SDS100/SDS200: I'm trying to figure out if I should get a preamp

[funsutton]

Here's my situation. I live in the very low point of my neighborhood and thus I have reception issues unless I get some altitude. To put this in perspective, I have a 2m/440 10ft Comet up on a 40ft mast and you can barely see the 50ft tip of the antenna from the main road.

For my SDS100 I am using a Larsen NMO150-450-800 with a ground plane kit. It's at the top of a 32 ft mast and connected to 75ft of LMR400.

I have reception issues at times with this setup. Sometimes I pick up the 800mhz NC Viper tower I listen to with great ease. Other times I'm watching the signal fall to one bar. I listen to other Viper towers as well and they also go up and down, depending on conditions.

I'm simply wondering if I would benefit from a good preamp. I feel I must be losing some of the signal in the length of cable when the signal gets weaker, and that perhaps a good low noise preamp would benefit me. I have a preamp on my tv antenna (Antennas Direct ClearStream Juice) and it works great. So I am familiar with preamps.

I've also been told that a preamp could damage my scanner? Could the receiver get so much signal that it would actually damage it?

Your thoughts on this would be greatly appreciated.

- Brian

 

[Ubbe]

A low noise amp will help. A cost effective one that use PGA-103+ are a good choise with a 1dB noise figure. There are circuit boards types for $25 and ones in a metal enclosure for $100, both requiring a weather protecting box. They'll need a $10 bias-T to power it thru the coax and a 12V power source.

75ft of LMR400 will attenuate less than 3dB at 800Mhz but the bandpass filter in the scanner will also attenuate at least 3dB until the internal amplification in the scanner can boost the signal. So a minimum of 6dB increase of reception can be had. What's important are to use a variable attenuator between scanner and coax to reduce the signal to a level where the scanner can handle it. Uniden has set the scanners internal amplification as high as it could, as the SDR receiver in it are not sensitive enough for scanner use, and adding gain to that could make it work worse. If you have strong cellular transmitters then that can be a problem as SDS scanners are sensitive to interference.

SDS100/SDS200: - Global Auto Filter Vs. Global Attenuation

I was reading through some previous posts and I am unclear on the difference between Global Auto Filter and Global Attenuation. What I have been doing recently is scanning groups of frequencies in sequential order to determine what DMR Color Codes are being used. On a few UHF frequencies in...

forums.radioreference.com

SDS100.mp4

drive.google.com

I assume you have tried all settings of the filter and also tried IFX to the frequencies to see what setting that gives the best reception.

When the signal fluctuate like that it's usually that you do not have a clear line of site to the transmitter antenna and it gets more dependent of weather and other environmental conditions. I have the same situation as one day an analog signal barely opens squelch but the next day it's a clean reception without any background noise. It's also very dependent of season, how much leafs there on the trees and if it's freezing temperatures.

Scanners doesn't get damaged by increased pre-amplifier signals but the actual pre-amplifier can easily fail. Prepare yourself and the installation to let you climb up to the amplifier, or other means to reach it, and take it down and solder in a new $5 PGA-103+ amp circuit in it. I have four pre-amps and in one of them I had to replace its PGA103 twice over a 3 year period. It's easy to solder a new component in as long as you have the amplifier on your table.

/Ubbe

 

[buddrousa]

Before you spend money are there any Radio Towers near you that may be causing your scanner to suffer from desense (VERY STRONG AJACENT SIGNALS)?

 

[donc13]

I would suggest you try a Yagi first. The Larse.. n is an excellent antenna but it does need a good horizontal ground plane. It was mainly designed to be mounted on a vehicle via a well grounded NMO mount. We're it me, I would try to couple the Yagi and the Larsen.

 

[N9JIG]

Pre-amps are a multi-edged sword. They often make reception worse. While the indicated signal strength might be higher the signal quality itself might be worse. If you want to try one be sure you can return it if it doesn't help. Remember that a pre-amp also amplifies the noise floor and signals that are not of interest.

You might try using the opposite approach: use an attenuator. If you are suffering from signal overload that might actually work better.

Another option is a preselector or band pass filter. Since you have a specific target in mind you can filter out the fluff and force the radio into ignoring the bands other than the one you want.

 

[nessnet]

I would suggest you try a Yagi first. The Larse.. n is an excellent antenna but it does need a good horizontal ground plane. It was mainly designed to be mounted on a vehicle via a well grounded NMO mount. We're it me, I would try to couple the Yagi and the Larsen.

Yagi was exactly what I first thought also.
Gives you some gain without upping the noise floor.

 

[funsutton]

I assume you have tried all settings of the filter and also tried IFX to the frequencies to see what setting that gives the best reception.

I've played with it some but generally I just turn the filters off because it doesn't seem to make much difference. And I don't understand IFX to know what I am doing.

A low noise amp will help. A cost effective one that use PGA-103+ are a good choise with a 1dB noise figure. There are circuit boards types for $25 and ones in a metal enclosure for $100, both requiring a weather protecting box. They'll need a $10 bias-T to power it thru the coax and a 12V power source.

I'll look into this but it sound more complicated than what I was expecting. I was hoping for something akin to the ClearStream Juice I mentioned, just for 800mhz.

When the signal fluctuate like that it's usually that you do not have a clear line of site to the transmitter antenna and it gets more dependent of weather and other environmental conditions. I have the same situation as one day an analog signal barely opens squelch but the next day it's a clean reception without any background noise. It's also very dependent of season, how much leafs there on the trees and if it's freezing temperatures.

I am very susceptible to conditions, unfortunately. And all of the towers I listen to in my city are in the direction of the worst areas of the sudden elevations around my home. Height is might, as they say, and what I really need is a tower. But that's not going to happen any time soon.

Before you spend money are there any Radio Towers near you that may be causing your scanner to suffer from desense (VERY STRONG AJACENT SIGNALS)?

I don't think I suffer from any strong interference like that. But I do have a power line tower that looks like it's used for cell stuff about a 900 ft from my house.

I would suggest you try a Yagi first. The Larse.. n is an excellent antenna but it does need a good horizontal ground plane. It was mainly designed to be mounted on a vehicle via a well grounded NMO mount. We're it me, I would try to couple the Yagi and the Larsen.

A yagi could be interesting. I'd have to turn it a fair amount given where the towers are that I'm listening to.

You might try using the opposite approach: use an attenuator. If you are suffering from signal overload that might actually work better.

I wish I was suffering from signal overload. That would be an easier problem to solve I think. But my problem is on the other end of the reception spectrum.

 

[buddrousa]

Bingo you have interference

 

[nessnet]

"... tower that looks like it's used for cell stuff about a 900 ft from my house.

buddrousa beat me to it.
VERY likely the source of RFI.

Search here for how to properly experiment with filters.

 

[Ubbe]

I've played with it some but generally I just turn the filters off because it doesn't seem to make much difference. And I don't understand IFX to know what I am doing.

IFX are just as with the filter settings, you try it and see if it helps. The scanner uses two different SAW filters, one at about 250MHz and the other at about 350MHz and are automatically selected depending of the frequency used but you can force them to switch by the IFX function and it will change how the reception works. You have to stop scan, push hold on a conventional frequency and do Function+7. For trunked frequencies you have to enter the frequency by Channel+frequency+Channel and then do Function+7 and then continue scan. Do that to each frequency, control channel and all voice channels, to see if it improves reception. For digital systems you configure a display field to show Digital Error Count and you aim to get the lowest error.

I'll look into this but it sound more complicated than what I was expecting. I was hoping for something akin to the ClearStream Juice I mentioned, just for 800mhz.

It's just like your clearstrem, that also use a bias-T and a weather proof metal box with the amplifier at the antenna. The clearstream isn't bad with a 1,8dB noise and the PGA103+ has 0,6dB which will add less noise and probably can handle bigger signals without degrading. These amplifiers often comes with SMA connectors but you can buy jumper cables on Ebay with SMA at one end and the correct connector for the antenna at the other, and the same for the feedline.

This $25 amplifier can be powered from coax with 12V using the same bias-T that you have to your clearstream. Put that in a weather proof box. You can test drive it when connected at the scanner and have your bias-T between scanner and amplifier. But you will also need a variable attenuator to be able to reduce the signal by some 10-15dB before entering the scanner. It will be the same if you use a clearstream amplifier that has 19dB gain that you will need to attenuate at least 10dB or you will get worse reception. Practically all amplifiers will improve scanner reception when the correct attenuation are dialed in, even the $5 CATV ones I've tested and then I only have the amplifier in my room and not installed at the antenna, which will give a better result.

A yagi could be interesting. I'd have to turn it a fair amount given where the towers are that I'm listening to.

If the towers are in different directions then a yagi needs to have a rotor to turn it. If they are in within a 45 degree angle then it probably will work. The higher the gain in the antenna the narrower the beam will be. You will loose most of the reception at other frequency bands if the yagi are a single band 800MHz model. You could try a multiband TV antenna if installed vertically instead of the normal horizontal position. See to that you get an antenna that covers high channel numbers that include the frequencies for you scanned systems, as performance usually drops quickly in TV antennas when you go above their designed frequency range. If you don't have a lot of interferences then you can buy another set of that clearstream amplifier and use, as long as you also make some kind of adjustable attenuation to reduce the signal between 10-15dB.

/Ubbe

 

[dave3825]

This $25 amplifier can be powered from coax with 12V using the same bias-T that you have to your clearstream. Put that in a weather proof box.

Is that a passthru? Meaning it will pass signal without the bias t enabled?

 

[prcguy]

Is that a passthru? Meaning it will pass signal without the bias t enabled?

No power, no signal.

 

[Ubbe]

The amplifier use something like 100mA and a reed relay use an additional 20mA at 12V that you can solder in the amp to switch the RF input to instead go to the RF output when there's no power. Or simple bridge RF input to RF output for an easier modification. Then you will still have some signal if the amplifier fails. Check that the bias-T used can handle 200mA or more. They are usually designed for at least 500mA so shouldn't be a problem.

I look at reed relays and you want a 1C type that will have a switching function and a coil voltage of 12VDC or whatever your bias-T power supply outputs. They are usually $5 and there's also $20 types that only needs 10mA but in this case it doesn't matter if the relay even uses 100mA.

In the amplifier there will be a coil connected to the coax that the DC voltage goes thru but stops RF signals and you solder the relay coil to the that coils side that goes to the amp circuits voltage regulator to more isolate the relay coil from RF. If the relay have a protection diod, some reed relay do, then it it's important that the polarity to the relay coil are correctly connected to power and ground. The relay contacts to use are the NC ones, Normally Connected when there's no power.

/Ubbe