D-error - RSSI relationship

bobruzzo

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I have been closely watching my D-error and RSSI numbers and would like some input to what I am seeing. I've been using a 6 element 800mhz yagi for the past week or so for my sds200. The antenna is quite an improvement over the small vertical I was using. I live in a simulcast area and even with the SDS200 I get the occasional garbled audio. Playing with filters seemed to help quite a bit. The system I monitor has 4 sites. 2 of those sites I set filter "OFF" and experience pretty much clear audio. The other 2 sites seem to work with filter set to "AUTO" on one and "NORMAL" on other and one has the ATT on. I see that my D-error is usually "0" but at same time the RSSI varies between 50 and 95. What is the relationship between these 2 numbers? The highest I have seen my RSSI is about 117. I didnt bother to use the "NOISE" option because the numbers were erratically changing too fast to be of any use. As I watch scanner go thru the system the RSSI numbers are between 50, 60 up to between 92 and 110.
So I see that making adjustments to these settings does help in the long run even tho it does take time. I really want to experiment with aiming that yagi in different directions and record my results. But where I have it mounted is too inconvenient to do this. I may order another yagi, the same one since it's affordable and mount it on a simple mast that I can easily make adjustments. The 4 sites I monitor on my state system are located with one site to my south, one to my east and the other two to my north, So its just a matter of nulling to get best results on all 4 sites......but with these sites being in all different directions, it may be a waste of time. Not all departments broadcast on all of those 4 sites all at the same time. I read that by pointing antenna at one of the sites, which in my case would be the south site which has very strong signal, it would null out the others and any offending simulcasts. I have been wrestling with this for months and I think my location is part of the problem. But when you have 4 sites with a bunch of antennas and towers for each one scattered all over it is very hard to troubleshoot.
 

Ubbe

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Stockholm, Sweden
There's no relation between the RSSI level and D-error in a SDS scanner. RSSI can be anything as it uses a broadband signal detector. I tested and it displayed -45dBm when receiving a -80dBm signal when added a -60dBm signal 8MHz from the monitored one. Forget about RSSI being an accurate measurement.

/Ubbe
 

bobruzzo

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Cranston, Rhode Island
OK that makes better sense. Still annoying trying to troubleshoot. I am within 5 to 8 miles of certain antennas within the groups of antennas in each site. I dont know why I would get a weak signal being that close unless something is desensing what I receive. I just did a Google maps waypoint for the 5 Providence (RI) Zone sites....The map has a straight line going from my antenna to each of the 5 antennas in that zone. All the antennas are more or less northeast, and east northeast of my antenna. Looking at map, 3 of those RISCON PROV antennas have some high hilly areas between me and them. And 2 of them are the ones more northeast which bypasses those hills. I may be overthinking this a bit. I might try aiming yagi at the tower thats in a more northerly direction just to see what reception is like. But there are so many variables that affect this. Like when there is a transmission on the PROV site, the signal goes out over all 5 of those towers....But all 5 are in close proximity of each other, and its hard to null out some of them. Plus a 6 element yagi still has some beamwidth thats still going to pick up overlapping signals. Well it's worth a shot and the signals from the other state zones are pretty strong so antenna will pick 'em up regardless of direction yagi is pointing. It's just this 1 PROV zone I am trying to get better reception.
 

Ubbe

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5,122
Location
Stockholm, Sweden
I dont know why I would get a weak signal being that close unless something is desensing what I receive.
It's an automatic desense in SDS scanners as it has a very broadbanded sensor for signal strenght that not only affects RSSI readings but also reduce the sensitivity of the whole scanner due to its automatic gain control in the receiver chip. You can read about the AGC action in my test:

"The desensening of the receiver, how the RF AGC adjust the gain by the result of signals off frequency, where that the reciever lost 5dB of the signal when injecting -80dBm signal 100KHz from the monitored frequency.
Injecting a -60dBm signal gave 20dB loss and reception where almost completly lost and could only be heard faintly with the squelch open.

With the -80dBm injected signal it gave intermod at two frequencies 300Khz and 600KHz from the monitored frequency and totally killed the reception.
"


/Ubbe
 

bobruzzo

W1AV
Premium Subscriber
Joined
Nov 4, 2019
Messages
826
Location
Cranston, Rhode Island
It's an automatic desense in SDS scanners as it has a very broadbanded sensor for signal strenght that not only affects RSSI readings but also reduce the sensitivity of the whole scanner due to its automatic gain control in the receiver chip. You can read about the AGC action in my test:

"The desensening of the receiver, how the RF AGC adjust the gain by the result of signals off frequency, where that the reciever lost 5dB of the signal when injecting -80dBm signal 100KHz from the monitored frequency.
Injecting a -60dBm signal gave 20dB loss and reception where almost completly lost and could only be heard faintly with the squelch open.

With the -80dBm injected signal it gave intermod at two frequencies 300Khz and 600KHz from the monitored frequency and totally killed the reception.
"


/Ubbe
Well I have spent a good part of the day running up and down ladder adjusting my yagi and the bottom line is I get the best reception, no or low D-errors with yagi facing due EAST. I tried a few other directions and it was mostly give and take with the 4 zones in my state simulcast system. But with yagi facing due east I can get much better results. Thats that part of it. Filter settings seem to help. One zone in particular is VERY stubborn and nothing I tried works so I may delete it. Not much to hear on it anyway. I guess with all the trial and error tests I have been doing this is as good as it gets. I have found filter adjustments DO help but vary from place to place. I have tried putting the ATT on all sites. I ended up keeping it on 2 of them and it seems to do well. But wow there are SO MANY variables that this gets to be like a job making adjustments on everything.....radio adjustments, antenna adjustments.....I think I have tried all I can try. With the P25 system here RAISING antenna doesnt seem to matter since I am close to many of the towers. Then who knows if I am getting desense issues from some other transmitters, or cell towers? Theres really no way to know. But I am basically happy with the results I got today and will leave antenna alone. BTW I did read your test results a few times!
 
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