SDS100: Any VHF channel shows -45dBm level

This morning I turned on my SDS100 and had it set to scan all VHF Railroad channels.
After about a half hour passed, I realized I'd not heard a peep. Definitely not normal as there is almost a transmitter on the air here somewhere in the VHF rail band!
I grabbed the SDS and opened the squelch, no audio at all but the radio did stop on whatever frequency it was scanning when I opened the squelch.
I've had this happen before where the radio would not receive until I power cycled it. So I power cycled it when an open squelch produced no white noise. I had to power cycle it twice and then I was getting audio again.
It started receiving signals as well. Then I noticed that all signals on any frequency were showing a -45dBm RSSI on the radio and in ProScan which I'd had running for logging.
I then put the SDS into manual mode and stepped through several rail channels. Any frequency shows a -45dBm signal level and the signal strength bar meter shows full bars when I open the squelch regardless if there is a signal on the frequency or not. I fired up a couple other radios and found that the paging sites were also on the air. If all paging goes silent on their 152.xxx MHz frequencies, the SDS will show a weaker signal strength on quiet channels. It never goes any weaker than -107dBm though. The only way to get it to display a weaker signal is to remove any antenna. I discovered it's definitely being swamped by any moderately strong signal anywhere in the VHF band and not just paging like I'd found in previous tests.
I can tune the SDS to any VHF frequency and then open the squelch while there is no signal on the frequency it's tuned too and the signal RSSI will show -45 to -55dBM when I manually open the squelch!
When I let the radio scan, it does scan and does stop on weaker rail band signals but the RSSI value is showing a full quieting signal level plus the bar graph signal meter is always full bars. So it does pickup signals even when the 152 MHz paging sites are transmitting but its signal level functions are way off. My ProScan log is full of full strength logs.
The other thing I see is the SDS RSSI value nor the bar graph signal meter will show anything when there is no signal on the tuned frequency as long as you leave the squelch closed. It's acting like it will not display a signal strength value in its bar graph or in the RSSI value if the squelch is closed.
This is mainly when using an external antenna as that is needed for the statewide VHF P25 system here. I can't hear that system indoors with the rubber duck antenna so I must use an outdoor antenna.
Like my other findings involving the local 152.xxx MHz paging sites, this SDS is very poor for VHF.
I did run some tests today across the VHF Air Band and I find those frequencies do not show a -45 dBm signal strength reading when I open the squelch. Air band all show a RSSI of -107dBm when I open the squelch on a free frequency in the air band. Maybe something to do with AM mode but I did not test switching to FM.
I find this high dBm level across the entire range from at least 144.000 to 174.000 MHz. I did not test outside that range but did test some samples in the 450.xxx UHF band. UHF band behaved normally and does not show a false high RSSI value when I open the squelch.

I also thought that maybe I was getting some common mode noise from noisy power lines. I've had my share of noisy power lines before and they will definitely cause a RSSI value that reads stronger than -127dBm but today there is no power line noise present as shown on any other receiver including my Icom R9000 and the likes. The Icom's show no signal at all across the board yet the SDS100 shows -45dBm across the board anywhere in the 144 to 174 MHZ range!

I'd send this SDS in for service for the problem I've seen where it does not receive anything at times when first powered up but that does not happen often. If I sent it in, it would be returned with a note that there is nothing wrong as they would likely not see the problem.
The problems I see with VHF signals causing desense and now, a -45dBM RSSI can easily be seen as long as there is a nearby strong VHF signal somewhere in the VHF band but I doubt they test for that. They probably tune the radio to the frequency they are testing with and if it receives, it passes their test and is sent back. Do they test for desense and selectivity issues? Something tells me they do not, at least not for scanner products. So getting this problem seen and fixed is probably not going to happen.

For anyone that feels like they have poor VHF performance, try opening your squelch on an open VHF frequency and see what the RSSI value is showing when hooked to an outdoor antenna and in a high RF environment with strong paging or other local and strong VHF signals on the air nearby. Chances are you may see the same as I find.
I think this has probably been going on ever since I discovered the local paging sites were wiping out my VHF P25 signals but I'd never noticed the false RSSI values before.
I looked through some of my ProScan logs and I found a ton of false -45dBm readings over the past couple months. None of what I monitor should be anywhere close to a full quieting signal level of -45dBm!

Something is seriously wrong with my SDS100. Not sure if this is a common problem amongst others that own a SDS100 or not but I sure see the above problem. The bad thing is it does still receive VHF signals so this would make getting the problem resolved, near impossible. At least with the tests Uniden's techs probably run when this model is sent in for service. They would find that it receives their test signal(s) and send it back as fixed or problem not found.
I wish I had a proof positive way they could see this problem but I'm worried they will not read my notes so sending it in may be pointless.

While typing this, I left my SDS sit on an unused UHF frequency of 462.525 with the squelch open and it did not ever show any false RSSI values. In fact, it did not show an RSSI value at all. With that, it appears my findings only exist in the VHF high band and UHF is fine.
None of my other Uniden scanner radios show any odd or false signal strength levels on any bands. This problem only exists with my SDS100.

If you add a filter to reject FM broadcast to the antenna feed does it improve VHF-Hi at all? Does anyone make a tunable VHF-HI notch filter? It would be a good tool to determine what signal(s) are overloading the SDS. It might also be worthwhile to survey all the bands the SDS receives to see if any other range is adversely affected by strong signals.


I appreciate reports like yours to help me decide if this radio is worth consideration for my monitoring station. So far it appears that its usefulness is limited.

What happens when you tune to weather freqs.?

I just checked with my SDS100, and it does the same thing, -45dBm/full bars, on an unsquelched VHF channel without a signal present. It might be nice if we could get a few more reports.

I did try adding an FM BCB filter back inline but no help.
I've also tried putting notch filters inline for the common VHF paging ranges but also not much help. Paging filters did allow me to see a normal RSSI for a short period though. I suspect that other VHF transmitters are still swamping the front end. I can't find a quiet period where ALL strong VHF signals are off the air at the same time however!
I suspect I was lucky and the strong signals all went quiet while I had the paging filter inline and that is when I saw the RSSI return to normal for a very short time of about 10 seconds. Then it went back to the normal -45 to -55dBm levels again. There's just so much VHF activity in the area that finding a quiet time may be impossible. Maybe at 2 or 3AM.

I'm watching a spectrum analyzer also but so far I've not been able to track down any single source. It seems to be any strong signal in the VHF high range. As long as one strong transmitter is on the air, I see the false RSSI. Notching out paging took those signals out of the equation but left all the other strong RF in the mix. I also have a WX notch filter but again, no help. The WX site is a distance away but its signal is still very strong here. The fact I saw the RSSI return to normal for a short period should be proof that the NOAA channel is not a cause though as I did not have the WX filter inline at that time.

If I can catch that short period of time where the RSSI drops back to a normal no signal level, I'm hoping to see just how far away (in MHz) a strong signal must be before it quits affecting the SDS.
The cause really appears to be any strong signal in what appears to be the VHF high range but it could be strong signals outside that range as well.
I did put a high pass filter inline to remove everything below 30 MHz but no help again. I also have a low pass filter set at 700 MHz to filter all above 700 but no help.

Do you have a switched attenuator? It could help to determine how severe the overload is. The one I have is made by Coline and does 1dB steps from 0-31dB from DC-500MHz.

I just checked on my SDS100 and I have --- showing up on all unsquelched VHF railroad frequencies. I tried both with and without the graph and if there is no signal, there is no reading, just the ---

pro92b said:

Do you have a switched attenuator? It could help to determine how severe the overload is. The one I have is made by Coline and does 1dB steps from 0-31dB from DC-500MHz.

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I was just coming to post about this!

I did put an attenuator inline. I started dialing it down until the false RSSI returned to normal. The value varied depending on what VHF frequency the SDS was tuned too but for all practical purposes, an attenuation of 13 dB or greater seemed to tame things down. Or so I thought.
I was using the 7 NOAA frequencies to test with just to make sure I was not killing reception with the attenuator. Of the 7 NOAA freqs, I get a signal on 3 that is always there but does have slight static. One is full strength of course. I tested the 3 weaker stations. What I found was interesting. With the external attenuator inline and set to -13dB, I found that the RSSI value is totally blank on the SDS screen. No reading at all even though I had perfectly intelligible signals. The loss of 13dB of signal was not enough to kill reception at all.
Take the attenuator out of the picture and the RSSI shoots back up to -45dBm.
I then tried using the SDS's built-in attenuator. It has the same effect in that it knocks out any RSSI value display and also drops the signal bar meter to no bars. So the internal and an external attenuator both alter the RSSI values as displayed on the SDS to the point it reads no value and the signal level bargraph has no bars.

When I tuned my strong local NOAA signal, the RSSI read a normal value with the attenuator turned on or with my external attenuator inline and dialed to -13dB. I think the local NOAA station gave me an RSSI of -75dBm which would be about right after 13dB of attenuation.
Why an RSSI value is not displayed for the weaker NOAA signals with an RF attenuator inline is a mystery. I definitely don't have too much signal nor is the signal so low that an RSSI can't be obtained! Something is truly weird for VHF high in the SDS.
One thing I did notice on two of the weaker NOAA signals was the total loss of signal for a fraction or two of a second up to maybe a full second. It was not signal fade but the sudden loss of signal for a short duration and then it would pop back at the same level it was before it went away. It sounded like what overload would cause if another transmitter came online. Possibly from some of the short data burst some of the utility companies use for telemetry here as the loss of signal did not seem to be long enough for a voice transmission.
My SDS100 is the only radio that has this issue with all hooked to the same antenna system.

I did hook an outdoor antenna up directly to the SDS just to rule out any issues with a multicoupler or any other equipment but that also did not help the SDS in any way. I don't run pre-amps or anything but do use multicouplers from Stridsberg when things are all normal.
For those that may not know, multicouplers usually have RF amps in them to overcome the loss from their internal splitters when you get above two ports. These amps can fail or produce unwanted noise but mine are all working correctly as the direct to radio tests proved.
Most of my testing was done with an antenna hooked directly to my SDS though just to be on the safe side. I needed to keep everything out of the equation just to make sure something else was not going on and I was blaming it on the SDS!

I'd love to test VHF Low but that band has a high noise floor here so testing signal levels with real signals is hit or miss.

I did run some tests in the 450m MHz range again but again, I could not duplicate any of the issues I see with VHF. UHF seems fine for me so I probably won't test it anymore unless someone else reports a problem that I may have missed.

N9PBD said:

I just checked with my SDS100, and it does the same thing, -45dBm/full bars, on an unsquelched VHF channel without a signal present. It might be nice if we could get a few more reports.

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Mine shows ---

denrodg said:

I just checked on my SDS100 and I have --- showing up on all unsquelched VHF railroad frequencies. I tried both with and without the graph and if there is no signal, there is no reading, just the ---

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Is that test with an outdoor antenna? And do you live in a high RF area like around several hospitals for example?
Those conditions are what I'm testing with.
I also get no reading with the rubber duck antenna or no antenna or if I hook to an 800 MHz yagi.
Not so lucky when I hook to any rooftop multiband antenna or just a single band VHF antenna though. That's when my problems start.

cellphone said:

Mine shows ---

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These kind of results from others make me wonder if there is an unknown component change or revision somewhere that we don't know about or if some SDS100's are truly defective.
I think I only recall one person here who was finally sent a refurbished radio after they had send there's in for similar problems more than once. The refurbished radio worked fine while the original never did. I'm pretty sure that user was having VHF issues as well and that was the reason they were sending it in for service. Can't recall who that was but they post here fairly often.

Ok, I just read the original post. I apologize, but I skimmed it earlier and missed the "open squelch" part.

I get odd readings with open squelch. I am currently getting -107 to -112dBm and 1-2 bars of signal on my RS 800mhz antenna. If I remove the antenna, I get ---. If I hook to external antenna, I get -45dBm and full signal.

kruser said:

Is that test with an outdoor antenna? And do you live in a high RF area like around several hospitals for example?
Those conditions are what I'm testing with.
I also get no reading with the rubber duck antenna or no antenna or if I hook to an 800 MHz yagi.
Not so lucky when I hook to any rooftop multiband antenna or just a single band VHF antenna though. That's when my problems start.

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I have the same thing with my outdoor discone or with any of my 3 portable antenna, the stock, the watson 881 and the remtronix 800 mhz, on any antenna I have --- for a signal reading on any channel of the railroad band I turn to with the squelch set at 0, and I do live very close to 1 hospital, its roughly a quarter mile from my house, and then we have a larger hospital about 2 to 3 miles from me as well. As far as any other interference I have an FM radio station roughly a quarter mile in the opposite direction of the near by hospital and I never get any interference from that station, but on shortwave I get a lot of bleed over from a local AM station that is nearby. I hope this info helps.

Haven't noticed any VHF issues on either two SDS100s. Happened to hear 155.085 MHz before shutting down for the night on the BCD436HP so tuned it up on the SDS100 and both sound about equal sharing the same external antenna. The signal strength hovers around -80 dBm when active. It's about -105 dBm when silent and squelched open. Believe I'm about 45 miles away.

Thanks everyone, for the report.

I think this does show something is going on with this model but not for everyone. It's definitely not a problem that you would normally find in Uniden scanner type products.

My various newer digital type radios using the GRE design suffer from the same problem with a "S" for Signal always being displayed when scanning VHF systems. With the GRE's, it is usually caused by FM Broadcast stations and can sometimes be solved with FM traps but if you also live near high powered paging sites like are often found at hospitals with tall buildings, an FM Trap will not help.
I'm seeing the same thing with my SDS100. It is suffering from all the RF in and around the VHF high band from somewhere below 144 MHz up to around 170 MHz or so. Mine SDS signals readings do seem to settle down when I tune the VHF band above 170 Mhz. I don't know if my outdoor antenna is just not as resonant at that frequency or if it's because there is usually much less activity in the higher end of the VHF High band but something does allow the SDS100 front end appear to start working normally again as I get to the upper end of the band.
I've not yet figured out where things start to improve on the bottom end of the band as I've only tuned down to the bottom of the two meter band but the false RSSI values are still reported at 144 MHz. If I block the main part of the VHF band from about 145 to 165 MHz, things seem to improve but of course that is not really an option if you also want to scan that band!
I have a very old tunable notch filter of sorts sold by Grove eons ago. It is more of a tunable stub type filter and uses a small inductance of wire coupled to a variable capacitor to allow for tuning its notch frequency. It does not offer a very deep notch however. Nothing close to what a true notch filter can offer. This old Grove filter attenuates signals about 10 MHz either side of its center frequency so narrowing it down is hard.
I've played with both open and shorted stub filters before and while they have a narrower bandwidth compared to this Grove filter, they are also too wide for most practical purposes. This Grove filter also attenuates higher frequencies like 700MHz and up so it can't be left inline if you also monitor those ranges. If all your signals are strong, the Grove filter can be used with some success but it's really not much better than using a good step attenuator that covers it all. They both kind of defeat the point of using an external antenna though!
I myself am into weak or weaker signal reception in all bands so I need as much signal as I can get. Using external signal amps are not usually an option here unless I'm working with some satellite based signals that need more gain over what the radio can offer from it's sensitivity and what my antenna's can offer from their gain.
I was very surprised when I first tuned my states statewide VHF P25 system and I had no signal. All my other Uniden scanner radios receive this trunked system just fine.
The state system is not a simulcast system for most sites so I did not really need the SDS for that problem. I think I bought the SDS more for its cool customizable display more than anything. That feature I really like!

In all fairness to the SDS100, its simulcast performance for 7/800 MHz sites is fantastic in most cases. It will probably cure the LSM problem for those that suffer from LSM in the 7/800 bands. But... it is a very poor performer for those that want to monitor distant VHF sites and use an outdoor antenna who also live in high VHF activity areas with a ton of RF in the air.
I know a couple users that live in rural areas or "out in the sticks" as us city dwellers call it, that get great performance from their SDS100's using outdoor antennas. They all must use outdoor antennas to bring in the signals from the large cities they live near but not within. Being as they have a low noise floor, they can get away with using nice outdoor antennas without any problems but there same setup would likely cause problems if they moved back within the city.
I also own several GRE made or designed models. They all worked very well when I was located out in the sticks but they suffered badly from overload when I returned to the city. I'm actually well away from the city itself but my area of the county is loaded with tall hospitals. Each one has some form of UHF, VHF and 930 MHz paging antennas on their rooftops. This can really wreak havoc with many scanner models. It's a great location if you want to test how bulletproof your radio's front end is however! My SDS100 really reminds me of the issues I have with the GRE designs.
I don't know if Uniden can do anything about this problem with firmware but maybe they can help if they change the AGC settings or allow the users some control over these settings. Maybe via a hidden menu so the average user stays away from them? I think that would be a good thing for those that suffer from the problems I see.
Or, if it is truly a hardware shortcoming, maybe they will offer a fix if many suffer from problems and complain but I don't know if we will ever see that. If it's a component problem that only affects a certain batch of these models, then repair may cover it. I can only hope.

Thanks again for the replies.

kruser said:

None of my other Uniden scanner radios show any odd or false signal strength levels on any bands. This problem only exists with my SDS100.

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That's all you'll need to know and don't have to check your antenna system that is common to several other scanners.

If you don't have a $10 SDR USB receiver stick then get one and check how it receives. It will behave the same as a SDS100 in VHF as they use receiver chips of the same standard. The only solution to improve reception would be a hardware change to make many more and narrower antenna pre-filters but would still not reach up to "normal" scanner standards.

SDS100s are extremly location critical in VHF and low-UHF and are not suited for everyone that monitors those frequency ranges. It excells in 700-800MHz and of course if there are simulcast issues.

/Ubbe

Land of the Good and Plenty

kruser said:

...
In all fairness to the SDS100, its simulcast performance for 7/800 MHz sites is fantastic in most cases. It will probably cure the LSM problem for those that suffer from LSM in the 7/800 bands....

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I must be lucky since I haven't had any LSM issues with BCD436HP, BCD536HP, TRX-2, SDS100 scanners. Must be a combination of location and antennas.

kruser said:

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But... it is a very poor performer for those that want to monitor distant VHF sites and use an outdoor antenna who also live in high VHF activity areas with a ton of RF in the air.
...

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I'd have to disagree some there. I've had plenty of VHF activity on the SDS100 and while not the greatest, can't say it's the worst either. While reading this I got a hit on VHF Low Band on the SDS100. RSSI ranged between 45-50 dBm and depending on transmitting station, some audio was less scratchy than others. I set the BCDx36HPs on the same frequency as well as an IC R-30 and R100. They were all inconsistent on capturing the different transmitting stations (any activity and also readability) except for the R100 that picked up every transmission with the best clarity. They all shared the same external antenna.

kruser said:

...
I also own several GRE made or designed models. They all worked very well when I was located out in the sticks but they suffered badly from overload when I returned to the city. I'm actually well away from the city itself but my area of the county is loaded with tall hospitals. Each one has some form of UHF, VHF and 930 MHz paging antennas on their rooftops. This can really wreak havoc with many scanner models. It's a great location if you want to test how bulletproof your radio's front end is however! My SDS100 really reminds me of the issues I have with the GRE designs.
...

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I never knew I had paging intermod issues at my location until I got a TRX-2 a year ago. It was so bad on VHF that I returned it. No other equipment, to include amateur gear, ever had that issue in the 20 years I've lived here. And now it's gone again