Tagged: direct digital sdr receiver


Direct digital receiver – when HAM radio can save your life

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Staying safe with SDR in a hurricane

We Europeans can not imagine how terrible a hurricane can be. Watching the news on TV or surfing the internet, I see several heart-smothering pictures of the destruction caused by storms. Collapsed houses, broken trees, people who lost their homes and all belongings due to something they can not stand up against. I feel deep sorrow for all of them.

These days Patricia - lovely name for a Category 5 hurricane – reached Mexico. According to meteorology experts, she became the strongest hurricane on record, passing both Linda in the eastern Pacific and Wilma in the Atlantic. This morning I was wondering what I could do in a dangerous situation like this.


Source: cnn.com

Keyword is: up-to-date

In case of heavy wind, rain, and extreme weather, the communication infrastructure could brake down. TV, cell network, and commonly used news sources may become unavailable. Without up-to-date news one can not prepare for impending emergency situations. HAM radio has always been the last line of defense for these scenarios. It is independent of infrastructure, and can reach the other side of the globe.

The signals of the disaster recovery forces can be found somewhere around 125-135MHz in the VHF band. This information can be used to warn friends, relatives, or your local community if necessary.

Check out these pages for more useful frequencies:

If you are using a direct digital receiver equipment, like DRU-224 or Quadrus, you can receive VHF and UHF signals not only HF. Thus, you can get the latest news about the upcoming catastrophe from a fail-safe, trusted source. However, you need a pre-selector filter to receive a VHF band in direct digital receiver mode. You can learn more here:


The filter design and layout files are available from here:


The essentials for hurricane news reception

Let’s summarize the basic equipment for a state of the art radio:

  • direct digital receiver
  • pre-selector filter
  • wide band antenna

If you receive something interesting, strange, or amazing, don’t hesitate share with us on our Facebook page!

Stay safe!

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r1anr_28mhz_1501151040ut1-830x945 - feature

WSPR Quadrus SDR

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WSPR Quadrus SDR

What is WSPRNet?

In my last post I’ve introduced the WSJT receiver software and mentioned WSPRnet.
Weak Signal Propagation Reporter Network is a group of amateur radio operators using K1JT’s MEPT_JT digital mode to probe radio frequency propagation conditions using very low power (QRP/QRPp) transmissions.

WSPR Quadrus SDR on WSPRNet

Registered user can log in to the site, and their client software will send automatic updates on the currently received radio stations to the database. The connections are visualized on a map. Again, thanks to Andy, HA6NN, we have some pictures about the stations he was receiving with the WSPR Quadrus SDR.

Connecting WSPR Quadrus SDR

I’ve used the virtual audio cable connection in this experiment as well in order to send audio samples from the SRM-3000 SDR software of the Quadrus SDR platform to the WSJT software.


In this post, you see an example of using Quadrus SDR with external software connected through a virtual audio cable. The setup received some DX stations with the DRU-244A SDR hardware, which has enough sensitivity to receive signals from around the word with a simple wire dipole antenna.

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Direct digital SDR receiver for satellites

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[Direct digital SDR receiver for satellites]

Direct digital SDR receiver principle

As I’ve introduced it in an earlier post, the DRU-244A phase coherent SDR receiver digitizer card has no significant input pre-selection. Thus, it can implement direct digital SDR reception. The bandwidth of the input network makes it possible to digitize the signals in the upper Nyquist bands (which is referred to as under sampling). The platform may be used as a Direct Digital Radio (DDR) receiver for the VHF and UHF bands. We just need to add some input gain to compensate for the slope in input sensitivity at higher frequencies. The SRM-3000 Software-Defined Radio (SDR) application is prepared for this type of operation, and can tune to the equivalent frequency in the baseband.

Preparation of the VHF/UHF direct digital SDR receiver

I’ve already made some sensitivity tests in the 70 cm HAM radio band. See earlier post: Direct-digital-uhf-sdr-radio-receiver-dru-244. I’ve waited for an opportunity to test it on a real target, which ended up being the MASAT-1 - the first Hungarian cube sat. Yesterday, I had a chance to visit the ground control station of the university, where the folks have a tracking antenna with 20 dB gain. I’ve connect my direct digital SDR receiver to the split antenna signal.
To prepare I’ve tested two input pre-selector filters around the 437.345 MHz downlink frequency. One was a Mini-Circuits HPF-LPF cascade, the other was a ceramic filter for the 433 MHz ISM band for radio remote controllers. Both showed 1.5 dB insertion loss, which seems be acceptable; there is no significant input noise figure reduction, and hence significant loss of sensitivity.

LPF-HPF filter response for direct digital SDR receiver
LPF-HPF filter response for direct digital SDR receiver
LPF+HPF for direct digital SDR receiver
LPF-HPF pre-selection for the direct digital SDR receiver
BPF filter response for direct digital SDR receiver
BPF filter response for direct digital SDR receiver
BPF for direct digital SDR receiver
Monolit BPF for direct digital SDR receiver pre selection

I’ve also prepared a Mini-Circuits connectorized block LNA with 20 dB gain and <1 dB noise figure. This seemed to sufficiently improve sensitivity, and thus provided reception capability for direct digital SDR receiver.

Visiting the satellite control ground station

I’ve checked the satellite tracking information on-line, and showed up at the station at the right time to set up the rig. The station operator told me that they had a high-selectivity coax resonator filter installed before their 20 dB low noise preamp, so my pre-selector filter proved unnecessary. We had set up a computer display with incoming packets from some other stations, which helped us checking the reception in the area. We had nothing else to do, so we just waited for the satellite signal to appear on the display. I’ve utilized the +/- 12.5 kHz bandwidth to cover the doppler shift.

IMG_0396 IMG_0397

Receiving the satellite with the SDR receiver

At the predicted time, we’ve observed the first signals at the high side of the display. The observed doppler shift was more than +10 kHz.

rec01 rec02

Later, as the satellite got closer to us, the doppler shift got smaller. I’ve slowed down the waterfall display; this way we could see the doppler shift during the whole transmission.

rec04 rec05

On the last picture, we can see the uplink command packet right below the zero frequency. Our receiver seems to have been tuned a couple kHz below the exact frequency. However, the DRU-244A SDR receiver platform has an external 10 MHz reference input, so next time a GPS clock reference can be employed to keep the frequencies more accurate.

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