SDS100/SDS200: SDS200 - Overload?

[VA3ADP]

I recently noticed some odd activity with my SDS200, I have my scanner hooked up to an external discone antenna with a preamp. On my Local p25 system I am receiving no signal strength but an RSSI of -119dBM on the global filter. When I swich to the "wide invert" filter everything appears to work fine. Not sure if there is overload in the front. I tried using the standard telescopic antenna and the system appears to be working fine. This model is less than a year old and i had something similar occur with my SDS100. I am not entiruly sure as to what I should do. Thank you

 

[buddrousa]

The Preamp with a strong signal will overload the front end and could cause you strong signal to become degraded or even distorted.
Preamps only work if you are in the middle of nowhere with no transmitters around for several miles. Turn the gain down will help with signals.

 

[dave3825]

I tried using the standard telescopic antenna and the system appears to be working fine.

If it works fine with the stock antenna, why are you even using a preamp with an external antenna? Where is the discone mounted? Outside or in an attic?

 

[G7RUX]

I recently noticed some odd activity with my SDS200, I have my scanner hooked up to an external discone antenna with a preamp. On my Local p25 system I am receiving no signal strength but an RSSI of -119dBM on the global filter. When I swich to the "wide invert" filter everything appears to work fine. Not sure if there is overload in the front. I tried using the standard telescopic antenna and the system appears to be working fine. This model is less than a year old and i had something similar occur with my SDS100. I am not entiruly sure as to what I should do. Thank you

A wide band preamplifier with significant gain on an external antenna with a wide operating frequency range is very, very likely to introduce issues. A preamp should really only introduce sufficient gain to overcome the cable and distribution losses and even then can cause problems with strong out-of-band signals on its input.

For example, listening to VHF airband with this system would likely seem much more noisy than it should as the preamp will be receiving and unnecessarily amplifying already strong FM broadcast signals.

Personally, I would suggest taking the preamplifier out of circuit for a while to see how that goes but also to consider using some suitable band pass and/or bandstop/notch filters to deal with anything problematic.

 

[G7RUX]

That said, a preamplifier with significant gain *could* be useful if there is a bandpass filter before it to confine the system gain to the band of interest; for VHF airband then pop an airband pass filter in front of the preamp and you'll get generally better behaviour from it than putting the filter after the amplifier. I have put together just such a system for someone which had very high-quality (and expensive) bandpass filters both before and after the preamplifier before a fairly long run of feeder to the monitoring location, combined with a dedicated VHF airband antenna and the system performance was, and still is, extremely good.

 

[a727469]

I recently noticed some odd activity with my SDS200, I have my scanner hooked up to an external discone antenna with a preamp. On my Local p25 system I am receiving no signal strength but an RSSI of -119dBM on the global filter. When I swich to the "wide invert" filter everything appears to work fine. Not sure if there is overload in the front. I tried using the standard telescopic antenna and the system appears to be working fine. This model is less than a year old and i had something similar occur with my SDS100. I am not entiruly sure as to what I should do. Thank you

All the responses concerning preamp usage are 100% right. I have experienced similar issues in the past. However, theOP’s questions raises others….since he appears to have other receivers like the 996,536,436 etc, does this happen with them? The rssi of -119 would reflect very weak signal but usually an overloading preamp would show -70 or more. The switch to wide invert might be moving the signal just far enough from local interference to clear up. Personally I would just stay on wide invert if it works and if the preamp helps with other signals

 

[VA3ADP]

All the responses concerning preamp usage are 100% right. I have experienced similar issues in the past. However, theOP’s questions raises others….since he appears to have other receivers like the 996,536,436 etc, does this happen with them? The rssi of -119 would reflect very weak signal but usually an overloading preamp would show -70 or more. The switch to wide invert might be moving the signal just far enough from local interference to clear up. Personally I would just stay on wide invert if it works and if the preamp helps with other signals

I've been running my preamp for about 6 months now and it's great pulling in all my local systems/frequencies. My 536 seems to be working just fine and shows full signal strength. I'm guessing with the SDS it could be a sensitivity issue?

 

[tvengr]

What signal strength are you receiving using the stock antenna on the scanner. You probably have a strong transmitter or transmitters nearby which is overloading and desensitizing the receiver. The wide invert filter is probably decreasing the level of the overloading signal so that you are able to receive. I have had very good results with the wide invert filter on weak signals. It shouldn't reduce the level that much. Preamps also can have intermod problems with high powered transmitters and create other frequencies as a result. Your scanners probably all use different IF frequencies. An interfering signal created by intermod may affect the SDS200 and not the other scanners. I bet that if you turn off the preamp, the signal on the SDS200 will come up.

 

[Ubbe]

I am receiving no signal strength but an RSSI of -119dBM on the global filter. When I swich to the "wide invert" filter everything appears to work fine.

The squelch have to open to register and display a signal strength. It appears to open, as you see -119, but the noise squelch that should unmute doesn't agree as it probably see a too distorted audio signal. A signal strength of -119dBm are pretty much at the level where a squelch open and close when its set to 2.

Program your local sites control channel to a memory and set it to analog only and set to Wide Invert filter and listen to the audio. You can create a new department in the Quick Save Favorite system and call that Test to be used for evaluating signals and filters.

You probably will hear the control data nice and clear. Then switch that department to use Global, or Normal, filter and compare with the squelch set to 0. If you get audio it probably sounds distorted that will then not open the squelch when set to its normal 2 setting. Set the audio mode to Digital or All and set a field on the display to show Digital Error and try different filter settings as well as IFX to find a setting that gives the least amount of errors.

If you get garbled audio or none at all when you monitor calls, then you have to enter the frequencies of the sites voice channels in that test department and try and find the least amount of digital errors for those channels. IFX can be set individually to each frequency but filters are common for the whole site, so you might need to compromise to get all voice channels to work properly as well as the control channel. Then make that filter setting to the actual site for the system you have programmed. Remember to set both IFX and filter in Sentinel so it will be kept the next time that you'll program your scanner.

Amplifiers usually have a gain of 20dB or more and are too much for most scanners and needs to be attenuated before it enters a scanner. Maybe +6db are what a scanner can handle. SDS scanners already have a preamplifier in them so they are more suspect to overload than other scanners. If you now use no additional attenuation, like a splitter or a fixed 10dB attenuator, then try and use the scanners own attenuator setting, at least for that local system that probably doesn't need any stronger signal than what you already have without a preamplifier.

/Ubbe

 

[VA3ADP]

The squelch have to open to register and display a signal strength. It appears to open, as you see -119, but the noise squelch that should unmute doesn't agree as it probably see a too distorted audio signal. A signal strength of -119dBm are pretty much at the level where a squelch open and close when its set to 2.

Program your local sites control channel to a memory and set it to analog only and set to Wide Invert filter and listen to the audio. You can create a new department in the Quick Save Favorite system and call that Test to be used for evaluating signals and filters.

You probably will hear the control data nice and clear. Then switch that department to use Global, or Normal, filter and compare with the squelch set to 0. If you get audio it probably sounds distorted that will then not open the squelch when set to its normal 2 setting. Set the audio mode to Digital or All and set a field on the display to show Digital Error and try different filter settings as well as IFX to find a setting that gives the least amount of errors.

If you get garbled audio or none at all when you monitor calls, then you have to enter the frequencies of the sites voice channels in that test department and try and find the least amount of digital errors for those channels. IFX can be set individually to each frequency but filters are common for the whole site, so you might need to compromise to get all voice channels to work properly as well as the control channel. Then make that filter setting to the actual site for the system you have programmed. Remember to set both IFX and filter in Sentinel so it will be kept the next time that you'll program your scanner.

Amplifiers usually have a gain of 20dB or more and are too much for most scanners and needs to be attenuated before it enters a scanner. Maybe +6db are what a scanner can handle. SDS scanners already have a preamplifier in them so they are more suspect to overload than other scanners. If you now use no additional attenuation, like a splitter or a fixed 10dB attenuator, then try and use the scanners own attenuator setting, at least for that local system that probably doesn't need any stronger signal than what you already have without a preamplifier.

/Ubbe

thank you Ubbe, I do have a spliter installed to run the Preamp to all my scanners. However, I will try a bit more ATT and see what happens

 

[wtp]

far away signals on the preamp and one radio.
local stuff and no preamp on the other.

 

[Ubbe]

thank you Ubbe, I do have a spliter installed to run the Preamp to all my scanners. However, I will try a bit more ATT and see what happens

It might not help as SDS scanners starts to have issues with other signals in the spectrum at -80dBm or stronger that can step right over your monitored frequencies. Filter settings and IFX will move the interference in frequency to a place where it hopefully doesn't matter to your monitored frequencies.

/Ubbe

 

[VA3ADP]

Well the Attenuarator seemed to clear things up

 

[pb_lonny]

Well the Attenuarator seemed to clear things up

That is great, at times it can make a huge difference.

 

[dmfalk]

Using a preamp and attenuator settings on an SDS100...... Can i just make a no-comment here? 😶 I mean, just use a good antenna.... Not like you're gonna receive signals from the moon or anything.......

But then, stranger things have happened...... 🫥