Antirez weblog

antirez 640 days ago.
# Software defined radio is cool

About one week ago I received my RTLSDR dongle, entering the already copious crew of software defined radio enthusiasts.

It's really a lot of fun, for instance from my home that is at about 10 km from the Catania Airport I can listen the tower talking with the aircrafts in the 118.700 Mhz frequency with AM modulation, however because of lack of time I was not able to explore this further until the past Sunday.

My Sunday goal was to use the RTLSDR to see if I was able to capture some ADS-B message from the aircrafts lading or leaving from the airport. Basically ADS-B is a security device that is installed in most aircrafts that is used for collision avoidance and other stuff like this. Every aircraft broadcasts informations about heading, speed, altitude and so forth.

With software defined radio there are a lot of programs in order to demodulate this information (that is encoded in a fairly simple to decode format). I use "modes_rx" that is free software, just Google for it.

However this transmissions happen in the 1090 Mhz frequency. My toy antenna nor other rabbit ears antennas I had at home worked at all for this frequency, so I read a few things on Google and tried a simple design that actually works well and takes 10 minutes to build using just a bit of wire and a wine cork.

# Cork wine dipole antenna

Sorry but I know almost nothing about antennas. However I'll try to provide you the informations I've about the theoretical aspects of this antenna.

Technically speaking this antenna is an Half Wavelength Dipole. In practical terms it is two pieces aligned parallel wires with a small space between them, with a total length that is half the wavelength of the frequency I want to listen to.

Speed of light = 300 000 000 meters per second
Frequency I want to listen to = 1090 Mhz, that is, 1090 000 000 Hertz
Wavelength at frequency = 300 000 000 / 1090 000 000 = 275 millimeters.

The half of 275 millimeters is 137 millimeters more or less, so this is the length of our antenna:



Now you can ask, why half the wavelength? But even for a n00b like me this actually makes a lot of sense, look at this:



Basically if you imagine a sinusoidal wave,  when one of the two pieces of the antenna is invested by the *high* part of the wave, the other is in the *low* side, and I guess that for induction this creates the current.

Ok, end of broscience for today.

# How to build it

Simply take two pieces of wire of 10 centimeters each, and insert then into the cork. Then blend the two wires to emulate the design in the picture trying to make the space between the two wires small enough. Finally cut the two wires so that they are more or less the same length, and for a total of 137 millimeters.

In the other side of the cork I connected my two wires that go to the RTLSDR. I'm so lazy that I not even soldered the wires... You probably should!



Finally the two connected wires go to a PAL-style connector like this:



That in turn is connected with an adapter for the much smaller connector in the RTLSDR USB dongle. Even with all this interruptions along the path I can receive many aircrafts like a boss, even from indoor, like this:

(-51 0.0000000000) Type 17 BDS0,9-1 (track report) from 3c6313 with velocity 299kt heading 129 VS -320
(-49 0.0000000000) Type 17 BDS0,5 (position report) from 3c6313 at (37.500388, 15.005891) at 9300ft
(-51 0.0000000000) Type 11 (all call reply) from 3c6313 in reply to interrogator 0 with capability level 6
(-51 0.0000000000) Type 17 BDS0,9-1 (track report) from 3c6313 with velocity 298kt heading 129 VS -320
(-51 0.0000000000) Type 17 BDS0,5 (position report) from 3c6313 at (37.499863, 15.006681) at 9300ft

... And so forth.

Have fun! And if you have tricks to make the antenna better while retaining the simplicity, please let me know.
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