SDS200 SDS vs cell phone tower: Best filter to use?

Anderegg

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If you have a -50dBm signal and then enable the scanners 20dB attenuator you should receive a -70dBm signal, not a -100-110dBm. That's almost no signal at all. It's probably the multicoupler that gets overloaded so try and add an attenuator between it and the antenna. Stridsberg multicouplers doesn't take kindly to strong signals and gets easily overloaded. There are fixed 20dB attenuators to be had for $5 and adjustable ones for $20. You'll probably need to reduce the signal to a level just under where the multicoupler doesn't get into problems. It can be done by expensive filters that only take out the cell phone frequencies or you could try to attenuate all signals coming from the antenna to a level that works and probably still receive the systems you want to monitor.

As the scanner seems to partly receive and decode the control channel but then on the voice channel it looses reception, try and set IFX to the voice channels and try the filter settings again. Cellular frequencies can be either in 750, 800 or 850MHz band and the system you try to monitor are probably at 860 or 770MHz that are very close to those cellular frequencies.

The filter setting in the scanner are in the RF side of the receiver, at the first IF frequency, but could be too late in the receive chain as the receivers front end have already been hit by the full force from the whole frequency band, as also the multicoupler are exposed to.


/Ubbe
I will try to take a video of the issue, maybe I can identify particular problem freq's on the system that I can throw an IFX at. I think that if I throw a freq at IFX, the scanner will apply that IFX to any time the scanner hits that freq, even if it comes up in a trunking CC grant? I know I have IFX enabled for another systems control channel, but I can't remember how I did it. The SDS's are really really bad with selectivity...on 800 they perform similarly to old pre triple conversion scanners, where I can hear paging signals and data bursts from nearby 800 frequencies beeping and squaking over the voice traffic of analog.

Paul
 

Anderegg

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So, I will set up a video camera to recoird the screen of the SDS200, so I can review it to see the "trouble" frequencies on the system. I will create a new conventional system and enter the trouble frequencies in as conventional, then apply IFX to each of them. When you apply IFX to a conventional frequency, it WILL apply it to the same frequency should it ever pop up on a trunked system. I just tested this now, seeing IFX on my trunking control channel.

Paul
 

jonwienke

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You can add the problem frequencies to the IFX list in Sentinel, as opposed to making a conventional system and all that other hassle. Make your video to document the problem children, then enter them into the profile in Sentinel.
 

Anderegg

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I can't locate an IFX list in the menu, where do they hide it?

Paul
 

jonwienke

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In the scanner:
Hold on the problem freq and hit function-7 to toggle ifx.

Is Sentinel, there's a button you click on the miscellaneous tab in the profile editor iirc. Clicking the button opens a list of ifx frequencies, you can add and delete as needed.
 

Anderegg

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Ah, no wonder I couldn't find it in the menu. You can't add takgroups to an IFX list :p


Paul
 

kruser

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Ah, no wonder I couldn't find it in the menu. You can't add takgroups to an IFX list :p


Paul
Nope, because talkgroups can show up on any frequency on a given site or system. So it's done at the frequency level which is where the problem needs to be corrected if it can.
 

Ubbe

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Are you sure about that? If they were physical filters, they would likely have been available when the SDS100 was first introduced, as opposed to a feature added months later via firmware upgrade.
The filter is a SAW physical one but Uniden added functions in the software to alter the 1:st IF frequency in the exact same way you have IF shift in shortwave receivers and some Icom scanners/communication receivers. You can google that to get more detailed info as some people in the forum doesn't like detailed info being posted.

The different filter settings are a shift in IF frequency to the lower edge of the SAW filters bandpass frequency, Normal, and to the middle between center and outher edge, Wide. Then Invert switches if from lower to upper edge of the SAW filter to make it low pass. Off is in the center of the filters 10Mhz bandwidth.

/Ubbe
 

Anderegg

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So...to block 870MHz cell phones from my 850MHz system, I would want to bandpass low, so NORMAL should technically be suited for that.

Here is a video showing the issue, I clipped out the working freq's and just left the bad apples...it's the same set of freq's that keep doing the blinking dance and never receive.


So far I have identified 4 offending freq's, but that was only from a few minutes of monitoring. identified 4 happy freq's as well. :)

Paul
 

jonwienke

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You're still going to get better results if you put a filter on the antenna coax that blocks the offending signal before it enters the multicoupler.
 
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RRR

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Do you have one you could suggest, Jon? One that doesn't hurt 700 ~ public safety freqs?
 

jonwienke

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Not without knowing the frequency range(s) used by the cell transmitters on the tower. I' assuming 700MHz cell frequencies, but it would be best to verify with a SDR dongle.

If the scanners only need to monitor one system, a bandpass filter passing only the freq range used by the system would be an alternative.
 

Ubbe

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So...to block 870MHz cell phones from my 850MHz system, I would want to bandpass low, so NORMAL should technically be suited for that.
Normal are high pass, it passes frequencies higher than the one you recieve. Invert is low pass, it pass frequencies lower than the one you are monitoring and are the one to use. At least that are what Upman says but it could be bugs in the function and it also changes if the monitored frequency are below the IF frequency, which is either 265MHz or 380Mhz depending of the IFX setting and the frequency band used, that the scanners software must take into consideration.

Before buying expensive filters, try a stub filter just to see if it helps. Try that before and after the muclticoupler. You'll need a T connector and a short lenght of coax that needs to be cut to the frequency it should attenuate. I use RG 6 as it is cheap and good quality that gives a more narrow filter effect compared to RG58. Measured from the end to the exact point in the T connector where the coaxes meet, should be 69mm for the middle of the cellband, 880Mhz. It isn't sharp enough as a proper filter but should indicate where the problem are and if a notch or low pass filter will help.

The video seems to show that the scanner receives 852Mhz frequencies just fine with 0-20 in D-error but the voice channels at 854 and 855Mhz doesn't receive at all even when the signal strengt is full with a -50dBm level.

/Ubbe
 

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RRR

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So high pass, or low pass (upper or lower) but no filter to zero in on the one we need?
 

Anderegg

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With built in filter set to off or normal, the scanner will not even lock the control channel...invert seems to work OK, wide normal and wide invert also seem to "work" ok. Auto doesn't work either. IFX set to the naughty freq's, seemed to help them maybe receive 50% more voice, but still troublesome.

Paul
 

jonwienke

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The best solution is going to probably be a filter upstream of the multicoupler blocking the cell signals. That's a worst-case RFI scenario you're dealing with. Adjacent band, with RX antenna co-located on TX tower.
 

Anderegg

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Trying to use all the "available" options prior to reverting to more drastic measures. It took me 4+ years to get them to replace the daisy-chain of a single antenna feed with 20 BNC T splitters...was getting -120dBm instead of -50dBm with that rats nest! We might try to re-aim the yagi, but even pointed away from the cell antennas, there is just sooooo much radiation sooooo close.

Paul
 
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