Cannot tune below 24MHz using Nooelec NESDR 5 on Raspberry Pi 4

[Robin Greig]

Just purchased my first SDR, and am installing it on a fresh Raspberry Pi 4

1. Flashed a fresh install of the 64 bit Raspberry Pi OS

2. sudo apt update && sudo apt upgrade -y

3. sudo apt install rtl-sdr

4. reboot the Pi

5. plugged in the dongle and ran rtl_test and it found the device

6. used add-remove programs  to install version gqrx-sdr-2.15.9-1+b2(64-bit)

7. rebooted Pi and started GQRX from the GUI

8. I/Q Input
Device: Nooelec NESDR SMArt v5 SN: 0000001

Device string: rtl=0

Input rate: 1800000

Decimation: None

Sample rate 1.800 Msps

Bandwidth: 0.00000 MHz

LNB LO 0.00000 MHz

Whether I scroll on the frequency setting or right-mouse click, it will not go below 24.000.000 Hz

I can tune an FM station 95.9 and it plays the audio but I wanted to test it on AM stations as well.

The dongle is supposed be able to go down to 100 KHz, how can I fix GQRX to go below 24MHz?

Thanks,

Robin Greig

VE6RBN, Calgary, Alberta, Canada

[Ian Brooks]

I suspect that this is a problem with the rtl_sdr driver. I get a similar problem when trying to use a RTL-SDR v4 dongle with sdr apps on Android. Although the v4 tunes below 24 MHz. the Android rtl-sdr driver treats it like a v3 which only tunes above 24MHz.

I believe that the Linux rtl-sdr drivers have been updated to operate with the v4, so these might also work with the Noolec unit. Is there a Noolec users forum? If so try posting your question there. Meanwhile I will check if my GQRX installation on Ubuntu 24.04 Linux works with a v4 rtlsdr dongle.

Regards, Ian Brooks, Dorset, UK.

[Ian Brooks]

My version of GQRX which is v 2.17 does tune below 24MHz with a rtlsdr v4 dongle. It was installed using the GQRX App image, which should include the necessary driver files. Raspberry OS is a version of Linux, so it's worth a try. I have got it to work on a 64-bit version of BookwormPup, which is a small Linux distro like Raspbian, so there's a good chance.

After downloading the App image, use your file manager and right-click on the Appimage, select Properties and change the Permissions so that it can be executed. After you have done this you will get a message box asking you whether you want to execute the file when you click and select the "Open" option. This shouldn't affect your existing version of gqrx. In the past I've had several AppImage versions of gqrx on my PC and they co-exist quite happily. I hope this helps.

Regards, Ian.

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[Richard Andrews]

What you want is to configure before receiving a signal. 

look here:  https://osmocom.org/projects/gr-osmosdr/wiki/GrOsmoSDR#RTL-SDR-Source

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[Charlie Hotel]

Hi,

I thought there was a ‘no limit’ Check box somewhere that allows to tune whatever frequency (ie only Limited by the hw, not by the driver)

Cheers

Bernard 

Le 24 janv. 2025 à 16:38, Ian Brooks <ian.bro...@gmail.com> a écrit :

I suspect that this is a problem with the rtl_sdr driver. I get a similar problem when trying to use a RTL-SDR v4 dongle with sdr apps on Android. Although the v4 tunes below 24 MHz. the Android rtl-sdr driver treats it like a v3 which only tunes above 24MHz.

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[Dave Baxter]

AFAIR

You need to configure GQRX etc, to use direct sampling below 24MHz for RTLSDR sticks "if" they  (& the driver supports it.)

Or use an upconverter.

Dave B G8KBV

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[Ian Brooks]

The RTL-SDR v4 dongle is able to tune below 24MHz using an internal upconverter. The problems I was having with the Android rtlsdr driver were due to direct sampling being switched on. Switching this off enabled tuning below 24MHz with SDR Touch on my Android tablet. The other SDR program which I use on the tablet seems to configure the driver correctly when the v4 is used. The Noolec v5 dongle seems to have a similar spec to the RTL-SDR v4. There's an article on RTL-SDR Blog which compares the performance of the v3 and v4 particularly on HF.

Regards, Ian.

To view this discussion visit https://groups.google.com/d/msgid/gqrx/CABDW77LfZF2hsGKaxsqO%2B1yX7290nRN56qFWh1uAnxxRcueQzQ%40mail.gmail.com.

[Robin Greig]

Thanks for your help Ian;

Since I posted this message I've downloaded SDR++ and found that it will allow me to tune down to the AM band.

Again, I'm very new at this and am "assuming" that both GQRX and SDR++ are using the same rtl-sdr drivers?

So I'm thinking that Richards feedback of changing my configuration for my dongle while using GQRX may be the way to go

However I have a spare SD card so I'm going to go thru the same process but install the up-to-date version of GQRX and then play with the drivers

Thanks, Robin Greig, VE6RBN

[Robin Greig]

Thanks Richard;

I did play a bit with the xtal_freq and tuner_freq input strings but that broke my connection to the dongle.

This morning I'm going to do a fresh install of rtl-sdr and up-to-date gqrx and play a bit more.

I found a great site for folks starting out to install GQRX on a pi 5

https://www.aaronrombaut.com/build-gqrx-on-raspberry-pi-5/

Again thanks for the link to the osmocom webpage

Robin Greig, VE6RBN

[U Soso]

I had same issue and contacted NESDR support. Here their answer:

To tune below 25 MHz using your SMArt v5, you will need to enable direct sampling 'Q' mode in your SDR software. This will enable the device to tune to an extended range of 100 kHz.

So, you have to open device tab, and in field where rtl=<id> is given, you add "direct_samp=Q".

(I tried already yesterday with parameter "direct_samp=2", which let me tune below 24MHz, but the results were not as expected (could tune, but fldigi didn't recognize anything.))

[Roger David Powers]

Use of direct sampling mode pretty much guarantees poor results, because direct sampling bypasses the rtl-sdr's internal low-noise amplifier.  This is why it is called direct, it goes directly to the A/D without going through the LNA.  

Unlike RTL-SDR.COM V4, this device does not appear to have a built-in up-converter.

Their web site ( https://www.nooelec.com/store/sdr/sdr-receivers/nesdr-smart-sdr.html ) says:

Though the direct sampling implementation of NESDR SMArt v5 is much better than any other RTL-SDR, we still recommend using an upconverter like the Ham It Up for a more fulfilling HF experience (sold separately).

I don't know why they say it is much better than others, but if they are still recommending an up-converter it can't be very good.  It probably does need an external amplifier and filter setup to be decent, but then you run into the problem all rtl-sdr devices have on HF, poor dynamic range due to 8 bit sample size.  

I know I tried direct sampling on MF and HF with RTL-SDR.COM V3 and it wasn't very good.  RTL-SDR.COM V4 did work well for me.  Other USB dongles in a higher price point such as AirSpy HF Discovery+ and the various SDR Play devices are IMO a big improvement above the RTL devices for MF/HF work.

[U Soso]

Ok, i followed this video: https://youtu.be/cdKhxicZHGs?feature=shared and

got, inspired by video above, this antenna and this receiver (hoping, that v5 is always better than previous versions).
I'm bloody new and unexperienced in this area (as you might already have guessed), so "an upconverter like the Ham It Up for a more fulfilling HF experience" doesn't help me much.
So, i better send back the receiver i got and search for RTL-SDR v4 (Amazon didn't come up with any hit), or

what would be the best solution to receive WeatherFax and NavTex (518 kHz)?

Many thanks for your answer and suggestions.

PS:
If someone has helpful beginner guide links, it would be highly welcome.

[Roger David Powers]

Ok, i followed this video: https://youtu.be/cdKhxicZHGs?feature=shared and

got, inspired by video above, this antenna and this receiver (hoping, that v5 is always better than previous versions).
I'm bloody new and unexperienced in this area (as you might already have guessed), so "an upconverter like the Ham It Up for a more fulfilling HF experience" doesn't help me much.

Unfortunately, getting experience in this area does involve some pain points.  

These devices all use the Realtek chipset, thus the RTL part of the name.

These devices were originally designed for Digital Video Broadcast and the frequencies they needed to cover were in the tens to thousands of megahertz, yet you are wanting to use the device on 518 kilohertz.

https://osmocom.org/projects/rtl-sdr/wiki/Rtl-sdr shows that the lowest frequencies they natively cover is around 22 MHz.  As mentioned the direct sampling change puts the parts into a mode where the low noise amplifier is bypassed which allows them some HF coverage but in my experience this makes the results unsatisfactory.

One approach to using these parts is to have a second device that takes the 518 kHz signal and moves it to somewhere above 22 MHz where the RTL-SDR can receive the signal.  That is what an up-converter does.  Also since that up-converter knows that it will be receiving signals in the HF bands it might have amplifiers and filters designed to work well at HF frequencies.

Ham It Up ( https://www.nooelec.com/store/sdr/sdr-addons/upconverters-downconverters.html ) is a product that is an external up-converter.  RTL-SDR V4 also has a built-in up-converter.

So, i better send back the receiver i got and search for RTL-SDR v4 (Amazon didn't come up with any hit), or

what would be the best solution to receive WeatherFax and NavTex (518 kHz)?

I agree with sending it back.

As mentioned my main recommendation would be to move away from any of the RTL-based products and try something like Airspy HF Discovery ( https://airspy.com/airspy-hf-discovery/ ) or SDR Play RSPx ( https://www.sdrplay.com/rsp1b/ ) products.  These product typically aren't on amazon, you can/should use an approved dealer for your region since there are counterfeits out there. 

The reason is what I said, poor dynamic range.  RTL's analog to digital converter can only produce the values 0-255 (i.e. it is an 8 bit ADC) so it has a hard time representing strong signals and weak signals at the same time.  This matters a lot at lower frequencies.  Most other products have 10 bit (0-1024) or 12 bit (0-4096) sampling resolution so do a much better job at dealing with the wide range of signal variation encountered at lower frequencies.  

And the RTL-SDR devices need to fit into a small form factor (thumb drive size) so typically don't have much front-end filtering, and what they have is often designed for VHF and UHF frequencies, because they were designed to be digital TV devices.  The other recommended devices have bigger form factors and are designed around MF/HF work.

Digital TV signals are totally different than HF signals.  They are usually line-of-sight, often without interference from adjacent signals, sent by powerful transmitters, not subject to interference by lightning, and very broad band.  WEFAX on HF/MF is by comparison really narrow band and often not line-of-sight, therefore it benefits from the use of different technologies.

So while RTL-SDR V4 is a big improvement over RTL-SDR V3 and it's cheap, it still IMO has noticeable flaws for copying signals in the HF and MF bands.  

If you want to find out for yourself, https://www.amazon.com/RTL-SDR-Blog-RTL2832U-Software-Defined/dp/B0CD745394 is the link for V4.  Like I said, it is a big step over V3, but pretty soon you discover its limitations.  Yet it's only $35 whereas the ones I'm talking about are somewhere in the $100-$200 range depending on various factors.  So if you can only spend $35 it might be worth your time to give it a try and decide if it's working good enough for you. 

Note the V4 needs specific drivers for its up-converter to work correctly, https://www.rtl-sdr.com/v4/ gives more info.

[Dave Baxter]

Hi.

Personally, I found that things like the "HamItUp" converters just swamped the RTL sticks with local oscillator leakage/pass through, causing desense at the best, or overload at worst.   Not what you want when you're looking for a weak signal 1/2 a MHz from it!

I tried two of them, other than the LO frequency, the LO leakage was not much different.   Both went to the recycling bin.

Those "MiniWhip" E Field antenna clones, though OK, you do need to put them far away from "stuff" that creates "noise like" interference.  Computers, monitors, other SMPS containing things, LED lights, and any copper phone or power cable that carries a DSL signal.

As well as possibly loading the feeder cable with ferrite, to stop noise getting in that way!

A shielded active loop would be better.

Better choices for such LF reception, could be one of the "SoftRock" SDR kits from the likes of FiveDash.com (sadly not any more.)

There are lots of similar things about now too, plus they are so simple, they are not difficult to build from scratch!  All you need is a good (quiet) soundcard that has a Stereo Line input.

GQRX (and other software) will work with that just fine.

If you want something "general coverage" as a RX, it rapidly gets more complex, if just because of the filters needed, to prevent image issues, as well, as a local oscillator that can work at several frequencies.

Heck, grab an old MW AM radio, add a BFO, and use that for the 500 kHz RX.  All you need for Fldigi, is an audio translation of the off air signal.  It can be told too, if you in effect are using a RX in Upper or Lower sideband, depending which side of the IF the BFO is running.

The RTL stick things are very good at VHF and up, but pretty poor at HF and below, unless you have a good antenna.  (When they can be kicked into direct sampling mode.)

Lastly, if you are serious about going the SDR route, then the SDRPlay products are good, not that expensive, and can (with some effort*) work with GQRX or SDR++ on Linux.

(*Not difficult, but takes a lot of time and reading, to do it right.  You will likley need to build some software library from source at least.)

You do get coverage from 10's of kHz to nearly 2GHz though !

73.

Dave GOWBX.

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[Roger David Powers]

Personally, I found that things like the "HamItUp" converters just swamped the RTL sticks with local oscillator leakage/pass through, causing desense at the best, or overload at worst.   Not what you want when you're looking for a weak signal 1/2 a MHz from it!

I tried two of them, other than the LO frequency, the LO leakage was not much different.   Both went to the recycling bin.

Interesting data.  To decode the jargon a bit, using an up-converter is another stage of signal processing, and it can/does introduce unwanted signals.

I personally haven't operated "Ham It Up". I only have limited experience with RTL-SDR V4.   I did test that it worked on HF, but soon moved back to using the Airspy since it's a lot better.

Those "MiniWhip" E Field antenna clones, though OK, you do need to put them far away from "stuff" that creates "noise like" interference.  Computers, monitors, other SMPS containing things, LED lights, and any copper phone or power cable that carries a DSL signal.

As well as possibly loading the feeder cable with ferrite, to stop noise getting in that way!

A shielded active loop would be better.

All good points.  I will point out that I read the transcript of the u2b video and it says "3:58 let's try with frequency 7.880 kilohertz" which is in the HF band as opposed to MF or LF.  This means the required antenna is much shorter than one for 518 kHz.  It also may be subject to less interference, depending of course on local conditions.

Lastly, if you are serious about going the SDR route, then the SDRPlay products are good, not that expensive, and can (with some effort*) work with GQRX or SDR++ on Linux.

(*Not difficult, but takes a lot of time and reading, to do it right.  You will likley need to build some software library from source at least.)

I think the "SDR Play" products are good, but one reason I prefer AirSpy is they release the source code for their drivers whereas "SDR Play" only releases binary versions.  Personally, I try to avoid binary-only products.

[Dave Baxter]

Hi again.

I've not used any AirSpy devices, just an older SdrPlay V1 that I was given (the original owner replaced it with I believe an AirSpy model, for some reason I know not.)

The SdrPlay V1 might be an older unit, but when I can join the dots (and yes, their "driver" is a "large binary blob") it works very well indeed.   (I do need to resurrect all that...)

I'll look into AirSpy products perhaps sometime in the future.  At present, I have no other need for any other SDR kit.  I have "several" RTL based sticks too of course!  (One gets regular use as a DAB RX, using software from https://www.welle.io/   It works very well.)

As well as several of the earlier "SoftRock" SDR front ends.  One a dedicated 60m beacon RX (Redundant now, as the three UK 60m beacons are "no more".   Another was used as a pan-adapter for my TS870s.   They work well enough, and the bang/buck ratio is better than the later expensive commercial kit in many cases.

Thanks for the tip however re AirSpy.  Much appreciated as yes, I also prefer open to closed software.  (Not that I understand much of the internals, but I do contribute a little code to Flrig.  C/C++ is not my native language!   But I stumble onwards.)

73 Roger.

    Dave.

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[Roger David Powers]

A point that needs to be made: NavTex is on a very low frequency, 518 kHz, so you need a very different antenna than WeatherFax which is typically at higher frequencies, as per your first video it was using 7 MHz.  

As mentioned in Dave Baxter's email, loop antennas are often used for receiving the 518 kHz signal.  A full sized long wire antenna would be hundreds of meters long.  The loop antennas do their best at avoiding the need for such a huge antenna, but can be hard to tune.

The WeatherFax signal around 7 MHz would probably be heard with a good whip antenna, although a full sized long wire antenna would be around 20 meters long.  

Some links I think may help:

https://www.youtube.com/watch?v=DjRZJ3uOPm8 -- Automated NavTex Receiver Using A Raspberry Pi & SDRPlay SDR, by Tech Minds

https://www.youtube.com/watch?v=nSRwEHpYBfc -- Decoding marine weather fax using FLdigi on HF by Kevin Loughin

[Dave Baxter]

Hi again.

FYI, the "Loop" antenna I use, is an active wide band loop, not a narrow band passive unit, that can indeed be "interesting" to tune.

Mine is one of these:-  https://www.george-smart.co.uk/projects/wellgood-loop/

The history of it is here :- https://www.george-smart.co.uk/projects/wellgood-loop/wellgood-loop-history/

I bought the built up boards for both the loop amp itself, and the Bias unit.  I had to build them into boxes. (Waterproof for the outside unit of course.)

The "loop" itself, is a 3.3m length of scrap LD F 4-50 coax, bent into a circle.  The outer jacket cut back from the ends, then they were squashed flat in a vice, and 8mm holes drilled through the flats to mount on the brass M8 bolts I used on the walls of the "outside unit" box.

Weather Fax and Navtex? (Sitor3)   Fldigi does well enough for me.   There are of course many other options, but Fldigi is free (as in no monetary cost) and cross platform.  (Windows, Mac, Linux etc.)  And is still in full support and maintained.

See:-  https://www.w1hkj.org/

73.

Dave.

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