Matthew R. Smith

Radio Maintenance, Inc.

Reading, PA

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Hi Matthew,

That's interesting. We purchased 6 complete links of the PTP820S and starting installing them 2 weeks ago. We have 2 of the links online and are experiencing a similar problem. LinkPlanner predicts -37db and the best we can get is in the -50 range. We have a lot of experience with 11GHz radios and alignment and spent quiet a long time making sure they wern't misaligned. The strange thing is these 2 links and another, which is mounted and aligned but isn't online, are having the exact some problem: the prdicted LinkPklanner RSSI and what the radios actualy receive is very different. We have opened a case with Cambium and are waiting.

I don't know if version has anything to do with it but this is the first time we are using this version. Other than that we don't know what might cause such a low RSSI but will post here if we hear anything new.

Best regards,

Mike Whelan


I am running the same firmware version, and this is the first time I am running this version as well.

In the case of your link which is hovering around -50 but LinkPlanner predicted much better, are you experiencing -50 at both sites, or are the signals significantly imbalanced like mine?

In my case, I know the tower crew has not yet successfully aligned the antennas. Both LinkPlanner and the datasheet from the coordinator predict around the same RSL, so the tower crew simply hasn’t found the main lobe. It is a short hop of less than two miles, but the three foot antennas have a half power beamwidth around two degrees, so we simply haven’t found the main lobe.

Regardless, the differential in RSL of 10 dBm is concerning because I saw it during staging and I am seeing it in the field, albeit on a path which is not yet correctly aligned. I suspect an issue with the filtering in the branching drawer. I have a note into Cambium Support and hope to speak with them on Monday.


The links are not significantly imballanced - they have a 1db to 2db difference, I normally blame this on our tower technicians. :-)



Well, at least yours are balanced. It almost has to be an alignment issue, wouldn’t you think?

After two and a half hours of path alignment yesterday afternoon, the tower crew advised me that they had to install one of the antennas 180 degrees opposite of the other in order to mount to the respective tower. I immediately asked if the feedhorns were oriented the same, and the answer was yes. Since I still can’t wrap my head around what they did, they are returning on Monday with different mounting accessories to ensure the dishes and feedhorns are oriented the exact same.

So, we shall see.