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Thread: Performance limited by ADC bit depth?

  1. #1
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    Performance limited by ADC bit depth?

    Hi All,

    I'm using an RTL-SDR dongle, so somewhere around 43dB of ADC dynamic range. I find that I rarely see aircraft more than 80 nautical miles away. Being located midway between Heathrow and Luton and under several airways, I have a large number of aircraft flying nearby.

    Assuming I've got a setup that has aircraft 1 nm away at -3dBFS, is 100 nm range the absolute best I can reasonably expect? (100 being 40 dB more than 1 and 80 not being much different to 100 on a log scale).

    If the receiver was somewhere different and so didn't have to cope with aircraft so close, could I tweak the gain up and see further as a result? For the purpose of the question I'm ignoring that this will raise the noise floor. I'm trying to understand if it's the ADC and nearby a/c which are limiting the receiver's range rather than something at RF.

    Freddie
    JFreddieF
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  2. #2
    Passenger jackthom's Avatar
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    You don't say what sort of aerial is in use and whether it is mounted high up "in the clear".

    A colinear antenna for example would have much higher gain closer to the horizontal and provided it's view of the horizon isn't too resticted it will help bring in aircraft at a greater distance.
    More local planes, which are generally at a higher angle, would be relatively attenuated by the response of this aerial allowing use of higher RF gain in the receiver where necessary for better distant reception.

    A dipole or a ground plane antenna will to some extent produce this effect but AIUI it is especially noticeable with the colinear.

    I haven't been playing around here for long but living only a couple of miles from Newcastle airport I haven't yet come across signals so strong that they appear to be causing decoding problems.
    The highest RSSI I've spotted so far has been -1.5dbFS from a plane taking off 2 miles away and visible from the garden, just above the tree line.
    At the same time I regularly receive signals around 130NM and I'm still only using a discone aerial mounted well above the house roof.
    Last edited by jackthom; 2018-09-06 at 12:49.

  3. #3
    Captain abcd567's Avatar
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    Quote Originally Posted by jackthom View Post
    At the same time I regularly receive signals around 130NM and I'm still only using a discone aerial mounted well above the house roof.
    Have you tried these DIY antennas for 1090 MHz?

    EASY ANTENNA #2 : CANTENNA


    EASY ANTENNA # 3 : QUICK SPIDER

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    The antenna is the small whip that is part of the bundle linked from the FR24 "build your own" page. It's in a loft (say 10m AGL?). It's in a large dormer window, on a metal box. Unfortunately the window faces a large building (shopping centre) maybe 75m away. On the other side there are houses, but at a slightly lower elevation than mine. The other problem I have is the mobile phone mast less than 50 m away.

    I haven't tried any other antennas yet, I'm only 6 weeks in to this. I'm thinking about whether to buy a slightly bigger one, or to try building one. As it's just a fun experiment at this point, I'm reluctant to spent lots of money. Also, the engineering aspects of DIY antennas appeals to me, and I should I get more serious I can get out my copy of Skolnik and talk to colleagues who are actually antenna designers. They probably won't let me use their test chambers though!
    Last edited by JFreddieF; 2018-09-06 at 16:10. Reason: Auto save missed the end of the last sentence
    JFreddieF
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    Passenger jackthom's Avatar
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    Quote Originally Posted by abcd567 View Post
    Have you tried these DIY antennas for 1090 MHz?

    EASY ANTENNA #2 : CANTENNA


    EASY ANTENNA # 3 : QUICK SPIDER
    I haven’t tried those myself but really enjoyed reading the cantenna/spider thread.
    I did build a coaxial cable colinear which was working very well, until the self amalgamating tape at the base lost adhesion in our unusually hot weather a few weeks ago and the entire cable assembly slipped out of the plastic tube and onto the roof.
    I reinstalled the original discone on the mast to get things going again and haven’t gone back to experimenting with aerials yet.
    Last edited by jackthom; 2018-09-06 at 20:44.

  6. #6
    Captain abcd567's Avatar
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    Quote Originally Posted by jackthom View Post
    I haven’t tried those myself...
    For trial, you dont have to take these diy aerials up the mast or roof. Just place these aerials inside the room besides a large window.

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    Yes, same here. I've spent many hours reading the various antenna threads. They're fascinating.
    JFreddieF
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    After further thought I've realised that there is a big unwritten assumption in my original question: that all transponders transmit the same power. This won't be the case, even if there is a nominal value. I've no idea if that's the case but I'm assuming not.

    I've continued to tweak my setup, and I'm getting better range/coverage each time. So I've answered my original question with a "no". Although clearly in some setups the ADC could become a limit, as any part of the rig could.
    JFreddieF
    T-EGWU97

  9. #9
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    Quote Originally Posted by JFreddieF View Post
    After further thought I've realised that there is a big unwritten assumption in my original question: that all transponders transmit the same power. >SNIP<
    Not only the power, but different installations may be more efficient radiators than others. There are a lot of older generation jets operating in my region and I consistently lose a small number of aircraft of one type, while another type flying though the same area is a solid signal.

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