I’ve been busy for a year and updating this blog has been delayed. I’ve anyhow slowly been adding and replacing components in my radio equipment. I purchased an improved receiver, namely the Airspy (airspy.com). Airspy has similar silicon tuner chip (Rafael R820T2) as the RTL-SDR receivers (R820T). The difference between the two receivers really is in the ADC/interface chip. RTL-SDRs use the famous RTL2832U from Realtek whereas the Airspy uses more advanced LPC4370. You can find a good review of Airspy by Radio User Magazine (PDF here). In my opinion, Airspy certainly is a good SDR to purchase when you are moving up from RTL-SDR dongles.
Airspy has a frequency range of 24-1800 MHz and if you want to go lower for HF listening, you need to use an upconverter. I am using Ham It Up from NooElec. This is one of the available upconverters, you can find more alternatives from this blog by KF7LZE (updated 2015). Airspy makers are currently in process of designing and testing their own upconverter called Spyverter.
Here is a picture of my current HF setup. It has a PC connected to Airspy in series with Ham It up.
For HF (3 to 30 MHz), you need to use different antenna than on VHF-UHF. General rule would be that the antenna dimensions (length) should be comparable to the wavelength of the radiation. This is required for the efficient reception. The wavelength corresponding to 3-30 MHz would be 10-100 meters. Antenna with these dimensions would be difficult to build in our apartment house. However, using a magnetic loop antenna provides not only reasonable dimension but also some measure of immunity to local electric noise. Electric noise is usually plentiful within domestic, urban setting and it includes sources such as televisions, fluorescent lights, all sorts of domestic appliances and car ignition systems. Even the elevator in our house generates significant noise when the electric engine is running. So while small magnetic loop antennas are suitable only for reception, they do have some benefits as well.
I am using an active loop antenna ALA1530S+ from Wellbrook Communications. It has a loop diameter of about 98 cm. There is an antenna interface unit which handles the power feed to the loop amplifier and has an amplifier of its own. You can read more details of this active loop antenna from the review by Radio User Magazine (PDF here).
The recommendation from the antenna manufacturer is to place the antenna at least 6 meters away from buildings. As you can see, I have not done so as I have been quite happy with the reception even indoors. The active loop antenna is directional, meaning that it picks up signals originating close to plane of the loop. Signals coming perpendicular to the loop are rejected as the antenna has the figure-8 radiation pattern. My antenna plane is located roughly in east-west direction. This way I am able to reject some of the interference coming from indoors.
The first choice software for AirSpy is SDR#. The software is written by the AirSpy developer so it has significant optimization benefits.
So, what can you discover using your SDR system on HF band? My current personal interest points towards number station and military traffic monitoring. My geographical location at southern Finland gives a good access towards both St. Petersburg and Kaliningrad areas. Other areas at relatively reasonable distances are Moscow and Pskov. An excellent resource for number station enthusiasts is Priyom.org which even has a Google calendar with upcoming number station transmissions. Other site which I have found are Numbers-stations.com and Numbers & Oddities. There are of course many other sites available with more information. At least Priyom.org and Numbers-Stations have active Twitter accounts (@ and @).
If you want to try-before-buy, the amateur radio club ETGD at the University of Twente (Netherlands) has an excellent free web-based HF SDR available at websdr.ewi.utwente.nl:8901. With a click of that link, you will be connected to a web interface and receiving HF radio.
Here are some short videos I have made: