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Author Topic: Do I Need A Filter?  (Read 613 times)

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abcd567

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Do I Need A Filter?
« on: October 29, 2017, 12:09:41 am »
(1) INTRODUCTION
In addition to receiving ADS-B signals, the ADS-B antenna also receives Cell/Mobile/Pager signals in the range 800 MHz to 1200 MHz. The Cell/Mobile signals are generally much stronger than the ADS-B signal. When these strong signals reach the receiver, they overload its front end tuner. This overload severely affects performance and reduces maximum range and number of planes. Stronger the Cell/Mobile signal, lower is the max range & number of planes.

To overcome this situation, a hardware called “Filter” is used. As its name shows, it filters out unwanted signals and passes a narrow band of frequency around 1090 MHz (say 1075 MHz to 1150 MHz), blocking most Cell/Mobile/Pager signals from reaching the receiver.

In order to determine if a filter is needed or not, it is necessary to scan frequencies in the range 800 MHz to 1200 MHz. If the scan shows very few and weak interfering signals, adding a filter will not give any appreciable improvement. On the other hand if the scan shows a lot of strong interfering frequencies, adding a filter will give substantial improvement in ADS-B 1090 MHz reception.

(2) How To Find Out RF Signals Affecting ADS-B Reception in Your Area
Software developers have developed softwares which use a very low cost ($10) hardware (DVB-T) to perform this scan. There are following 3 ways it can be done. Please click the options below to see the details.

Please note that you have to connect an antenna to DVB-T. It can be either the whip antenna supplied with the DVB-T, or preferably your regular ADS-B antenna to get more realistic results. Since DVB-T's antenna socket requires a MCX-male fitted to antenna cable, you will need a pigtail with MCX-male connector at one end, and at other end a connector which matches the connector on your antenna cable.


OPTION-1:
DVB-T is plugged into Raspberry Pi (or is plugged into a Desktop/Laptop running Linux Debian/Ubuntu)

OPTION-2:
DVB-T is plugged into a Windows Computer - Using GUI rtlpan,exe.

OPTION-3:
DVB-T is plugged into a Windows Computer - Using Command Line.
« Last Edit: December 01, 2017, 06:00:59 pm by abcd567 »

Runway 31

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Re: Find Out Existing RF Signals (Cell/Mobile/Pager etc) In Your Area
« Reply #1 on: October 29, 2017, 08:51:54 am »
Very interesting

abcd567

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Re: Do I Need A Filter?
« Reply #2 on: November 06, 2017, 10:36:35 pm »
OPTION-1:
DVB-T plugged into Raspberry Pi.

This method uses software tool rtl_power which is part of software package rtl-sdr.

Credits:
Kyle Keen (keenerd): For rtl_power and heatmap.py
Oliver Jowett (obj): For guide how to use rtl_power and heatmap.py

STEP-1 of 6:
Install required packages (rtl-sdr and python-imaging), and get the python script (heatmap.py) to create image scan.png from scanned data stored in file scan.csv
Code: [Select]
sudo apt-get install rtl-sdr
sudo apt-get install python-imaging
sudo wget https://raw.githubusercontent.com/keenerd/rtl-sdr-misc/master/heatmap/heatmap.py
sudo chmod +x heatmap.py


STEP-2 of 6:
Free the DVB-T Dongle which is in use by a decoder like dump1090, or dump1090-mutability, or dump1090-fa.
Code: [Select]
sudo systemctl stop dump1090
sudo systemctl stop dump1090-mutability
sudo systemctl stop dump1090-fa
sudo systemctl stop rbfeeder

STEP-3 of 6:
Once DVB-T Dongle has been made free, run following test
Code: [Select]
sudo rtl_power -f 800M:1200M:100k -i 30 -c 50% -e 30m -g 30 -F 9 >scan.csv


The above command will generate following output.

Code: [Select]
Number of frequency hops: 286
Dongle bandwidth: 2797202Hz
Downsampling by: 1x
Cropping by: 50.00%
Total FFT bins: 9152
Logged FFT bins: 4576
FFT bin size: 87412.56Hz
Buffer size: 16384 bytes (2.93ms)
Reporting every 30 seconds
Found 1 device(s):
  0:  Realtek, RTL2832U, SN: 00001003

Using device 0: Generic RTL2832U
Detached kernel driver
Found Rafael Micro R820T tuner
Tuner gain set to 29.70 dB.
Exact sample rate is: 2797202.148434 Hz
[R82XX] PLL not locked!


Wait for 30 minutes for scan to finish.
When scan is finished, it will say "Exiting.... Canceled by user".
Scan will create a file "scan.csv" in current folder and save scan data in it.

This will produce a very wide image (4000 pixels) that shows spectrum power from 800MHz - 1.2GHz.
If you want a narrower image, increase “100k” above to something larger, it controls the bandwidth that corresponds to one pixel on the x axis.
-i controls the integration interval (time for one pixel on the y axis)
-e is the total runtime, longer gives you a taller image.


STEP-4 of 6:
Don't forget to Reboot Pi so that system is restored to normal, and data feeding is restored.
Code: [Select]
sudo reboot


STEP-5 of 6:
Create an image "scan.png" in current folder from data stored in file "scan.csv"
Code: [Select]
./heatmap.py scan.csv scan.png

This will produce a very wide image (4000 pixels!) that shows spectrum power from 800MHz - 1.2GHz.


STEP-6 of 6:
Copy the image "scan.png" from Pi to your Win/Mac Computer.  To copy a file from Pi, you will need to install on your Win/Mac computer an SCP software. Some popular free of cost software are FileZilla (win and mac) or WinSCP (win) or CyberDuck (win and mac).


Example of Scan of Existing RF Signals by rtl-power

The original image is very wide (4000 pixel), but displayed in this forum smaller (1280 pixel). As a result details are not clear.




Three 500 pixel wide cut-outs from above 4000 pixel wide image. These cutouts show details better.





« Last Edit: November 29, 2017, 12:19:43 am by abcd567 »

abcd567

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Re: Do I Need A Filter?
« Reply #3 on: November 08, 2017, 02:05:48 am »
OPTION-2:
DVB-T plugged into Windows Computer - Using GUI rtlpan.exe

Important: If you want to plug in your DVB-T Dongle into Windows Computer, you should install its driver Zadig.

STEP 1 of 7:
Create a new folder of a name of your choice and at a location of your choice. For this guide, I will use new folder name "RF Scan", location inside "Download" Folder.

STEP 2 of 7:
Download RelWithDebInfo.zip from here:
Download Link: http://osmocom.org/attachments/download/2242/RelWithDebInfo.zip

Web Page: https://osmocom.org/projects/sdr/wiki/rtl-sdr
(scroll down to bottom of page to find download link)


STEP 3 of 7:
Un-zip RelWithDebInfo.zip. It will create a folder RelWithDebInfo and inside this folder another folder rtl-sdr-release which contains several files and 2 folders named x32 and x64 . Copy ALL files of folder x32 into newly created folder RF Scan.

STEP 4 of 7:
Download rtlplan.exe from the site given below, and save it inside the folder RF Scan
https://sourceforge.net/projects/guiforrtlpower/





STEP 5 of 7:
Double-click rtlpan.exe  to start the software.

STEP 6 of 7:
In the rtlpan window, make settings as follows (see screenshot below)
start freq: 800Mhz (800000000)
end freq: 1200Mhz (1200000000)
step: 100kHz
Gain: 49.6



STEP 7 of 7:
Press "START" Button. The scan will start and the scan image will start building gradually. Wait for about 15 to 30 minutes for scan image to build to sufficient height. Click "STOP" button to stop the scan. In addition to scan image, a file scan.csv will be generated inside folder "RF Scan"




« Last Edit: November 29, 2017, 12:18:52 am by abcd567 »

abcd567

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Re: Do I Need A Filter?
« Reply #4 on: November 12, 2017, 04:06:58 am »
OPTION-3:
DVB-T plugged into Windows Computer - Using Command Line


Important: If you want to plug in your DVB-T Dongle into Windows Computer, you should install its driver  Zadig from  http://zadig.akeo.ie/


STEP 1 of 7:
Download Python Installation File python-2.7.14.msi  from the site given below, and install it on your computer
https://www.python.org/downloads/

STEP 2 of 7:
Download Python Imaging Library Installation File Pillow-4.3.0.win32-py2.7.exe from the site given below, and install it on your computer
https://pypi.python.org/pypi/Pillow/4.3.0

IMPORTANT NOTE: Pillow and PIL are not compatible. If you have already installed PIL, first uninstall it, and then install Pillow




STEP 3 of 7:
Create a new folder of a name of your choice and at a location of your choice. For this guide, I will use new folder name "RF Scan", location inside "Download" Folder.

STEP 4 of 7:
Download RelWithDebInfo.zip from here:
Download Link: http://osmocom.org/attachments/download/2242/RelWithDebInfo.zip

Web Page: https://osmocom.org/projects/sdr/wiki/rtl-sdr
(scroll down to bottom of page to find download link)


Un-zip RelWithDebInfo.zip.
It will create a folder RelWithDebInfo and inside this folder another folder rtl-sdr-release which contains several files and 2 folders named x32 and x64.

Open folder x32




Copy following 3 files of folder x32 into newly created folder RF Scan.
(1) libusb-1.0.dll
(2) rtl_power.exe
(3) rtlsdr.dll





STEP 5 of 7:
Download file heatmap·py by Right-Clicking on the link below, and choosing "Save Link As..."
Move the downloaded file heatmap·py into folder RF Scan.

https://raw.githubusercontent.com/keenerd/rtl-sdr-misc/master/heatmap/heatmap.py



STEP 6 of 7:
Make sure no program is using DVB-T Dongle. Stop any such program to free DVB-T.

Open Command console by typing cmd in search. Once the Command Console is open, do following:

Code: [Select]
# First CD to folder RF Scan
#Once in folder RF Scan, give following command
rtl_power -f 800M:1200M:100k -i 30 -c 50% -e 30m -g 30 -F 9 > scan.csv


The above command will generate following output.




Wait for 30 minutes for scan to finish.
When scan is finished, it will say "Exiting.... Canceled by user".
Scan will create a file "scan.csv" in current folder and save scan data in it.

This will produce a very wide image (4000 pixels!) that shows spectrum power from 800MHz - 1.2GHz.

If you want a narrower image, increase “100k” above to something larger, it controls the bandwidth that corresponds to one pixel on the x axis.
 -i controls the integration interval (time for one pixel on the y axis)
 -e is the total runtime, longer gives you a taller image.




STEP-7 of 7:
Create an image "scan.png" in current folder from data stored in file "scan.csv"
Open Command console by typing cmd in search. Once the Command Console is open, do following:

Code: [Select]
# First CD to folder RF Scan
#Once in folder RF Scan, give following command
heatmap.py scan.csv scan.png

The above command will generate following output.



This will produce a very wide image (4000 pixels!) that shows spectrum power from 800MHz - 1.2GHz.


Example of Scan of Existing RF Signals by rtl-power




This image is very wide (4000 pixel), but displayed smaller. As a result details are not clear.
Click over it to see bigger size.


Three 500 pixel wide cut-outs from above 4000 pixel wide image. These cutouts show details better.







[/font]

« Last Edit: November 29, 2017, 12:17:43 am by abcd567 »

Tom N Jerry

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Re: Do I Need A Filter?
« Reply #5 on: November 30, 2017, 12:17:48 am »
This is a very useful thread. Deserves to be made Sticky by the Admin.