My understanding of an RSSI reading is that it should equal Tx power - Tx feed loss + Tx antenna gain - pathloss + Rx antenna gain - Rx feed loss. If this is the case, if the radios at either end are identical and have the same Tx power, shouldn't the RSSI experienced at both sides be the same?
I have a scenario where on one side, the radio is receiving at around -100dB, at at the other side, the RSSI is -85dB. What could be the cause of this?
I have noted that both of the antennas could benefit from being raised away from the nearby metal roof, but wouldn't that still factor the same in both transmitting and receiving? Also, note that on one end there is an omni directional antenna and on the other end a yagi (same vertical polarisation). Unfortunately I do not have VSWR readings of either side at this stage.
@tomnexus, your answer is insightful. There is some additional information which I have discovered though, which might confuse the matter even more. At this stage I cannot measure the VSWR as the site distant from here - all I have are some computer diagnostic logs which I am reviewing now, and the diagnostic utility doesn't have VSWR readings.
The radio network is in fact a point to multipoint data radio network. Let's say the base (omni antenna) is called "BASE", and the two remotes are R1 and R2, each with yagis pointing back to the base. They are not near one another. Communication is half duplex, so R1 and R2 can only hear B, not one another. BASE's RSSI from R1 is -80dBm. R1's RSSI from BASE is likewise -80dBm. BASE's RSSI from R2 is -85dBm, however R2's RSSI from BASE is -100dBm as explained above. The initial thought, seeing that R2 receives from BASE 15dB quieter than vice versa is as you said - bad VSWR at BASE, combined perhaps with the BASE radio automatically turning it's own Tx power down. However, if this were the case, I would expect to see the same phenomenon with RSSI at R1. The odd one out in this picture appears to be R2 alone. Is it ever possible that receive sensitivity is degraded more than its transmit performance? Does this point necessarily to an issue with the radio itself? As mentioned before, all antennas are each mounted rather close to the slightly angled roof of a metal shed. Is it at all possible that a radiation pattern or path loss can be influenced differently on Tx than on Rx?
I'm not sure if it's relevant or not, but the last thing is that the supply voltage at BASE is occasionally dropping slightly - this might be related or not. The nominal voltage for the radios is 13.8VDC, however these radios are commonly used in conjunction with batteries and solar radios and have a fairly wide voltage tolerance (will work properly at >10VDC, shuts down at < 9VDC). I have noted that the supply voltage for the BASE radio is 11.9, and occasionally it simply drops to 11.4 (all along, however, R2 receives at -100dBm). It might be that the power supply/battery combination is not adequately handling the required current draw, but I wonder if this current draw is increased on account of bad VSWR. That still doesn't explain it though. Thanks for you ongoing help. BTW, radios are running at roughly 5W.