Signal Strength Ratio what causes changes and when to start worrying?

I've recently deployed my first ptp670 link. The link seems to be fine, but a few weeks ago the SSR changed. What is the acceptable range of values for SSR, and when is SSR something to be worried about? Also, what could cause a fluctuation like I'm seeing in the graph below? We haven't moved or changed anything about either end of the link....


As you probably know by now, in linear units of W or mW, signal strength ratio is the ratio of the received signal strength in the vertical polarization to the received signal strength in the horizontal polarization. If we work in logarithmic units of dBm, then SSR is the difference in received signal strength.

A SSR of up to 5 dB is nothing to worry about.

In a non-line-of-sight link the H and V polarizations tend to fade independently, and it's common for SSR to be 15 dB or more. SSR is the main factor in selecting polarization diversity (single payload) or polarization multiplexing (dual payload). Single payload is used when SSR is extreme.

You ask about the abrupt 3 dB change in the graph below. If this link uses connectorized antennas, then it might be a change in the cable connection. Alternatively, it might be a change in antenna alignment, although this seems relatively unlikely because SSR was stable before and after the step. If you are using DSO then the SSR step is most likely the result of a change in RF channel. Antenna gain varies across the band, and there maybe a small amount of frequency-selective multipath fading.

Does this seem to be a possible explanation?


1 Like

Yep, this makes lots of sense. The link is using DSO. I wouldn't expect any cable changes on the connectorized antennas, but I haven't inspectedin a while. Thanks very much!

Hi Nate,

I'm glad that helps. What is your network management system?