hi, I used an SDR TV dongle to make an SDR Tetra detector. I perform an fft on the data collected. It works fine for stuff like a key fob where it shows me a spike at a certain frequency but when I use it outside to see if there are other tetra users around (e.g ambulance going past) it shows me nothing. How can I diagnose what my problem is? Do I need to get a new antenna? Can processing power be an issue? thanks. I just need some guidance as I am stuck.

 

I am going by very little experience but my first suspicion is that you are not using an antenna tuned to your frequency of interest.

It might also be the case that there is very little activity on the frequency you are watching.

 

I am going by very little experience but my first suspicion is that you are not using an antenna tuned to your frequency of interest.

It might also be the case that there is very little activity on the frequency you are watching.

I scanned across a range of 370MHz to 490MHz. As I understand the tetra can operate between these frequencies. I used a key fob which uses 433MHz and this showed up on the FFT but there were tetra users nearby and they usually always constantly send out signals. Maybe I don't understand the whole concept properly hence why my initial question.

The SDR picking up 433mhz made me think the antenna is sufficient. Other than the antenna length how does one go about tuning the antenna?

Thanks.

 

Length is the only thing that you can tune with something like a simple ground plane. It is a good idea to match the antenna and receiver impedance to the impedance of the coax to reduce signal loss. If you have a whip of the appropriate length, it is important to have a ground plane under it.

If your SDR dongle uses the popular RTL2832U you might find this article useful
Measuring the input impedance of the RTL2832U direct sampling input pins

You will probably appreciate the discussion of ground plane antennas and impedance on this site:
Antenna Notes For A Dummy