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Co-axial co linear antennas
In article 20151119002316.0a9bd48f@silent, 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 You really want to visit the flightaware forums: https://discussions.flightaware.com/...-tracking-f21/ you'll find very detailed discussions of antennas and how to get performance out of your SDR setup for receiving ADS-B. Executive summary: co-linear antennas can be very good performers. They are very precise beasts; a poorly made co-linear antenna really sucks. There are a number of very good, very easy to make designs available. You also need to consider the RF environment -- the SDR is a very wideband beast and you will probably have to consider bandpass filtering to prevent overloading the front end. All this and more in detail on the FlightAware forums! |
#2
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Co-axial co linear antennas
On Wed, 18 Nov 2015 17:00:11 -0800
artie wrote: In article 20151119002316.0a9bd48f@silent, 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 You really want to visit the flightaware forums: https://discussions.flightaware.com/...-tracking-f21/ you'll find very detailed discussions of antennas and how to get performance out of your SDR setup for receiving ADS-B. Yes, thank you. A quick read of that site suggests that it is full of interesting reading. None, however, deals with the theory of operation of the antennas. That is why I posed my question here. Executive summary: co-linear antennas can be very good performers. They are very precise beasts; a poorly made co-linear antenna really sucks. I think you have missed the point. I was talking about coaxial collinear antennas, made from coax cable and the design problems I see. There are a number of very good, very easy to make designs available. You also need to consider the RF environment -- the SDR is a very wideband beast and you will probably have to consider bandpass filtering to prevent overloading the front end. Indeed and quite possibly! But that is a separate issue and is not related to my question. All this and more in detail on the FlightAware forums! Do they discuss antenna theory there? |
#3
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Co-axial co linear antennas
In article 20151119014546.42105df7@silent, Steve
wrote: related to my question. All this and more in detail on the FlightAware forums! Do they discuss antenna theory there? Yes. look for posts by abcd567. He has a number of posts on theory behind co-linear antennas, and common pitfalls in their construction. Look for posts by other hams (usually identified by callsign) as well. There's a good article from colorado state on generalized CoCo antennas. I believe they're also covered in a number of RSGB antenna and VHF society publications. (while some discuss theory, a lot of what's discussed is of the "mine is bigger than yours" variety, and people wondering why the n-element co-co they built from salvaged wire gives them worse results than the stock little whip) -- Namaste-- |
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