Signal strength conflict?

[N8LHG]

I have an interesting signal strength issue I want to run by you folks. I'm new to the sds200, fighting with the learning curve. While monitoring P25 trunk systems, I noticed the signal strength in the top right corner shows 3 to 4 bars when monitoring trunk systems as long as I am in the whip antenna that came with the rig. As soon as I connect to my rooftop discone, the bar display shows zero bars (P25 trunk). Now if I'm monitoring conventional systems, like airport AM traffic, it works fine. I've even pulled the antenna off the roof, verified proper connectivity, cable condition, etc. I've even connected my 2 meter transceiver to the discone, and verified low SWR and it seems to work fine. Connected the discone to my old radio shack pro 2004, and conventional 800 MHz stations seem to receive fine (though the 2004 has no signal strength display ability). Seems to be isolated to the sds200 itself. What would cause the trunk signal strengths on the sds200 to show zero signal bars if the antenna seems to be functioning properly? Any idea?

 

[engine183]

Is the P25 trunked system you’re trying to monitor a simulcast location. If so it’s possible that the simulcast sites are causing the problem. If you are not sure if it is a simulcast system you can check in radio reference database.

 

[900mhz]

filter selection.

 

[N8LHG]

Is the P25 trunked system you’re trying to monitor a simulcast location. If so it’s possible that the simulcast sites are causing the problem. If you are not sure if it is a simulcast system you can check in radio reference database.

Yes, do you think they are interfering or overpowered on the outdoor antenna?

 

[900mhz]

Yes, do you think they are interfering or overpowered on the outdoor antenna?

I have never, ever, met a condition with a SDS device where overpowering signal affects reception. The only situation I have encountered is trying to decode a 800 system which wasn't very strong to begin with right near a cell tower...cell interference wacked out the front end a bit. Filter adjustments helped. My Unication however, purred right along, like a kitten.

 

[Ubbe]

It's an automatic gain adjustment in the front end of the receiver that reacts to a 10MHz wide window of signals. If there's a strong signal within that window it reduces gain to stop any overload issues but you might loose your monitored signal totally if that other signal are strong enough. The design of a discone makes it drop at least 10dB of signal strength at frequencies 7-8 times it lowest frequency range, that's usually 100MHz as the lowest range and 10dB loss at 700-900MHz Also coax attenuates more at higher frequencies.

/Ubbe

 

[n1chu]

What type and how long is your coax feedline on the discone? When you get up into the 800 MHz range you need something like the LMR400 or LMR600 (there are other offerings that aren’t as expensive (and of a smaller diameter?) available also. I use a 50 foot run with a short pigtail of smaller diameter coax for flexibility from the LMR400 to the SDS200. (The LMR400 is what has been referred to as “poor man’s hardline”, it allows for some flexibility over hardline but getting it into the house and bending it are difficult. I know of one installation where the scanner is connected to the LMR400 and the LMR400 supports it from drooping, it sits above the desktop, floats in mid air-not good for the radio’s antenna connection however.

 

[N8LHG]

Thanks for all the replies folks! The discone is a diamond D130J which I removed the vertical low band element from, fed with about 50 feet of rj8x cable. I use to use it exclusively on my Kenwood tm631a 2/220 transceiver. I sniffed around and do have digital noise coming from the cell phone band. It sounds loud, and quite messy. I don't have equipment that can go into those frequencies, so I can't nail down the exact location. The rig I'm using to sniff around isn't going to have a solid front end either, so its purely a guess. I'm guessing I'm going to need to attenuate that band with a tuned filter or something to prove that out.

 

[Ubbe]

I sniffed around and do have digital noise coming from the cell phone band. It sounds loud, and quite messy.

You usually don't hear cellphone signals as those are some 200KHz wide in modulation. If might be DMR and NXDN systems that interfere. What happens if you enable the attenuator to reduce the risk of adjacent channel bleedover? Try IFX to channels and also the whole set of different filter settings.

/Ubbe

 

[N8LHG]

You usually don't hear cellphone signals as those are some 200KHz wide in modulation. If might be DMR and NXDN systems that interfere. What happens if you enable the attenuator to reduce the risk of adjacent channel bleedover? Try IFX to channels and also the whole set of different filter settings.

/Ubbe

I'll give that a try and see what happens, and post results.

 

[GROL]

N1CHU has your answer. RG8X at 850 MHz attenuates the signal 6.5db which is about 78% less signal to the scanner from the antenna, and as also mentioned, a discone is wideband, but not efficient so there is loss there as well. At an aviation frequency of 125Mhz that cable inserts a loss of 2.3db which is about 38% less signal to the scanner. So much better at 125 Mhz. The discone may also provide more signal at 125 MHz than at 850Mhz. Better cable may solve your problem, but that discone is maybe a bit of the problem as well. I wouldn.t think the scanner is being desensitized by signal overload. You probably lost too much signal in the cable. Maybe the Pro-2004 is a bit more sensitive at 800Mhz.

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[N8LHG]

You usually don't hear cellphone signals as those are some 200KHz wide in modulation. If might be DMR and NXDN systems that interfere. What happens if you enable the attenuator to reduce the risk of adjacent channel bleedover? Try IFX to channels and also the whole set of different filter settings.

/Ubbe

Ok, well I have good news. I first messed with the filter settings. Tried all settings, and found that the "wide invert" gives me 3 to 4 blocks on the signal level. "wide auto" and "wide normal" is similar but feels less. The "normal" gives me zero blocks, the off gives me 0 to 1 block. So... normal, invert, auto, and off are the lowest performers. The wide options are the highest... I should also note the signal is not static, wiggles up and down +- one block I assume due to signals bouncing around the real world environment. Anyway, thats where I am now, haven't messed with the attenuation function. Kinda' wanted to avoid using that option. What I'm ecstatic about, is it seems to have (more or less) resolved the zero signal issue.

N1CHU has your answer. RG8X at 850 MHz attenuates the signal 6.5db which is about 78% less signal to the scanner from the antenna, and as also mentioned, a discone is wideband, but not efficient so there is loss there as well. At an aviation frequency of 125Mhz that cable inserts a loss of 2.3db which is about 38% less signal to the scanner. So much better at 125 Mhz. The discone may also provide more signal at 125 MHz than at 850Mhz. Better cable may solve your problem, but that discone is maybe a bit of the problem as well. I wouldn.t think the scanner is being desensitized by signal overload. You probably lost too much signal in the cable. Maybe the Pro-2004 is a bit more sensitive at 800Mhz.

I agree it isn't perfect, but I really didn't want to invest in hardline for this receiver. I knew there was going to be loss, but was shocked the outdoor antenna appeared to perform worse than the indoor antenna.

I want to than EVERYONE that responded and helped. Its what the hobby is all about. Just proves the whole old saying "the more you learn, the less you know".

 

[cwhill]

Just to weigh in here... Not necessarily with anything helpful but more of an observation. I had a Pulse Larson Yagi hooked up outside my window (basically ground level) on my SDS200. The simulcast in my area is about 15 miles away as the crow flies. The Yagi was down low and close to the building. I've always had good signal at 5 bars, decent RSSI (below 95 typically and usually no to low errors). I also have two analog radios (BCT15X) on a separate Larson antenna cut for those frequencies. I was hoping by getting an antenna up and clear that signal and what I could hear would only get better...So I recently installed an Omni-X on the roof (above trees and buildings) about 40 feet up. I ran LMR-400 down (60 feet of cable) and into a Stridsberg multi-coupler. It now feeds the SDS200 (for Simulcast only) and two BCT15X (for analog). The Analog is definitely better and I can hear more VHF at greater distances. The Simulcast system saw no improvement and actually became worse. What is interesting is with all filters except wide invert the signal bars were solid but the audio was all over the place. Error counts were higher, RSSI was higher and as I said I had a hard time understanding the audio. When the filter is set to Wide Invert RSSI drops and floats around 100, errors are lower and the audio is a little better BUT..my signal meter bounces like crazy between 2 and 4 bars. I thought it was odd that signal was higher with no or 'less' filter yet audio was lousy. Anyway my apologies for the long rambling explanation but it really reveals just how variable area and location really matter. What I thought would have been better by moving the antenna much much higher turned out not so much for 850 simulcast. (I knew the digital would take some hit) but I didn't expect that much. The change from that quality Larson Yagi and short cable run (even at ground level against a building) to the 60' LMR400 and a wide band antenna, even at altitude hurt the quality of the digital signal. Just for the record to defend that Omni-X it really is a quality antenna and the analog stuff I'm getting is improved greatly.

 

[N8LHG]

Just for the record to defend that Omni-X it really is a quality antenna and the analog stuff I'm getting is improved greatly.

I tend to agree. The digital 800 MHz signal isn't better on the discone compared to the inside reception, however, the lower band analog is considerably better. I've always loved the discone antennas because I could always throw a vhf/uhf amateur rig on it and guarantee a low swr. It simply became a default dependable does almost everything option, and at the time seemed to make sense as a wide band scanning antenna.

 

[Ubbe]

I thought it was odd that signal was higher with no or 'less' filter yet audio was lousy.

The Signal strength detector in SDS scanners are reacting to frequencies several MHz away. If you have interference problems you should probably aim to get the worse possible signal strength I.E. less interference from another signal. And always set the attenuator to On if signal are -70dBm or stronger like -60dBm. Signals levels at -90 and -80dBm are less likely to generate problems compared to -60dBm levels.

SDS100 signal strength measurements

/Ubbe

 

[N8LHG]

The Signal strength detector in SDS scanners are reacting to frequencies several MHz away. If you have interference problems you should probably aim to get the worse possible signal strength I.E. less interference from another signal. And always set the attenuator to On if signal are -70dBm or stronger like -60dBm. Signals levels at -90 and -80dBm are less likely to generate problems compared to -60dBm levels.

SDS100 signal strength measurements

/Ubbe

After reading your test results, the implication is that just because the meter shows higher, doesn't mean the monitored frequency is stronger. I should stick with the narrow filter, and not put such trust in the meter display?

That actually makes sense if its reacting to signals MHz away that I couldn't care less about.

 

[cwhill]

The Signal strength detector in SDS scanners are reacting to frequencies several MHz away. If you have interference problems you should probably aim to get the worse possible signal strength I.E. less interference from another signal. And always set the attenuator to On if signal are -70dBm or stronger like -60dBm. Signals levels at -90 and -80dBm are less likely to generate problems compared to -60dBm levels.

SDS100 signal strength measurements

/Ubbe

Thank you for this. It’s a terrific read. Am I correct in thinking then that the D-error reading is really the most reliable tell of the signal quality of the monitored frequency and any IFX or filter settings that cause higher error numbers will result in worse audio? Its funny even asking that as it seem logical… I a got stuck thinking the lowest RSSI was best unknowing other Frequencies slightly off from the monitored one were effecting the radio like that. My silly perception I guess that air wave were more like air lanes and more organized…LOL..
Also - my apologies if this is taken as hijacking from the OP’s question but I’m hopeful it’s only enhancing the response.

 

[N8LHG]

Also - my apologies if this is taken as hijacking from the OP’s question but I’m hopeful it’s only enhancing the response.

Please, hijack away 😄 I'm learning how to operate this receiver, and can use all the help I can get! I've read the enigma filled uniden manual cover to cover now several times and I still have many questions.

One thing I have learned is, I love this scanner! I'm impressed with the radio and SW tuning abilities.

 

[JoeBearcat]

I have an interesting signal strength issue I want to run by you folks. I'm new to the sds200, fighting with the learning curve. While monitoring P25 trunk systems, I noticed the signal strength in the top right corner shows 3 to 4 bars when monitoring trunk systems as long as I am in the whip antenna that came with the rig. As soon as I connect to my rooftop discone, the bar display shows zero bars (P25 trunk). Now if I'm monitoring conventional systems, like airport AM traffic, it works fine. I've even pulled the antenna off the roof, verified proper connectivity, cable condition, etc. I've even connected my 2 meter transceiver to the discone, and verified low SWR and it seems to work fine. Connected the discone to my old radio shack pro 2004, and conventional 800 MHz stations seem to receive fine (though the 2004 has no signal strength display ability). Seems to be isolated to the sds200 itself. What would cause the trunk signal strengths on the sds200 to show zero signal bars if the antenna seems to be functioning properly? Any idea?

I've seen this many times personally. Sometimes the signal is lost in the feedline. With the back-of-set antenna there is no feedline loss. Add to that the fact that Discones have no gain, and you have another potential issue. The Discone could also be in a null. Or it might be overloading your scanner with signals from other bands or even the same band. And or all of these could be the issue. You didn't say the length of type of feedline you are using, but 800 MHz signals (many P25 systems are on 800) are subject to loss much more than VHF or UHF signals - usually on the order of 2-4 times as much.

 

[GROL]

Somewhat obscure in a later post by him, he said he was using 50 ft RG8X which attenuates signal at 850MHz by 6.5 db so the feedline is losing about 78% of the signal at the antenna. I have a similar situation. I have 75 ft of RG213 to a multiband antenna and cannot pick up any of the 700/800 Mhz sites near me. It is a homespun multiband antenna that receives 800Mhz well on a short feed line, but where it is mounted 20 ft high, my 75 ft RG-213 feedline attenuates the signal 5.4 db. I use that antenna for other than 800. For 700/800 the portable antenna that came with all my Uniden Scanners is all I need.