Software-Defined Radio

I had been toying with the idea to get a Software-Defined Radio (SDR) stick for a long time now; even though they’re fairly inexpensive, I’m not one for making impulsive purchases. I mean, what am I going to do… Listen to the radio using my laptop? Seems a bit overkill.

When one of my colleagues mentioned he was starting a project and had ordered an SDR-stick and another coming forth as a licensed ham operator, it pushed me over the line and I ordered one as well. Some basic Googling showed that SDR is possible using DVB-T receiver USB-sticks. Some clever people figured out that the fairly inexpensive sticks can be used to receive a wide range of the radio spectrum, partly due to the lack of a frequency filter and because the sticks pass through the raw data over the USB connection.

Honestly, I am not very well-versed in electronics or radio, but I find it very interesting. The folks over at RTL-SDR.com seem to know what they are talking about and have good documentation, so this seemed like the stick to go for. In addition, the connector used by this SDR stick is an SMA connector, which is frequently used for things like satellite antennae. By ordering this one, I would be quite sure that it would not be too hard to find new and better antennae if I would stay interested. Unfortunately, the official retailer for the stick specified in their agreement they would send the device within 48 hours, they happily took my money and went silent for three days.

As I was still very excited to play about with SDR, I ordered a cheaper USB DVB-T stick based on the RTL820T2 chip off bol.com. It was due to arrive the next day. Fortunately, both arrived on Saturday, and Monday was the second day of Pentecost and I would have three days to play around with them.

The licensed ham operator advised me about setting up the dipole: make each leg a quarter wavelength and make sure both legs are in-line with eachother for the best reception. Wikipedia, YouTube and (again) the RTL-SDR site helped me with the rest. While antennas can seem very daunting, the basic principle is incredibly simple.

Radio transmissions are polarized and if you try to receive a vertically polarized signal with a horizontally oriented antenna, you’re going to have a bad time. RTL-SDR page mentions that most terrestrial (Earth) signals are vertically polarized, so a vertical setup would yield the best reception.

This also explains why car antennae are usually oriented with a slight slant to the back. The almost-vertical orientation picks up the best signal, while the slant is both aerodynamic and allows horizontally polarized radio waves to also be picked up, albeit with significant losses. FM radio is generally an incredibly strong signal.

FM-broadcasts around 96.8 MHz. The signal is so strong that the automatic gain control completely blocked out the other signals.

With this knowledge, I set up the antenna inside, using gqrx as a spectrum viewer and audio decoder. This worked extremely well for FM, but not so much for any other radio frequencies I was trying to listen to. For example, the Schiphol airport approach (west) frequency was almost inaudible. The next day had a lot better weather and I mounted the antenna outside using the supplied suction cup mount, positioning the antenna vertically and using the extension cable so I was able to sit comfortably inside.

Schiphol approach on 121.2 MHz, also marked in the screenshot. The signal is incredibly faint with an antenna placed inside.

The difference in signal reception was staggering, not only could I hear approaching planes for more than 50 kilometers away, the almost clear line-of-sight between me and Schiphol allowed me to listen to the air traffic controller as well.

Schiphol Approach West frequency (120.200 MHz). Communication between Tower and different aircraft.

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