How to Create a Linear Transponder using a RTL Dongle and HackRF with Gnu Radio.

How to Create a Linear Transponder using a RTL Dongle and HackRF with Gnu Radio.

I wanted to test the concept of a linear Transponder and how difficult would it be to build it with conventional components.

Hardware Configuration

Hardware required to build the Transponder.

1. SDR IQ demodulator with rtl dongle (RF In)
2. SDR IQ DSP processing system. Gnu Radio (DSP processing)
3. SDR IQ Modulator. HackRF (RF Out)
4. Background management system. (OBC) for additional telemetry.

Software required for Transponder.

1. SDR IQ demodulator. rtl dongle (RF In)
2. SDR IQ DSP processing system. Gnu Radio (DSP processing)
3. SDR IQ Modulator. HackRF (RF Out)
4. Background management system. (OBC)

Video of working Transponder.

Gnu Radio.

Transponder Block Diagram

RTL SDR Block.

RTL-SDR Source

Low Pass Filter Block.

 

Low Pass Filter Block

AGC Block.

AGC Block

Power Squelch.

Power Squelch

Osmocom Sink. (HackRF)

Osmocom Sink (HackRF)

FFT. (Spectrum View of input)

FFT Spectrum View

REF: Power point Slides at AMSAT SA

Ref:Source code is available on Github Version 4

VHF 2m Low pass filter design for 146Mhz ideal for Raspberry Pi

VHF 2m Low pass filter design for 146 Mhz ideal for Raspberry Pi 

I wanted to use the Raspberry Pi SDR TX capabilities using the IO pins to generate RF.
The following was possible.
APRS
FM and SSB and CV
WSPR
SSTV....

There is a very nice library that will run on Raspberry Pi one B called rpitx witch can generate CW,SSB,FM SSTV...

The problem in using a Digital IO pin on the pi to generate RF signal would mean it will generate a squire wave signal witch has lots of harmonics. To solve the harmonic problem you will need to build a low pass filter.

Harmonic about 25 db down with now filter.

This harmonic would not be in regulation with rf transmission and would therefore need to create a low pass filter to remove the harmonics.

Here is a filter I build to get rid of the harmonics.

RFSim99 application.

Here is the Low pass filter spectrum analyzer curve on my LWT500,
The Harmonic level should be about 25 db below the carrier.

LPF Curve for 146Mhz (1 Harmonic down about 25db)

Here is some photos of the veroboard test filter I am now busy make a PCB for Raspberry Pi with a small 10db amp.

146 Mhz Low pass filter SMD components

Still need to replace the two wires with Coaxial cable.

This filter board with 7 stage low pass filter will be available soon. (60db)

3D model of 2m Filter board

I will still add a screenshot with the Filter attached later.

Boards has arrived. Assembly and testing will start.