But before that, I decided I wanted a proper "hardware radio", and bought myself a portable wide band radio receiver  (great value for the price, I advice it). Then when I was about to get one of these dongles, as usual and as for the above radio, I got sucked into the reviews suggesting this or that other model for better sensitivity or extra features. Now I understand the price range for these cheap sticks is just in the order of a few tenths at maximum, but maybe there is a chance anyone has investigated a bit more on the available choices already?
For example.. the one that has been written about in the article has 4 stars and 159 reviews while the first alternative in the "Amazon recommends" box has some 940 reviews and same 4 star score for just £ +2 (but I'm not even sure they do exactly the same things.. sorry). So, just wondering, maybe someone has already gone through this search and can recommend the best one to get?
Thanks to the popularity of this family of DVB dongles, all the information you need is right here: http://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/
Also look into the Funcube dongle for a sdr dongle that is designed for ham radio and not just an adapted DVB dongle -- they are pretty much the same thing though. If you can find an E4000 dongle on ebay it's probably worth twice as much as the R820T dongles because it gives you a wider frequency range. I have an R820T dongle and it works just fine though.
One thing to keep in mind is that these dongles only have 8 bit ADCs in them, so no matter how great your signal path, you will top out at ~48db of sensitivity (rough formula is sensitivity in db = 6*bits of ADC). A "real" SDR like the USRP B210 has a 12 bit ADC in it, which means that it will top out at around 72db of sensitivity. Plus, spending the big money means that you will get a radio that takes less shortcuts than a cheaply produced dongle, so you are much more likely to get closer to that theoretical peak. But certainly start with a dongle, and play with it and see how far you get before you "absolutely need" some TX capability or have truly maxed out what you can do with only USB 2.0 and 8 bits of resolution (though there is a guy who was passively tracking aircraft with FM radio stations and two synchronized dongles so what you can do with these is pretty crazy).
Of course, the ~6 dB per bit rule applies only at the ADC's full (Nyquist) bandwidth. If the ADC is followed by a decimation and filter chain it can potentially do a lot better. Do these dongles perform their conversion at the final signal bandwidth, or do they digitize at a wider IF and downconvert to baseband digitally?
CamperBob you sound like you know what you're talking about and I'm mostly self taught in this area so if I'm wrong please let me know.
It sounds like the bandwidth of the 137 MHz satellite signal is in the 36 kHz vicinity, going by the article. So one possible strategy would downconvert 137 MHz to somewhere in the first Nyquist zone of the ~30 MSPS ADC (i.e., below about 15 MHz). The ASIC picks up 3 more bits of process gain by decimating to 3 MSPS, then the host CPU could pick up about 5 more bits by further decimation to about 100 kSPS. The 36 kHz signal would then have a dynamic range approaching that of a 16-bit ADC, which is quite adequate for most purposes.
Of course, IMD artifacts and other spurious tonal responses due to the cheap RF front end don't experience any such improvement, but you sure can't complain for $20.
Also I was wondering.. is there a trasmitting dongle? Yes I do know already about transmitting licensing and fines :)
With my Tecsun PL 660 I already receive air band and can listen to air traffic or tower, but I am not sure what you meant by 2 synchronized dongles... maybe you meant that one was listening on tower and another one on air traffic control?
If you want to transmit, I HIGHLY suggest you look into ham radio. It's cheap to get a license, under $50 to test and get your technician's license in the USA. That will give you a lot of legal opportunities to TX and you will learn quite a bit in the process. The ARRL technician study guide is a great book and if you get through it all you are pretty much guaranteed to pass the test (all the test questions are in the book). The test isn't a cakewalk but it's passable for most anyone who studies.
The E4000 is out of production, but I think it gives ~500 MHz more receive frequency on the top end. It's really not game changing and the new R820T2 dongles seem to be better overall, but the E4000 is worth the extra $30 to lots of folks. You can get to higher freqs without an E4000 anyway. A popular "hack" is to use an MMDS downconverter (another $25 Chinese purchase) to get access to the 2.4 GHz bands and sniff bluetooth and wifi packets. This is fairly advanced stuff that goes beyond just listening to voice broadcasts that you are doing now since you will be trudging through some cryptically written tutorials and writing some of your own code to get everything to work. Some people just want to listen to voice, and don't need the extra frequency since you can listen to AM/FM broadcasts, civilian air traffic, ham VHF/UHF, and even military VHF/UHF with just a regular dongle.
As far as synchronized dongles, a guy was running two dongles off of the same clock source (he soldered a wire between the two chips so they could use the same crystal). That way he could compare the differences from signals received at two physically separated antennas and track moving objects (like airplanes) that reflected radio transmissions. This is by far the most advanced project (I feel bad calling it a hack) I've ever seen with an RTL dongle, and most of the licensed hams I know don't have the brains to do it: http://kaira.sgo.fi/2013/09/passive-radar-with-16-dual-coher...