Do I Need A Filter?

[obj]

@chrislfa

Either way it’s an artifact of the sampling and rendering in that form when signal is very low.

IIRC this is mostly that you’re seeing the edges of one of the filters (I suspect the downsampling filter in the 2832). i.e. rtl_power is using, say, 2MHz wide bins, but the filter is a bit less than 2MHz wide so you see attenuation of the background noise at the edges of the bins. I think rtl_power maybe has some sort of option to overlap the bins and only take the FFT results from the central region when pasting them together, but it’s been a long time since I looked at it.

edit: ok, that’s -c, but there’s also some strange logic that ignores it in some cases…

[chrislfa]

That makes more sense and explains the presence at higher gains. I only see them at very low gains on the FA Pro Stick Plus; at higher gains they are scaled down into oblivion for the most part.

[JonMu]

@abcd567

YES

I thought so also (my question was a bit loaded). I had bought the FA 978-1090 MHz ADS-B Bandpass SMA Filter. But it didn’t help.

From the images above, most all of the bad stuff from 800-900 MHz disappeared. But little from 900-950 MHz. So I started looking for info about the FA 978-1090 MHz ADS-B Bandpass SMA Filter. I found some scans on Amazon including these two images below.

Anyway, it looks like the filter rejects up to ~900 MHz. And I am guessing I am having issues from 900 to 965MHz

My questions:

• Is there a published scan of the filter?
• Is there a recommended 978 MHz filter that rejects signal up to ~965 MHz?
• I am curious - does anyone happen to know what services occupy 900 to 960 MHz in the US?

Thank you! Jon

[abcd567]

I have RTL-SDR Blog Triple Filtered LNA. I will do following RF Scans this weekend and post their results:

• Scan 1: FA antenna + $9 Generic Dongle
• Scan 2: FA antenna + FA filter (Light Blue) + $9 Generic Dongle
• Scan 3 : FA antenna + Triple Filtered LNA + $9 Generic Dongle.

For the time being, here are the scan results using different combinations of filters & dongles using Noise Source. The last one is for Triple Filtered LNA

RF Scan of Various Combinations of Dongles, Filter, and LNA

[abcd567]

@abcd567

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

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

@obj
Can you please arrange to update above chunk of code by the one below?
I cannot edit this post as it is over 2 years old.
Thanks.

sudo apt-get install rtl-sdr 

## For Raspbian Jessie & Stretch use following command:
sudo apt-get install python-imaging  

## For Raspbian Buster use following command:
sudo apt-get install python-pil  

sudo wget https://raw.githubusercontent.com/keenerd/rtl-sdr-misc/master/heatmap/heatmap.py
sudo chmod +x heatmap.py

[Jerryh47]

@JonMu

questions:

• Is there a published scan of the filter?
• Is there a recommended 978 MHz filter that rejects signal up to ~965 MHz?
• I am curious - does anyone happen to know what services occupy 900 to 960 MHz in the US?

I tested the FA filter with a spectrum analyzer and the scans posted on Amazon are close to what I found.

To reject 965 and pass 978 calls for a selective SAW or cavity filter. I’m using a 4 element cavity filter with good success on 978 on my very noisy urban location.

In the USA the 902-928 band is full of Part 15 devices and in urban areas is quite noisy. “Smart meters” have been using the band too. There are SMR repeater and trunking systems in the 900 MHz band, too.

[JonMu]

Hi Jerry - thanks for your response!

@Jerryh47

In the USA the 902-928 band is full of Part 15 devices and in urban areas is quite noisy. “Smart meters” have been using the band too. There are SMR repeater and trunking systems in the 900 MHz band, too.

Thanks for the reminder about Part 15 devices! I have an electrical smart meter in my backyard though I have not found the exact frequency (still searching). I see LoRa is near 915 MHz.

Too much noise for a wide open front end receiver!

 

@Jerryh47

I’m using a 4 element cavity filter with good success on 978 on my very noisy urban location.

Did you purchase or build the 4 element cavity filter for 978 MHz? If purchase, where was it purchased from?

[abcd567]

@abcd567

@obj
Can you please arrange to update above chunk of code by the one below?

@obj
Thanks a lot Oliver

[abcd567]

@markrrichardson

EDIT: Think i found the answer in the FR24 forums…just change to

sudo apt-get install python-pil

1. It was very kind of Oliver (@obj) to update it today, as I cannot edit my old posts in Flightaware forum.

2. As in other forums there is no restriction on updating old posts, today I have updated “Do I Need A Filter” in both the Flightradar24 and Planefinder forums.

[JonMu]

@JonMu

• Is there a recommended 978 MHz filter that rejects signal up to ~965 MHz?

Hello! I am looking for suggestions for a 978 MHz filter. A cavity filter or SAW filter should work…

[chrislfa]

Uputronics makes one, here’s the page with the datasheet. I and a few others have their 1090Mhz filtered preamp which is good; I expect the UAT version will be equally good.

[JonMu]

Hi Chrislfa!

I did look at the Uputronics 978MHz UAT Filtered Preamp but the data sheet doesn’t have any technical info about the filter (e.g., what frequencies it rejects and what is accepted, and how much attenuation at those frequencies). I’ll write them a note. Hopefully they will write back. Thank you!

[wiedehopf]

They normally have pictures with the specifications.

Anyhow it works well for UAT, i know at least one setup that uses it.

[chrislfa]

@JonMu

I’ll write them a note. Hopefully they will write back. Thank you!

I’ve always had good responses from Anthony there. If you get details or a link to more tech data would you mind posting the info back to this thread please, it’ll be handy for others in the future? Thanks.

[JonMu]

Just sent the question. I will let all know!

[wiedehopf]

It’s right there on the product page

978MHz UAT Filtered Preamp

[JonMu]

@wiedehopf

They normally have pictures with the specifications.

I found that image and counter pixels (not fun!)

EDIT: I am guessing (SWAG) it is 953 MHz @ -25 dB and 1003 MHz @ -25 dB.

[wiedehopf]

@JonMu

I found that image

I mean, there is literally no other product for 978 MHz available as far as i know.

Well i suppose the new 978 MHz dongle by AirNav will have a SAW filter builtin as well.
But for that you probably won’t get any data.

Just curious which specs you are looking for.

To stop out of band intermodulation an 978MHz SAW bandpass filter, which has a 15MHz bandwidth is also present. The insertion loss of this filter is approximately 2.3dB, which does increase the overall noise figure of the unit to 0.75dB.

Anyhow if you need more specifications, writing them is the right approach

[JonMu]

@wiedehopf

Just curious which specs you are looking for.

To stop out of band intermodulation an 978MHz SAW bandpass filter, which has a 15MHz bandwidth is also present. The insertion loss of this filter is approximately 2.3dB, which does increase the overall noise figure of the unit to 0.75dB.

15 MHz bandwidth would be great!

 

I am looking for something similar to this. If 966 MHz & 990 MHz were 25 dB down instead of 35 dB down that would be great! None of the below is written in stone.

[wiedehopf]

@JonMu

15 MHz bandwidth would be great!

I copied that text from the product text from the uputronics 978 MHz amp.
It’s a bit hidden in all that text, but it’s there

Bandwidth is usually specified using the -3 dB points, isn’t it?

They should offer an option with a 2nd SAW filter on the input, that would be nice!
A tiny bit less sensitivity but much better suited to high RF environments!