Narrow Band Low Noise SDR-RTL Amplifier

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MillbankPWS
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Joined: Mon Oct 10, 2016 9:20 am

Narrow Band Low Noise SDR-RTL Amplifier

Post by MillbankPWS » Mon Oct 10, 2016 9:57 am

Hi, I have recently set up a weather satellite receiving station in the north of Scotland using the R820T2 dongle and a home made QFH antenna. I am using SDRSharp and WXtoimg software. Unfortunately my location is 8 meters above sea level and is surrounded buy hills. I have set the record threshold at 20deg above the horizon, but the picture quality is only good when the satellites (NOAA15, 18 and 19) are directly overhead. The QFH antenna is 7 meters above the ground on a mast and the cable length between the antenna and the R820T2 is 50 metres. SDRSharp is set to Narrow Band.

I suspect I am loosing signal because of the cable length. Can anyone recommend a ‘Narrow Band Low Noise SDR-RTL Amplifier ‘that can be used to boost the signal, please? I understand that amplifiers work best at the antenna end, but as this is 50 meters from the R820T2, there is no power source at the antenna. The satellite frequencies are; 137.6200MHz, 137.9125MHz & 137.1000MHz

Many thanks.

Gary

joe36
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Joined: Wed Jul 20, 2016 11:09 pm
Location: Cass co. in. 46994

Re: Narrow Band Low Noise SDR-RTL Amplifier

Post by joe36 » Mon Oct 10, 2016 6:08 pm

I am going to do the best I can with this one as it is not as strait forward as it might seem.
Yes you are losing considerable signal with a 50 meter run of coax. There are several questions that must be answered. No. 1 is. What type of coax are you using? It most likely can be improved but this possesses more questions. What is your budget? Both in cost and physical limitations that work for you, size flexibility and so on.
My go to coax is RG6 quad shield or if I did not have quad shield. RG6 standard is very close and will work almost as well. This is one of the cheapest and easiest coax types to find.
Next is the amplifier its self. Most amplifiers for this type application are wider banded than you will want and can often cause more problems than they solve. Even with a general purpose built in band pass filter.
For this reason I would suggest building or buying a filter for the specific band you want to receive. In applications were there are very strong nearby signals. It is usually not sometimes needed but sometimes beneficial to put band pass filters before and after the amplifier.
Next is to get the filters and amplifier as close to the antenna as possible. If it is on the antenna end of the coax the signal to noise ratio will be greatly improved.
Although you do not have power to the antenna. You can put DC on the coax either through a bias T arrangement or by simply putting the DC on the coax and isolating it from the desired signal with a bypass capacitor. Google is your friend but I as others will be more than willing to help with the details if need be.
Joe KA9UCN

rtlsdrblog
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Re: Narrow Band Low Noise SDR-RTL Amplifier

Post by rtlsdrblog » Tue Oct 11, 2016 12:21 am

Yep what Joe said is right. Even with high quality RG6 at 50m of length you're loosing about 5.4 dB's. If you're using lower quality cable it could be much higher.

Have a look at the uputronics store https://store.uputronics.com/index.php? ... ry&path=59. They sell some 137 MHz LNA + filter units. You will probably need to power it via bias t from the dongle.


MillbankPWS
Posts: 7
Joined: Mon Oct 10, 2016 9:20 am

Re: Narrow Band Low Noise SDR-RTL Amplifier

Post by MillbankPWS » Fri Oct 14, 2016 11:58 am

Many thanks for this information, very helpful. I have purchased RG6 cable, and I will try moving the antenna closer to the R820T2. I can reduce to about 25 metres although the antenna will be about two metres lower and closer to buildings and trees. I have now received the 137.5MHz NOAA/Filtered Preamp from Uputronics. I must add that the service and help provided by Anthony at Uputronics was second to none. The unit arrived within 24 hours.

https://store.uputronics.com/index.php? ... duct_id=94

I will try re-wiring and see if any improvement. I will also try at first using the preamp at the dongle end and will then order the BAIS_TEE ZFBT-6G+ by Mini-Circuts.

Many thanks again.

Gary

rtlsdrblog
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Re: Narrow Band Low Noise SDR-RTL Amplifier

Post by rtlsdrblog » Sat Oct 15, 2016 4:08 am

MillbankPWS wrote:Many thanks for this information, very helpful. I have purchased RG6 cable, and I will try moving the antenna closer to the R820T2. I can reduce to about 25 metres although the antenna will be about two metres lower and closer to buildings and trees. I have now received the 137.5MHz NOAA/Filtered Preamp from Uputronics. I must add that the service and help provided by Anthony at Uputronics was second to none. The unit arrived within 24 hours.

https://store.uputronics.com/index.php? ... duct_id=94

I will try re-wiring and see if any improvement. I will also try at first using the preamp at the dongle end and will then order the BAIS_TEE ZFBT-6G+ by Mini-Circuts.

Many thanks again.

Gary
Yep have heard great things about their service and products. Just remember that the LNA needs to go close to the antenna, not close to the RTL-SDR. Only that will help overcome the coax losses.

MillbankPWS
Posts: 7
Joined: Mon Oct 10, 2016 9:20 am

Re: Narrow Band Low Noise SDR-RTL Amplifier

Post by MillbankPWS » Mon Oct 17, 2016 6:15 pm

Thank you for the tip, but I was reading that as the QFH antenna is a continuous loop if a Bias-Tee Injector is added the DC will be shorted by the antenna.

Can anyone confirm if a Bais-Tee injector can be added to RG6 that connects to a QFH Antenna, or does the DC just power the Filtered Preamp with no through current to the antenna?

Many thanks. Gary

rtlsdrblog
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Re: Narrow Band Low Noise SDR-RTL Amplifier

Post by rtlsdrblog » Tue Oct 18, 2016 5:05 am

MillbankPWS wrote:Thank you for the tip, but I was reading that as the QFH antenna is a continuous loop if a Bias-Tee Injector is added the DC will be shorted by the antenna.

Can anyone confirm if a Bais-Tee injector can be added to RG6 that connects to a QFH Antenna, or does the DC just power the Filtered Preamp with no through current to the antenna?

Many thanks. Gary
If you place an LNA in between the dongle and antenna, then there will be no shorting as the LNA will not pass current through.

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