I haven’t posted here in about a year and a half, which is shameful! But, my last post was about low-cost SDR, and since then I’ve ventured into it. In this post I want to describe how to build a collinear antenna, which can be used for receiving ADS-B signals on 1090MHz from aircraft. I use a NooElec R820T tuner dongle on my laptop as my radio receiver (I’ll write up more about this later), and as I love quite close to an airport, it’s ideal for capturing signals from airlines.ADS-B carries navigational information about aircraft such as their heading, location, altitude, airspeed, groundspeed, and so on. This is enough information to make a pseudo-“radar” system with your computer, where you can plot aircraft and their details on a map on your computer.
Being so close to an airport, I’m able to use either the stock antenna that comes with the USB tuner, or attach a bit of wire to the stock antenna and hang the other end from a curtain rail; both give me good results and I’m easily able to detect and plot airplanes flying around Dublin, to and from the city’s airport.
However, to extend the reception range and quality (and this generally applies to all radio reception) there’s a special antenna called a collinear which is tuned to the wavelength/frequency of ADS-B transmitted by aircraft. A collinear antenna is an array of dipoles, made up of as many elements as you want, then fed straight into the receiver. It’s rather inexpensive to make, as it simply uses readily available coaxial cable used it television aerial setups.
Building the Collinear
Ideally, the wavelength of your antenna (or antenna element) should be the same as the wavelength of the radio signal you’re trying to receive. For 1090MHz that aircraft transmit, this corresponds to 27.5cm. Thanks to those amazing properties of radio, you can use certain fractions of that length; for this antenna, we’re going to make half-wave elements, which still has excellent reception quality, but doesn’t take up as much space.
Coaxial cable has certain properties that allows us to adjust the half-wavelength even further: radio waves travel through the cable at a different speed than in a vacuum, and so we can adjust for this velocity factor. A half-wave element using 75 Ohm coax cable works out at just about 11cm.
So, grab your coax and start cutting!
A good length to work with is 15cm, then strip off 2cm of the braid and foam on either end to reveal only the core wire. Make up a good few of these; I made five altogether.
When you have all of your elements cut, you now need to assemble them. Get two elements, and stick the center wire of one into the braid of the other, with a small square of insulating tape in between (this stops the braids touching).
Now, push the two together. When they’re joined, close up the joint with some more insulating tape.
Repeat this process so that all of your elements are connected. A diagram of your antenna should end up looking something like this.
Now. On one end of your array you might choose to terminate the antenna, like in the diagram above. If you do that, use a resistor that matches your coax cable, either 50 or 75 Ohm. That said, I did not terminate my array, and simply left an exposed core wire at the end. It seems that there’s a lot of discussion out there on what’s best, but my advice is, if you don’t have a resistor handy, don’t worry about it.
On the other end of the antenna you’ll need to attach your feed line. I used an RF lead for connecting a TV to a VCR/DVD player, with the female socket cut off. I simply connected it in just the same way as I joined the elements together: core to braid.
Finally, I fitted the whole thing into a bit of water pipe cut to length. I used a lot of insulating tape to seal and bung the ends.
Testing the Collinear
I’m literally testing the antenna as I speak. This is because the NooElec dongle uses an MCX connection, while my antenna used a ‘normal’ coax connection; I had to order an adapter that just got delivered a couple of days ago.
I’m also not testing near an airport, so I can’t really do a fair comparison just at the moment, but so far, I think I’m seeing better results. Sitting in my kitchen and holding up the antenna gives me signals from maybe five or six aircraft at a time, whereas when I used the stock antenna I’d get maybe one.
I’ll do a full test in the coming few days, but at the moment the signs are looking good. If I need to make any updates or edits to this post, I’ll do so, so please do check back again soon!
Test Results – Update on March 11th
I’ve just plugged in my antenna, and I’m located maybe 6 or 7 km from Dublin airport. Immediately, I got good results. The antenna picked up ADS-B signals straight away, and listed eight aircraft. Normally, I’d get one or two planes, and have to wait a few moments to pick up more.
The reception radius has increased hugely, and the diagram below is a rough comparison of what I’m getting now (green), compared to the previous longwire antenna (orange). It’s a screenshot from the rtl1090 scope, with my own radius circles overlaid.
As I said, I’d normally get relatively few aircraft picked up with the longwire – maybe a max of six or so at any one time. With this antenna, I picked up 22 aircraft only a few minutes ago! Most aircraft seem to be appearing towards the east of Ireland, which I guess is expected: Ryanair and Aer Lingus have a lot of destinations between them based in Europe. I’ll check this with www.flightradar24.com to see if my receptions correspond to theirs, particularly with planes towards the west of the country.
Overall, I’m very happy with this antenna. I’m getting very good signals and it was a very easy build. Definitely worth it, and I’d recommend it if you’re interested in tracking aircraft like this.