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Co-axial co linear antennas
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. 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. They all consist of 'accurately' cut and calculated equal elements that reverse the phase 180 degrees for each element so the theory goes. Pondering over this, it struck me that although the websites do take into account velocity factor of the coax, that is for the coax in normal operation. Once the outer conductor, or screen becomes an element in an array, exposed to the outside world it has a faster velocity of propagation which, in turn means that the outer of the coax must be longer than the inner which is there to maintain phase on each segment. In other words an impossible antenna to make. Those were my thoughts which have led me to have doubts about the cheap, simple designs that abound. Any thoughts, or have I missed something? Steve G8IZY As ADS-B is line of sight, I would think a gain antenna is overkill unless you are trying to get past coax signal loss without a preamp. I would first try a simple ground plane made from 5 stiff wires and a coax chassis socket with some decent coax and see what happens. If that proves insufficient, the only thing not recoverable is the 5 pieces of wire. -- Jim Pennino |
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