SDS100/SDS200: RSSI dBm Range

[JamesBrox]

I'm logging a system with 40 sites (tower locations). I know my base station, a 200 paired with a Diamond discone at 35' above elevation will not reach them all. But I don't want to delete a site that could help my logging. How can I determine which ones to keep in scan? By dBm RSSI number? And what is range dBm measurement threshold is considered good? Thanks

James
WRXU693

 

[hazrat8990]

Program them all, turn off location control for the system, and then check each site to see if they are able to be decoded. Once you've figured out the ones you can actually hear, you can either delete, or avoid the rest.

 

[prcguy]

Most commercial radios will receive useable speech down to at least .25uv and some a little lower and .25uv is -119dBm in a 50 ohm system. But that depends on where the squelch is set which could be higher at .5 to 1.0uv. .5uv =-113dBm and 1uv is -107dBm. I might use -110dBm for what you are doing. All this is assuming your receiver has an accurate readout of dBm.

 

[JamesBrox]

Program them all, turn off location control for the system, and then check each site to see if they are able to be decoded. Once you've figured out the ones you can actually hear, you can either delete, or avoid the rest.

I did that. I picked up more sites w/ location off then on.... of course. So, a shown dBm signal reading on any particular site means I'm hearing that tower even if no comms is being TX? Correct?

 

[JamesBrox]

Most commercial radios will receive useable speech down to at least .25uv and some a little lower and .25uv is -119dBm in a 50 ohm system. But that depends on where the squelch is set which could be higher at .5 to 1.0uv. .5uv =-113dBm and 1uv is -107dBm. I might use -110dBm for what you are doing. All this is assuming your receiver has an accurate readout of dBm.

I set my squelch on 2, noise floor is pretty low here. Oh, I don't know if the RSSI is accurate or not on this 200. Just getting ranges from -114 to -91dBM on any given site that I'm receiving. Forgive my ignorants.....so, in general, the higher dBm reading is the strongest and the lower the number means the weakest?

 

[prcguy]

oh, I don't know if the RSSI is accurate or not on this 200. Just getting ranges from -114 to -98dBM on any given site that I'm receiving. Forgive my ignorants.....so the higher dBm reading is the strongest and the lower the number means distant signal?

You can check your receiver with a service monitor to see if it tracks. Otherwise not knowing if your RSSI is accurate would be like driving past a radar cop using your hand out the window to estimate speed.

 

[JamesBrox]

You can check your receiver with a service monitor to see if it tracks. Otherwise not knowing if your RSSI is accurate would be like driving past a radar cop using your hand out the window to estimate speed.

Layman terms.... you mean the signal bar, upper RH corner. lol

 

[buddrousa]

-91 is alot better signal than -119

 

[dave3825]

I picked up more sites w/ location off then on....

That’s normal. The range info on sites in the database is generally for the sites intended area of operation. Often a sites rf will travel further than the range listed.

 

[JamesBrox]

That’s normal. The range info on sites in the database is generally for the sites intended area of operation. Often a sites rf will travel further than the range listed.

I saw that. General site ranges were 22-25 miles. I'm a lot farther than that.

 

[JamesBrox]

-91 is alot better signal than -119

Lower the better. Thanks

 

[dmfalk]

Keep in mind that RSSI is measured in negative dB/m, so the weaker it is, the higher the number appears. -114 dBm is about the minimum strength the SDS series will let through on a squelch of 2. -90 is decent. -40 dBm is dang near on top of you!

 

[JamesBrox]

Keep in mind that RSSI is measured in negative dB/m, so the weaker it is, the higher the number appears. -114 dBm is about the minimum strength the SDS series will let through on a squelch of 2. -90 is decent. -40 dBm is dang near on top of you!

Thank you, sir.

 

[Ubbe]

It will struggle to decode digital signals at -105dBm so it will need to be a stronger signal to not have too many data errors that makes decode impossible.

A discone usually have a 10-20dB loss above 700MHz so if the sites are in that range then the telescope antenna at the back of the scanner might even work better. If you have systems above 700MHz that are weak then consider adding another antenna tuned for that band and have a diplexer at the antenna to split up the signal so that everything above 600-700MHz goes to that other antenna. Or if everything you monitor in the VHF/UHF range are super strong then that 700-900MHz antenn will still pick them up.

If a suitable diplexer cannot be found then MiniCircuits got lowpass and highpass filters that can be used. One lowpass filter to the discone that stops signals above 500MHz and a highpass filter above 500MHz to the dedicated 700-900MHz antenna. They have different connector types on different filters but something like this lowpass and highpass can be used and a T-connector to a common coax. It could even be possible to have those filters at the scanner and have a coax go to a good antenna in a window.

/Ubbe

 

[buddrousa]

Ubbe both of my SDS100's and both of my SDS200's decode 100% at -115DB does not matter if it is Analog, P25, DMR or NXDN.
The only time my TRX scanners work better than my SDS scanners is on unknown DMR or NXDN Trunking Systems where the LCN's are unknown to me and we all know that is only because the TRX scanners only scan the Voice Channels and do not TRUNK TRACK.

 

[JamesBrox]

It will struggle to decode digital signals at -105dBm so it will need to be a stronger signal to not have too many data errors that makes decode impossible.

A discone usually have a 10-20dB loss above 700MHz so if the sites are in that range then the telescope antenna at the back of the scanner might even work better. If you have systems above 700MHz that are weak then consider adding another antenna tuned for that band and have a diplexer at the antenna to split up the signal so that everything above 600-700MHz goes to that other antenna. Or if everything you monitor in the VHF/UHF range are super strong then that 700-900MHz antenn will still pick them up.

If a suitable diplexer cannot be found then MiniCircuits got lowpass and highpass filters that can be used. One lowpass filter to the discone that stops signals above 500MHz and a highpass filter above 500MHz to the dedicated 700-900MHz antenna. They have different connector types on different filters but something like this lowpass and highpass can be used and a T-connector to a common coax. It could even be possible to have those filters at the scanner and have a coax go to a good antenna in a window.

/Ubbe

Great info. The system I'm logging is DMR Tier3 UHF.

PowerSouth Energy Cooperative Trunking System, Various, Multi-State

PowerSouth Energy Cooperative Trunking System Profile

www.radioreference.com

 

[ofd8001]

With digital signals, its really academic. You either got it or you do not. As long as you are getting understandable audio, you are good.

I get one site that has no indication on the signal strength meter, but I hear it like it was next door.

 

[Trilliumaire]

Besides getting as many sites as possible, based on what you can demod, also look at de-duping sites that overlap traffic. So many P25 sites around here are clones of each other, so in this case, I would just pick the one with the most reliable reception, measured by the DERR value displayed on the SDS100, not the RSSI. The less the FEC has to do, the better your digital quality. So go with lower DERR values like 0-5 as your gauge.

For analog the RSSI is a proxy at best because your antenna could be folding in a bunch of noise, and artificially increasing your RSSI. So look at an SNR based measurement. You can spot this with a waterfall unlock on the SDS100, or hook the same feed to an SDR that measures SNR. I have an SDRPLAY RSP1B for more detailed looks.

 

[JamesBrox]

Besides getting as many sites as possible, based on what you can demod, also look at de-duping sites that overlap traffic. So many P25 sites around here are clones of each other, so in this case, I would just pick the one with the most reliable reception, measured by the DERR value displayed on the SDS100, not the RSSI. The less the FEC has to do, the better your digital quality. So go with lower DERR values like 0-5 as your gauge.

For analog the RSSI is a proxy at best because your antenna could be folding in a bunch of noise, and artificially increasing your RSSI. So look at an SNR based measurement. You can spot this with a waterfall unlock on the SDS100, or hook the same feed to an SDR that measures SNR. I have an SDRPLAY RSP1B for more detailed looks.

Not seeing the D-ERR values.... even on detailed screen option.

 

[Trilliumaire]

Not seeing the D-ERR values.... even on detailed screen option.

You have to enable them in the menu. They are not on the default screens. I can walk you through if you need help.