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Co-axial co linear antennas
On Thu, 19 Nov 2015 00:23:16 +0000, Steve
wrote: With a RTL SDR type USB stick on its way, I turned my attention to looking for an antenna with high gain that would give me good coverage of ADS-B 1090 MHz broadcasts from aircraft, one that I could easily and cheaply make for myself. Here are some NEC2 models of some ADS-B antennas. We've built a few and they work well. http://802.11junk.com/jeffl/antennas/AMOS-3-INV-1090Mhz/index.html http://802.11junk.com/jeffl/antennas/AMOS-5-1090MHz/index.html http://802.11junk.com/jeffl/antennas/AMOS-5-INV-1090MHz/index.html The front end of the typical RTL2832U SDR gizmo seriously lacks dynamic range. If you install a high gain antenna, with a pre-amp to compensate for the coax losses, in a location with a good view of the sky, you're going to overload the receiver front end. You're also going to experience some front end overload and possibly intermod from other services using the same L band range such as TCAS radar, JTIDS/MIDS, and DME. Cavity filters are a big help. For starters, I suggest a very simple antenna with coverage of the sky. There's plenty out there to hear and you won't be bothered by overload and interference. Once you have a good idea of what software is the lesser evil, and you find your range to be inadequate, think about big antennas, preamps, and filters. A quick google produced loads of hits for this type of antenna. https://www.youtube.com/watch?v=TkUYdCPFXXs as a random one. You get the idea. Retch. Those are alternating coax cable collinear antennas. For a given length antenna, they have 3 dB less gain than a collinear antenna made with coils or 1/4 wave stubs. The coax flavor are also end fed instead of center fed, making it difficult to put the main lobe at the horizon, instead of a few degrees above the horizon (i.e. uptilt). The good news is that it will have a lousy pattern and low gain, which should reduce overload and interference problems. If you really want to build something like that, test the antenna to see where it's tuned with a sweep generator, directional coupler, RF detector, and an oscilloscope. The one's I've seen, that were built without a fixture, look awful. Good luck. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
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