Most of the ADSB feeders / enthusiasts are not being able to put up antennas on the roof covering the whole 360 degree. But as a go to antenna, most of the time ground plane vertical antennas are being used at first and then co-linear coaxial antennas are being advised as an improvement over ground planes. If you have the ability to cover ~360 degrees, using these antennas is off course good for staying omni-directional. So this way you can get frames from all directions.
But many enthusiasts are actually using them sticking out from a window - a balcony or sometimes even indoors near to a window. On those cases moxon is a much better antenna alternative compared to those omni vertical antennas as:
Closed loop type antennas get less noise (QRM) compared to vertical omni antennas which improves SNR and that means more decodes even without a filter and Lna. You can use more gain on the SDR without hurting the SNR.
It has the gain directed mostly on the forward direction so you have the gain where you need it. Forward! As that is the only direction you can go from a window or a small balcony on most of the cases.
It has a good front to back ratio so this also means you get less noise from your own house/ electrical equipment as those things are behind the antenna.
It still has quite good gain on sides so you can still get decodes from sides (even those side lobes has a better gain than a ground plane)
Small and easy to build and easy to mount, more forgiving than vertical omni antennas on cases that you can not mount it far enough from the building.
Details:
This moxon is calculated for 1090 mhz adsb frequency and if you do it right you should end up with a SWR between 1.3 to 1.7 through all ADSB band which is quite acceptable for a diy antenna on this high frequencies.
Just print it and put a 1.5 mm copper or aluminum wire around it and secure it with zip-ties. Cut the wire from the middle of forward point for a ~1.5mm aperture between 2 parts. Also gaps on the sides are following the calculation so if you cut approximately where the gaps are (as precise as you can) it will be close enough to be resonant on ~1090 mhz. You might want to do a little bit of filing after cutting the wires to make it straight.
Solder a thin coax like rg 174 or rg 316 and keep the coax leads to feed point as short as possible. Make a small single turn loop ~1.5 cm wide of coax as near as possible to feeding point for creating a choke impedance for common mode currents for a better performance-pattern.
From my window on a high building I'm easily decoding ADSB packets within a 400 km range with this antenna. And decodes are not only on the forward direction they are also on both sides! (on the paths that I'm not blocked by neighboring buildings). So I can say I have ~180 degree covarage for 400km's with this antenna without a filter or a LNA.
Mount it vertical with the wire that you soldered the inner of the coax looking upwards and you can also tilt the antenna slightly upwards to get more decodes from higher altitudes.
You can use the other stl file to print a small stick that you can superglue to antenna then you can easily attach it to a non conductive stick to create some space between the building and antenna.
And off course have fun!
73 de TA2SUA
pwm
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