RSSI Discrepancy...

[RadioDitch]

Hey all, figured I'd throw this out here for whatever good it might do.

The last two weeks, I finally got the opportunity to utilize my SDS200 mobile in it's fullest glory for the first time while my brother was visiting. It had really just been on my desk prior since up here I don't really need all it's features unlike when I go back to visit Jersey or into Cote-Nord, Quebec for work.

I immediately noticed that the RSSI reading was off and inconsistent when we started working on systems in Cote-Nord for the database. There were several instances where we were directly under a known trunking site location, and I mean arms length to the tower, and had a reading in the -60dBm range. Meanwhile, my APX8000 and the mobile Unitrunker setup were in the much more expected -25dBm range. I then noticed that as we were about 0.25-0.50Km away, the SDS would then show a lower RSSI reading momentarily, then quickly climb into the -90dBm range.

This wasn't unique to one system, but it does seem to be happening mostly with digital trunking systems. Conventional analog it seemed to be fine. Not sure if anyone else has noticed anything similar?

For me it's not critical cause my brother and I really only use the scanner when working on the Radioreference database for finding new or previously unknown sites' control frequencies. For the pertinent details it's SDR's and Unitrunker or SDR#. But I could see this being an issue for users who are just starting as amateur hobbyists into the more advanced parts of the hobby like system hunting or amateur interference hunting.

Just FWIW.

 

[jtwalker]

It's probably a metric-to-imperial conversion issue

On my SDS100, I've only seen -50dbm a few times when I was close to a tower. When I saw -40, it was because of cold solder joint issue showing it's ugly head. I consider a -60 a very strong reading.

Aren't these readings more relative and not absolute any way?

 

[RadioDitch]

It's probably a metric-to-imperial conversion issue

On my SDS100, I've only seen -50dbm a few times when I was close to a tower. When I saw -40, it was because of cold solder joint issue showing it's ugly head. I consider a -60 a very strong reading.

Aren't these reading more relative and not absolute any way?

Issue is usually on the imperial side.

Even as relative measure, the spread shouldn't be quite that much between the SDS200 and known reliable devices when I'm at the tower site. If an authorized portable and an instance of Unitrunker running an RTL are showing a relatively close signal reading in the mid -25dBm range +/-, then the SDS200 at worst should be showing in the -40's.

 

[nessnet]

Yes, but what did you have the gain on the SDR's set at?
Could be apples and oranges.....

 

[RadioDitch]

Yes, but what did you have the gain on the SDR's set at?
Could be apples and oranges.....

Fair question. Low for RTL's. Gain turned down to 10 with squelch at -60. The "standard" 16 gain is too much for hunting a site, particularly in the situation we were operating in. However, again, when you're sitting literally under a site, squelch off, the SDS200 should be showing an RSSI better than in the -60dBm range.

 

[Ubbe]

The receiver chip in SDS scanners are thought to be used in sat receivers where you have some 10MHz wide channels and channel steps. It's an automatic gain control in its front end that reduce gain at high signal levels and also the RSSI values are calculated from that 10Mhz wide window when it starts to go up in the -60dBm range.

RSSI findings in SDS scanner

/Ubbe

 

[GTR8000]

I wouldn't put any faith in the RSSI readings the SDS series show. My experience is that it's relatively useless as compared with other Uniden scanners and/or other hardware/software. The two SDS200 I operate pretty much show everything at -60, no matter how powerful or weak the signal is. For example, one TRS has a subsite a few hundred yards from the scanner's location, and the signal is devastating...shows -60. Another TRS 20 miles away barely breaks squelch, yet also shows -60 most times. 🤦‍♂️

 

[RadioDitch]

I wouldn't put any faith in the RSSI readings the SDS series show. My experience is that it's relatively useless as compared with other Uniden scanners and/or other hardware/software. The two SDS200 I operate pretty much show everything at -60, no matter how powerful or weak the signal is. For example, one TRS has a subsite a few hundred yards from the scanner's location, and the signal is devastating...shows -60. Another TRS 20 miles away barely breaks squelch, yet also shows -60 most times. 🤦‍♂️

Oh absolutely. I put no faith in it at all. Hence the tone of the post and my disappointment as it would be great if it functioned properly. Especially for less experienced users who are getting into signal hunting but don't really have the knowledge of using more advanced skills or tools. For our actual work neither my brother nor I have any plans of moving away from the Motorola and SDR/Software combination.

The receiver chip in SDS scanners are thought to be used in sat receivers where you have some 10MHz wide channels and channel steps.

That would make a lot of sense.

 

[brumey]

Hello:
I am not sure if this is relevant or not. I was questioning the RSSI readings on known P25 Phase 2 systems and others. My setup has an SDS-200 and SDS-100. My antenna is a Discone, outside, on mast with LMR400 running 30 feet. I am at 2000 feet above Sea Level and 300 degree line of site to most services. My antenna is connected to a Stridesberg 4 port multicoupler (Awesome Device). So, the signal at each radio is close to being the same.

I was getting -60db on the SDS-100 and -90-100db on the SDS-200. After much testing, swapping cables, using antenna direct on known busy system, I looked at the global filters. I changed the SDS-200 to Global-Wide and it started to read about the same as the SDS-100.

I have not gone through the trial and error filter testing yet but, I now have similar RSSI readings on both systems. Probably both inaccurate

 

[Ubbe]

I was getting -60db on the SDS-100 and -90-100db on the SDS-200.

Is that an older SDS100 using the older receiver chip? If you can send a dot . to one of the scanners USB ports, ProScan have a protocol command function or use puTTY or any terminal program, then the scanner will respond with a lot of info and one will say STD=R836 if it is the older SDR receiver and STD=R840 if it has the newer which is the same as used in SDS200. If you have a R836 receiver it might indicate that there is a difference between the chips.

/Ubbe

 

[brumey]

Ubbe:
Thanks for the response and yes, I would really like to know the CPU versions. I was not one of the first to buy the SDS-100, maybe a year after launch. Can I get a little more guidance. I typically have Proscan connected via IP to my SDS-200 but I connected to the SDS-100 today. I think its connected correctly, I am using USB (COM140), the display is echoing in Proscan. I found the Test & Extras tab and Protocol commands.

I then typed "." without the quotes and get ERR. I tried "DOT" too. Other included commands tried, EFM, UNIDEN rebooted SDS-100. GFM, UNIDEN responds with GFM, ERR.

Obviously I don't know what I am doing with these protocol commands and need a little more help.

I did try putty, Serial Mode, 115.2 baud, COM140 but no connection.

Any help appreciated.

 

[Ubbe]

Any help appreciated.

I was in error about using the mini USB port, that doesn't work. Use the micro USB port instead. It will be a different port number than what the mini USB port uses. Just put a . in the Protocol Command window and Send, or set puTTY to the correct port and 115200 baud in the settings menu and then select serial for the session and push the . key.

/Ubbe

 

[brumey]

@Ubbe Thanks for the additional information and apologies for my delayed response. Work got in the way of fun stuff. I was able to use the microUSB port and ProScan to issue a "." It responded with STD=R840 so I assume I have the newer. Newer is better, faster, greater, right

So, my original reply on this thread had me noticing a significant difference in RSSI readings, The SDS-200 being about 25-30 higher. Since changing the Global filter to Wide Normal, the RSSI reading on the SDS-200 is now very close to the SDS-100. I know Global Wide Normal may not work for everything and I'll have to go through trial and error to see what needs different filters.

I just don't fully understand why changing the filters changes the RSSI that much but I'll learn to just accept it. I now have both SDS100 and 200 on Wide Normal.

Appreciate your help. Now I need to utilize ProScan to better my scanning.

 

[Ubbe]

I just don't fully understand why changing the filters changes the RSSI that much

It's a receiver chip intended for 10MHz wide satellite channels. What you are seeing are the added signal strength from adjacent channels that interfere with the RSSI measurements and possible also with the channel you are tuned to. When selecting filters use the D-error and Noise values, they should be as low as possible, and RSSI should in most cases be as weak as possible, I.E. interferences are as low as possible.

/Ubbe

 

[JoeBearcat]

Will add to the issue list to research.