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My RTL-SDR + homemade QFH antenna = fairly rubbish NOAA images :(

Posted: Tue Apr 20, 2021 9:49 am
by DaveJaVu
Hi all,

Fairly new to all of this, so please bear with! I have the 'official' RTL-SDR dongle, an old DVT one (same chipset) and a Chinese HackRF One clone and am trying to capture NOAA / Meteor images. I've tested with three home-built antennas, V-dipole, X style dipole and now a QFH which I built based on the calculator at jcoppens which it seems everyone uses (and I'm really pleased with how it came out tbh)

Results... are not great. I can hear a strong, clear-ish signal but there is I feel far too much static and the image quality is no where near what I have seen is possible,

So - the issues. When using RPi 4 with either of the dongles and GQRX (which was a pain to get working with Pulseaudio but we got there) and set to NFM with 6db gain I am just getting swamped with overlap from other local FM broadcast stations (I am in the lowlands of Buckinghamshire in the UK) and the NOAA signal is basically non-existent.

If I use the HackRF One clone, I can receive a strong AM signal, but the NFM signal is very poor and choppy. If I load the wav file recorded from the AM signal into Wxtoimg, I get something between a non-existent and a poor image. Wxtoimg also complains with the 'Narrow IF bandwidth' warning. I understand from reading it should be recorded from NFM using about a 55k bandwidth filter (?)

These are from wav recordings with AM modulation.. (and are by far the best)

Image

Image

Image

I also have a NooElec LaNa, without which I get no signal at all on either AM or NFM

Any ideas why my NFM signal is so rubbish?

Do I need a SAW filter / notch filter? - There is a NooElec SAW + NOAA filter / LNA I have seen which seems potentially to offer a solution.

All suggestions for improvements welcome!

Re: My RTL-SDR + homemade QFH antenna = fairly rubbish NOAA images :(

Posted: Wed Apr 21, 2021 10:42 am
by rtlsdrblog
Got a screenshot of what your spectrum looks like during a pass?

You need to use NFM or WFM mode (WFM mode usually works better), and definitely not AM mode. The filter size should be just large enough to cover the signal, at least 34 kHz.
So - the issues. When using RPi 4 with either of the dongles and GQRX (which was a pain to get working with Pulseaudio but we got there) and set to NFM with 6db gain I am just getting swamped with overlap from other local FM broadcast stations (I am in the lowlands of Buckinghamshire in the UK) and the NOAA signal is basically non-existent.
This seems to be your main issue. The broadcast FM stations are so strong that they are overloading the dongle. In this case the LNA+filter might help you. But if it's strong enough the broadcast FM signals might also overload the LNA.

Also first before trying the LNA+filter, check that your using manual gain settings, and definitely with Tuner or RTL AGC turned OFF. Try to adjust the gain to keep the broadcast FM interference low, whilst maximizing the NOAA signal.

Re: My RTL-SDR + homemade QFH antenna = fairly rubbish NOAA images :(

Posted: Wed Apr 21, 2021 7:06 pm
by DaveJaVu
Thanks for the feedback.

I just set up Cubic SDR as was getting nowhere with GQRX.

There was a 90 degree pass that went over my head at around 7pm this evening. I got an *ok* signal for most of the pass. Super annoyingly though Cubic split the file up so I've not been able to see if there's a usable image.

The signal was still very very static-y tbh.

This is the screenshot...

Image

Re: My RTL-SDR + homemade QFH antenna = fairly rubbish NOAA images :(

Posted: Thu Apr 22, 2021 3:42 am
by rtlsdrblog
You've got the gain at maximum, is this where you get the best SNR?

Also are you sure that you have the LNA powered?

Re: My RTL-SDR + homemade QFH antenna = fairly rubbish NOAA images :(

Posted: Sat Apr 24, 2021 9:03 am
by DaveJaVu
Update: Major progress and several issues ironed out along the way! A homemade antenna and being in a low lying, built up area were certainly all contributing to the problems. I added a SAWBird + NOAA filter / LNA from NooElec and it made an instant improvement:

Image

I've also now added an FM bandstop as I do get swamped by local broadcast stations. It also appears that previous poster's comment was correct - the additional LNA was connected but wasn't actually helping. I discovered that a DC block was required between the SDR dongle and the LNA and with that installed the difference in signal level was pretty dramatic.

The result of all of this, and a load of tweaking with settings meant I was able to capture this one this AM:

Image

I'm frankly super chuffed with that! I still have loads I can improve I am sure. I think my biggest issue is now static and eliminating that from the set up. Will keep on keeping on. :)