I wanted to spend more time experimenting with HF before posting a follow-up about the RTL-SDR Blog V3 software defined radio. Due to shifting ionospheric conditions and such, a 5 minute snap evaluation is no evaluation at all. Here’s the scoop after really working with the V3.
Yes, the V3 does HF — with limitations. What it does, it does surprisingly well for $35 USD.
I configured the V3 with a nooelec 9:1 V2 balun (unun) and a 23 foot (7 meter) long-wire antenna. I did a number of experiments in grounding and eventually just went with the simplest solution: long-wire to the antenna input and no ground. Electrical ground (wall outlet) was unsatisfactory and cold water pipe didn’t produce any improvement. [More on these experiments some day.] I compared the V3 against my old Drake R8 communication receiver using both long-wire (23 feet) and Datong DA270 active dipole antennas. The old Datong DA270 is long in the tooth and I got slightly better results with the long wire. The Drake is in terrific shape for its age (25 years). Wish I could say the same for myself. 🙂
The V3 tunes in quite a few stations! It took a bit of time to find my way around SDR#, trying this feature (noise reduction) and that (audio filtering). Reception-wise, the Drake has the edge, but not by much. I can easily tune the stronger shortwave stations out of Asia, for example.
The SDR# spectrum display makes a good companion to the Drake. I could pick out the most likely candidates on the spectrum display, then turn to the Drake and dial them in. Using the V3, I could tune in some weaker stations like a Honolulu weather station and the U.S. Air Force High Frequency Global Communications System (HFGCS). You haven’t done nothin’ till you hear an EAM. 🙂 The SDR# memory feature made it easy to follow an HFGCS simulcast through its primary stations. I may stick with this productive workflow in the future.
The RTL-SDR blog documentation states the V3’s limitations clearly and accurately. The V3 has an analog-to-digital converter (ADC) that samples the baseband radio frequency (RF) signal directly. Quoting the data sheet and user’s guide:
The result is that 500 kHz to about 24 MHz can be received in direct sampling mode.
Direct sampling could be more sensitive than using an upconverter, but dynamic won’t be as good as with an upconverter. It can overload easily if you have strong signals since there is no gain control. And you will see aliasing of signals mirrored around 14.4 MHz due to the Nyquist theorem. But, direct sampling mode should at least give the majority of users a decent taste of what’s on HF. If you then find HF interesting, then you can consider upgrading to an upconverter like the SpyVerter (the SpyVerter is the only upconverter we know of that is compatible with our bias tee for easy operation, other upconverters require external power).
Note that [the V3] makes use of direct sampling and so aliasing will occur. The RTL-SDR samples at 28.8 MHz, thus you may see mirrors of strong signals from 0 – 14.4 MHz while tuning to 14.4 – 28.8 MHz and the other way around as well. To remove these images you need to use a low pass filter for 0 – 14.4 MHz, and a high pass filter for 14.4 – 28.8 MHz, or simply filter your band of interest.
I definitely saw and heard aliases. The best example is WWV at 15.0MHz. Yep, I could tune in 15.0MHz directly. But, what’s this strong signal in the 20 meter shortwave band at 13.8MHz? It’s a WWV alias. Hmmm, 15MHz is 600kHz above 14.4MHz and 13.8MHz is 600kHz below 14.4MHz. Not a coincidence? I also found aliases of strong medium wave AM broadcast stations up around 27 to 28MHz.
So, I would say that the V3 is quite a good low-cost HF receiver, especially in the range from 2 to 15MHz, where I spent most of my time. I have an AM band-stop filter on order and hope to attenuate the strong AM broadcast stations. I did a quick survey of local transmitters and discovered three powerful stations within a few miles of my location. All transmit several thousand watts or more — enough to be troublesome. In addition to the aliasing issue, the stations may be overloading the V3 and degrading its weak signal performance. [More on this some other time.]
I find RTL-SDR’s assessment of the V3’s HF capabilities to be fair and transparent. If you’re a serious radio hobbyist, I recommend an up-converter (e.e., the nooelec Ham It Up) or an upscale SDR like the SDRplay RSP1A/RSPdx or the AirSpy HF+. The upscale models cost more, but have better HF support (no aliases, better RF front-end, etc.)
I’m good with the nooelec baluns, by the way, and have purchased a second one for the Drake R8. Rather than buy another SDR, I’m going to spend time on antennas instead. As to workflow, I like getting an overview of the spectrum via SDR and then focusing through the Drake R8. I want to try and evaluate an AM band-stop filter, too. I will post results once I get more experience under my belt. If I didn’t have the Drake R8, I would probably look into an RSPdx or an HF+ as the next step.
Want more? Check out my short review of the nooelec Nano 2+ SDR.
Copyright © 2020 Paul J. Drongowski, N2OQT