Monday, 3 March 2014

An ADS-B receive antenna from scrap

Playing with this little SDR dongle, ive discovered that it should be possible to use it to monitor aircraft ADS-B 'virtual radar' signals.

Ive tried this with the dongle working from the loft mounted discone, and received nothing. Now, the discone if I remember rightly has an upper frequency limit of about 800MHz, but the ADS-B signal is at 1090MHz! So an antenna for 1.09GHz was needed.

I didnt fancy a dipole, or a little groundplane antenna, even though these are very quick and easy to make. I wanted something a bit more challenging, and sturdy. My thoughts went to the groundplane antennas I used to see on fire stations, with a folded driven element.

After some questions on, and a bit of research online, I found these are called 'folded monopoles', and have a feed impedance of 100-150 ohm. I needed 50 to 75 ohm. It was suggested to me that I shunt feed the antenna with a hairpin. AC6LA kindly modeled the antenna on EZnec and made the dimension adjustments for 75 ohm. Whilst I began building the groundplane

Using a spare sheet of 1.5mm anodized aluminium, I marked a 13cm diameter circle (1/2 wavelength) using a pair of compasses, and cut this out using my nibbling tool. This tool is invaluable for this sort of work. You could use a hacksaw, but the nibbler will follow the curve. If your sheet metal is thinner, of course, tin shears would work. Aviation types will follow the curve as well, so long as you use the correct 'handed' ones!

Also, If you do make one of these, take special care. Sheet metal is not forgiving of skin contact! You dont want your new antenna stained with your claret!

Once cut out,  the edges were filed down to exactly on the line of the circle, the disc having been cut out a little wide to allow for my cack-handed cutting. The edges were then smoothed with a sanding block to remove burrs, and the central hole drilled for the antenna connector.

The connector in this case is a panel mount TNC type taken from an old Marconi RC-690 (lots of good bits in a 690!) This matches well to the TNC used on the dongle. A flat was filed on the side of the connector to solder the element to.

I spent a very long time trying to solder the main element, getting nowhere, when it occurred to me that the socket was attached to a bloody great radiator! So I removed it from the disc and soldered it on the bench! The main element was then cut to length and the hairpin soldered to the pin of the socket. A bit of measuring, cutting, bending and remeasuring later, and it was time to solder the stud tap point.

The resulting antenna looks like this

It being dark now, theres not many planes about that will be transmitting an ADS-B signal, and its too late to go outside and test properly line-of-site, so proper testing will have to await daylight.

I also have a 3m USB extension cable for the dongle now, but havent tested it with this yet.

No comments: