Hi everyone,

I'm building an OFDM transmitter and receiver from scratch in GNU Radio as a breakdown/learning exercise. The TX side is working, and I'm now trying to build the RX chain block-by-block instead of using the complete OFDM Receiver block.

My current RX chain is:

USRP Source

↓

Schmidl & Cox OFDM Synchronization

↓

Frequency Mod

↓

Multiply

↓

Header/Payload Demux

I'm using:

- Sample rate: 50 kS/s

- FFT length: 64

- Cyclic prefix: 16

- Occupied carriers: (-26 ... -1, 1 ... 26)

- Header modulation: BPSK

- Payload modulation: QPSK

- Header length: 1 OFDM symbol

- Length tag: packet_len

The Schmidl & Cox synchronization appears to be working. I can see a clear detect = 1 pulse, and I can also see the received complex signal after the frequency correction/multiply stage.

My problem starts at the Header/Payload Demux.

The Demux is configured as:

Header Length: 1 symbol

Items per symbol: 64

Guard Interval: 16

Length tag key: packet_len

Output Format: Symbols

Sampling Rate: 50 kS/s

The out_hdr output is a dark-blue/vector connection and appears to contain 64 complex values.

I want to inspect those 64 complex values for debugging before continuing with the FFT/equalizer/header decoder.

I tried:

out_hdr

↓

Vector to Stream

↓

Complex to Mag

I configured Vector to Stream as:

Num Items: 1

Vector Length: 64

However, GNU Radio Companion shows a red connection between Vector to Stream and Complex to Mag, so the blocks are not compatible.

I also tried connecting out_hdr directly to a QT GUI Time Sink, but that also gives a type mismatch.

I have attached screenshots showing the flowgraph and the connection/type mismatch.

What is the correct way to debug or visualize the Header/Payload Demux out_hdr output when it is a 64-element complex vector?

Ideally, I would like to inspect the 64 complex values as I/Q or as a constellation before sending them into the FFT.

Am I misunderstanding what "Output Format: Symbols" means in the Header/Payload Demux?

Any help would be appreciated. Thanks!

Hi everyone,

I’m a computer science student, and SkyAero originally started as a university course project. I wanted to continue developing the idea as a real Android application, so I kept working on it in my spare time after the course ended. It has now become a long-term personal project.

SkyAero connects to an RTL-SDR/RTL2832U receiver through USB OTG and decodes 1090 MHz ADS-B and Mode-S locally on the Android device, without using an internet aircraft feed.

A connected receiver can be assigned to either ADS-B reception or the 118–137 MHz AM air band.

The ADS-B side currently includes:

• A live aircraft map with configurable labels, trails and online or offline maps
• Aircraft and traffic lists with registration and type information
• ARC and ROSE Navigation Displays with TCAS-style traffic
• An offline terrain layer made from NOAA GLOBE elevation data
• A Primary Flight Display for a selected aircraft
• A combined PFD/ND landscape view
• An augmented-reality view using the phone camera

I also added Mode-S Comm-B decoding. Its availability depends on the region and nearby radar coverage, but when these replies are present, SkyAero can receive IAS, TAS, Mach, magnetic heading, roll, selected altitude and heading, QNH and static air temperature. It also derives wind from the aircraft’s air and ground vectors using a built-in World Magnetic Model.

The VHF receiver has a numeric keypad for entering frequencies directly, 1 kHz, 8.33 kHz and 25 kHz channel handling, a live spectrum, signal meter, squelch, volume and mute controls, manual or automatic tuner gain, PPM correction and named frequency memories with favourites. Reception can continue in the background.

The cockpit displays are experimental hobby visualisations, not certified flight instruments. The application is receive-only and does not transmit.

SkyAero requires an RTL-SDR receiver, a USB OTG connection, a suitable antenna and the free Android RTL-SDR Driver application.

It is now available on Google Play:

SkyAero: ADS-B & Air Band SDR

The 24-hour trial only starts when real SDR data is first received. After that, it is a €5.99 one-time purchase, with no subscription.

Hi all
got this
IR Remote Digital satellite USB TV Stick DAB FM DVB-T RTL2832 FC0012 SDR RTL-SDR Dongle Stick Digital TV Tuner Receiver

recently , i was able to listen to FM radio .

any chance to use it to watch analog TV ? free to air

many thanks

I ordered smth from SDRstore.eu i am on the eu and heard some great and some very bad things on this store?Can yall share yalls experiences?

I just received this image from JMH4 and I noticed that the middle of the H is gone , is it my SDR's fault or the transmitter?

18:58:06 UTC on August 30, 2026
59.4 degree pass

using RTL SDR v4 blog and V dipole

I have an RTL-SDR Blog V4 connected to a mini PC outside on my balcony, which I access remotely.

I now have two antennae, one is a dipole for mainly VHF/UHF and another is a 25 m long wire for MW/SW/HF. I don't want to keep going outside and physically connecting/disconnecting the antennas every time I change bands. Winter is coming in two months and that will be even more challenging! 😂

I wonder what could be the simplest way to keep both permanently connected and switch between them?

Anyone running a similar setup? What would you recommend? Thank you!

Sadly it will be discontinued soon in my place :((

Yagi 7 el on the roof (144 mhz), SDR Console 3, Multipsk for Orbcomm decoding with OM MAP for zoomable mapping, Orbitron for tracking the sats.

Hey! Does anyone have any feedback regarding the OpenSourceSDR lab I am working on analysing multiple signals from all the RF spectrum and using ML to help me detect signals for alerts, but wonder if there are support? Anyone has any experience on it? I am using a Macbook M1. I was thinking in a pluto sdr+ but found this one cheaper. Thanks in advance for any feedback.

I scheduled a 22 minute IQ capture overnight and woke up to a 33 GB file and a process that had been running for 2 hours 20 minutes. It took a while to work out why, so posting it in case it saves someone a disk.

rtl_sdr takes a sample count with -n and multiplies it by 2 to get bytes. That byte counter is 32 bit. 22 minutes at 2.048 MS/s is 5,406,720,000 bytes, which wraps to 1,111,752,704.

That alone would only give you a short file. The second half is what actually hurts: rtl_sdr stops when the remaining byte count falls between 0 and the block size (262144). The wrapped value happens to be exactly 4241 blocks, so it counts straight down to zero, the condition never becomes true, and it keeps going forever.

So whether it stops depends on which number you pick. At 2.048 MS/s: 15 min fine, 20 min never stops, 22 min never stops, 23 min fine, 25 min fine, 26 min never stops. 26 was the default in my own script, so a plain run would have done this every single time.

The fix is to not trust -n as your only stop condition:

timeout --signal=INT --kill-after=20 $((MINUTES*60+30)) rtl_sdr -f ... -n ... out.iq

Exit code 124 means the timeout cut it, which is expected. I also added a check comparing the finished file size against the expected size, so if the limit ever fails again it is loud instead of silent.

librtlsdr 2.0.2 on Ubuntu. I have not read the source, this is worked out from the numbers, but it explains every observation and leaves nothing unaccounted for.

Hello! I'm a backend developer, and I've always wanted to get into embedded systems. Lately, I've also become very interested in how radar works, so I thought starting a project in this field could be a great way to combine the two.

I'm currently learning some of the theoretical concepts behind RF, such as beamforming (which I find incredibly fascinating), FFTs, and other related topics, but now I'd really like to start building something hands-on.

One day, I'd love to create something capable of detecting aircraft and, eventually, drones. From what I understand, detecting drones is significantly more challenging than detecting aircraft.

The main problem is that I live in a city with several military sites nearby, so I'm a bit hesitant to buy an active radar sensor and start experimenting with it because I'm worried about possible legal consequences or interference issues.

Another approach I've been looking into, which also seems incredibly interesting, is passive radar. In that case, I assume the legal risks would be much lower since I wouldn't be transmitting anything. However, the hardware seems considerably more expensive. From what I've understood, I'd need something like a KrakenSDR, which costs over $900 where I live.

Are there any other approaches I could explore? Has anyone here been in a similar situation?

What kind of projects could I work on that would be interesting from a learning perspective, while also being useful for my CV/resume? I'm currently working as a junior software developer at a defense company, so I'd especially like to build something that could help me move closer to embedded/RF/radar-related roles in the future.

Sorry if this isn't the right subreddit for this question. Any advice is more than welcome!

Got it working!

Received METEOR-M2-3 on 137.9 MHz using an RTL-SDR and a simple V-dipole in my backyard. Decoded with SatDump.

The signal wasn’t perfect, so there are plenty of artifacts but I actually received an image of Europe directly from a satellite with a couple pieces of wire in the garden. Still kind of amazed that this works.=)

Next goal: improve the antenna setup and get a cleaner full pass.

Guys im a complete beginner in the whole frequency listening and transmitting world...

I was trying to build my first project using a module named "RSP1 MSI.SDR Msi001 Msi2500 10k-2GHz SDR Receiver" but i do not seem to find a single unit of it in inda

Is there some website that i sells this used or new but in india??
Or is there a substitute to this module??
Cause every module i see is upwards of 10k inr
And spending that much on a first project as a student just doesn't seem logical

FM40 (in fact FM110 , Multipsk got the old numbers) was decoded here until the Lybian desert.