More On SDR Dongles

 

A Closer Look

We had an earlier article about SDR and SDR dongles. I recently played with a nano version. It is probably a knock-off of the NooElec.

I can tell you my findings:

• It has to be connected on the USB computer port with an extender, otherwise the electric noise generated by the computer makes it unusable and completely deaf.
  
  
• It has not much shielding it; acceptable if it is not case to case to the electric noise generator, but at several centimeters apart, it is fine. I tried to shield it in metal, and it did not make any difference, in various test situations. I suspect it is already shielded somehow inside, or partly shielded inside.
  
  
• In the commercial FM band it is a cheap stereo and more important, an RDS receiver. It knows to display the name of the station, the songs that are played in that moment and whatever digital info the station sends in addition to the analog signal. The sensitivity in FM is way worse than 2 microvolts. Any dedicated commercial receiver amplifier, including my roommate’s Yamaha 2 micro V (and every single FM radio in the apartment we have, including clock radios, MP3 portables (the radio part) are more sensitive than the SDR dongle. Also, I am using a proper dipole antenna on the balcony, connected with coax cable to the SDR dongle, while all other 7 receivers have just a small piece of wire. I estimate somewhere at 30 – 50 microvolts sensitivity in the 88 – 108 MHz band.
  
  
• The characteristics differ very much on the Rx bands and require adjustment from the RTL dongle settings. That means RF Gain; RTL AGC; Tuner AGC. It seems it does not like the 50 MHz band and the sensitivity is not great in this band. I confirmed the bad findings of everybody writing about this issue on the Internet.

• In the 144 MHz band, with a good dipole, it receives everything the Kenwood 7950 and the Chinese walkie-talkie receives. It likes this band and it has good sensitivity. All repeaters from Victoria, Port Angeles, Nanaimo, Cowichan are 59.

• It also likes the marine band, air traffic band and the weather band. They are all around 150 MHz and once the settings are done for one station, they can be kept in the weather, marine, 2 meter bands.

• It is stable. I did not feel the need for a more stable oscillator. It did require adjustment in the software, -200 ppt for my dongle. This is considered a huge adjustment. I verified with encapsulated quartz oscillators (32 MHz, 125 MHz, 150 MHz, the 28.197 CW beacon), and indeed it needs that huge adjustment.
  
  
• The CB band and the beacon on 28.197 MHz (VE7MTY, Pitt Meadows, continuous, CW) are in a band where the RTL dongle is not so sensitive. The beacon (nearby me) booms in my SONY ICF7600G portable radio, with its telescopic antenna. The SDR dongle with a CB whip on the balcony receives it almost OK, but only because I was hunting for the beacon and I knew where it is. The beacon’s signal barely produces a trace in the display spectrum, and I am nearby it (exactly 13.89 km away).
  
  
• There are images everywhere. The FM band (88 – 108 MHz) can also be received on 30-50 MHz. The worse thing to do is to use an upconverter, as I saw so many on the Internet, with an NE612, and wide non-tuned input. I tried, and the images kill any useful signal. In the end I did 2 converters, in order to cover 3.5 MHz to 30 MHz, one for the lower part and one for the upper part. I used NE612, attacked by an amplifier with a BF998 in front. I have a tuning circuit just at the antenna, and 2.4 K resistor + coil in the output of the BF998 drain. The source terminal is connected directly at the ground and the BF998 is power supplied with 9 V (12 V is max in datasheet, and it does burn-up beyond 12 V). The oscillator is an encapsulated 3.3 V powered oscillator, in a socket, to easily changed. The best it worked for me is at 150 MHz, so stay away from FM commercial band and upconvert the shortwaves into a sensitive band that the SDR dongle likes. I can adjust the signal from the oscillator to the value from the NE612 datasheet, but actually it does not make any difference even if it is provided with 3 V (NE612 has a buffer in it before the mixer).
  
  
• The only program that totally works in Windows is SDR Sharp. It has plugin to decode CTCSS tones and display their value. All other programs partly work (not all modulation types; there are workarounds for stereo; workarounds for drivers and so on). SDR Sharp simply works, all options, everything that the hardware is capable of.
  
  
• The noise of the first element in the SDR dongle must be better than the BFR91A. I tried a wide range untuned amplifier with 1 BFR91A, and it did not bring in anything, just noise. The situation changed when I put a SAW 88 – 108 MHz (3 pin filter) in front of the BFR91A, and it helped.
  
  
• It does not run hot. Whatever other users noticed with old SDR dongles is no longer an issue with my SDR small dongle.

[Right] This is my upconverter for the SDR dongle, inspired from many articles, but not a copy. I always put the dual gate MOSFET BF998 with the S at the ground and the D in a series 2.2 KΩ plus 1 mH molded shock. The BF998 has a different behaviour than a BF981, and very much different than a 40673.

The values are for the 10 MHz - 30 MHz upconverter. With this upconverter in front the combination SDR dongle + converter is more sensitive than the SONY 7600G - probably somewhere close to 1 micro V. But it has to be adjusted every 500 KHz or so, otherwise the 28.197 MHz beacon is lost .

Final conclusions:

• The SDR dongle is the cheapest 2 meter receiver a ham radio can buy, and works as a receiver on par with dedicated equipment, which is generally limited by the line of sight, not by sensitivity. A beginner can listen to the weekly nets for some $8–11 CAD, shipping and taxes included.
  
  
• The SDR dongle is the cheapest FM commercial RDS receiver one can have, capable of displaying the digital data continuously transmitted by almost all stations in Vancouver.
  
  
• The SDR dongle was not meant as a general coverage receiver. It was designed as a DVB-T television European standard receiver, and probably it is best for that purpose.
  
  

~ Daniel VE7LCG

19/01