An updated internal and external ADS-B comparison

It’s been well over a year since I moved my external ADS-B receiver to the top of my Alimast and it’s been performing very well, regularly sitting in the top five of the UK receivers according to the FlightAware statistics page and in the top ten world wide on the PlaneFinder sharers list.  A good location is paramount when it comes to tracking aircraft and I am ideally situated, being very near to major airways into the UK and also having good reception from aircraft in north west Europe.

Earlier this year I also made a slight modification to my internal tracker.  The aerial is the same, a two element homebrew J-Pole collinear but I’ve raised it up by about three feet by cable tying it to a rifle cleaning rod.  As well as increasing the height, this is a metal rod so would have effectively improved the ground plane as well.  Doing this has significantly increased the number of reports that the system receives each day.

For details on the construction of this J-Pole aerial, including all the measurements you’ll need, scroll down to the comments section of this entry where I’ve posted everything you’ll need.

Both receivers see approximately the same number of aircraft per day which is what I’d expect but the external system receives around 20% more position reports.  I put this down to the fact that the aerial is higher and in the clear but the internal receiver still works really well.

This fancy display (which you can drag from side to side) will show you the heatmap of the two receivers.  The main difference is that the internal system has a big gap in coverage in the direction of the chimney stack.  Apart from that, they’re fairly similar.

External
Internal
ADS-B statistics, December 2018 (averages at bottom)

ADS-B statistics, December 2018 (averages at bottom)

The two installations are very different from each other and I think this demonstrates that an internal receiver can work well,  If you get get a decent aerial mounted on a good ground plane with no coaxial cable, you can make a useful aircraft tracker.  Building your own aerial is cheap and effective.

ADS-B receiver mounted on a rifle cleaning rod

ADS-B receiver mounted on a rifle cleaning rod

ADS-B receiver mounted at the top of my mast, just below the Hexbeam

ADS-B receiver mounted at the top of my mast, just below the Hexbeam

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8 Responses to An updated internal and external ADS-B comparison

  1. Dan Streeter says:

    Hi, Do you have any details on how to make the small J-Pole you have there, it looks great and I’m after something to stick in my loft thats a bit of an upgrade from my current antenna.

    • g6nhu says:

      Hi Dan,
      This is what I worked from:

      j-pole design

      73 Keith.

      • Dan Streeter says:

        Amazing, thank you – Just need to find an upside down Y calculator and I’ll get going on making it this week and see what results I get! haha

        Being a COMPLETE newbie to anything related to radio related stuff, would you mind pointing me in the direction of the calculations I need to use to get maybe a ‘cm’ length for these values.

        I’ve read a few tutorials and have seen various calculations to do with the speed of light and 1090MHz then dividing the resultant of that by 2, or 4 depending on the antenna type – but my knowledge to figure that stuff out, or even what type of antenna this would be classed as is very much non existent!

        Thanks in advance for your help.

        Dan

        • g6nhu says:

          Hi Dan,

          I did a quick search on frequency to wavelength (the upside down Y is lamda which is the symbol for wavelength) calculator and this is what I came up with. The frequency is 1090 MHz and when you hit calculate, you’ll get a wavelength in metres. From that, you can calculate how long each length is.

          Does that help?

          73 Keith.

  2. Dan Streeter says:

    Thats great, thank you – so a 1090MHz wavelength is 0.27503895m, which means the differing values in that diagram should be:
    .5 Y == (0.27503895 * 0.5) = 0.137m == 13.7cm
    .2 Y == (0.27503895 * 0.2) = 0.055m == 5.5cm
    .25Y == (0.27503895 * 0.25) = 0.068m == 6.8cm
    Total antenna length of 39.7cm

    Do you think it matters how many curls/loops/turns there are in the middle curly bit?

    Lastly – (I think) – how have you terminated/connected the base of the antenna to the connector? Is the small horizontal section just soldered to the tip of the central connector?

    Thanks again, I’ll be sure to update you on how it goes! =)

    • g6nhu says:

      Hi Dan, yes, those measurements look about right to me. I think I wound 16 turns on a pencil so about 5mm diameter.

      The distance between the two vertical elements is around 6mm and the feed point is with the centre of the connector attached to the main vertical element, 6mm from the bottom and the ground of the connector to the shorter vertical element, again 6mm from the bottom.

      I’ve managed to find another diagram (with measurements), this should make it crystal clear 🙂

      Two element J-Pole

      • Dan Streeter says:

        Fantastic, thank you – I’m going to be having a go at this over the weekend.

        • Dan Streeter says:

          Oh wow…
          Finished it off – had a bit of trouble soldering it the way I wanted so bodged the ground solder to the ground on the connector, so ended up wrapping some wire right around it – might go back and fix another day – wanted to test it out really…

          Got a plane tracked downstairs on my desk so guessed it was working so went and threw it up in the attic.

          WOWZERS!!!!
          I’m picking up planes >160nM away over the coast of France and actually picking up all planes that I can see on FlightRadar24 also!

          Currently picking up 31 planes, I think thats the most I’ve ever had!

          Check out https://danstreeter.co.uk/g/flight_radar/dump1090_screengrab.png !!!! Its insane!

          Thank you for your post, and your guidance.
          I’m going to add a post (very low traffic and boring) site with some photos of my build and I’ll link back here if thats ok?

          Thanks again

          Dan

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