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  • #31
    Originally posted by tintin View Post
    There's a GSM antenna at 700m from my ADSB antenna. I've bought this filter on the internet. There are even better ones with less attenuation in the passband but this is really cheap. Still waiting for it to be delivered... Will let you know how things improve (or not )
    I decided to spend the extra $15 on one of the others, its currently on a slow boat from China.

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    • #32
      An additional issue has come up that is puzzling me. I am located at the top of an escarpment at just under 900' and about 20 or so miles inland from the coast. Between me and the coast are the two main airports where I can pick up ground traffic at one and circuit traffic at the other.

      So I should be maxing out signals over the sea to the West, just look at the profile below:
      profile-west.png

      But no. I see diddly squat until inside 50 miles, yet to the North and East I have topography and other obstructions, yet see consistently out to 160+ miles. Can't even blame cell towers as they are to the East of me.

      I can only think is related to propagation of signals at 1090 Mhz. There can be significant low level ducting of VHF signals at times here, allowing me to open up VHF repeaters over 200 miles up the coast with just an vertical antenna, but could this ducting be working against me at 1090 Mhz by preventing reception of low angle signals? Anyone found anything similar?

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      • #33
        Originally posted by JasuK View Post
        Near our house, about 300 meters away, there is a huge GSM mast. As it is well know GSM signal makes lots of distortion and makes it almoust impossible to listen aircraft transponder signal far away.

        Have you any of you made shield or any kind of Faraday's cage to block signal coming from specified direction?
        The GSM signal is coming in through your antenna and overloadng he eceiver front-end. You need a good bandpass filter, not a faraday cage.

        A SAW filter may help, but a cavity filter will be better.

        http://shop.sysmocom.de/products/cf1090-kt30

        With a good filter you will get full performance despite the GSM tower. We have receivers IN GSM towers where the 1090 antenna is just a few meters from the BTS antenna, and there we use a cavity with 80dB attenuation + a Prostick with very good results.

        /M
        F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
        P-ESGR, P-ESIA, P-ESIB, P-ESGF
        mrmac (a) fastest.cc

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        • #34
          Originally posted by Stealth View Post
          Can't even blame cell towers as they are to the East of me.
          Cell tower interference is not directional anyways.

          /M
          F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
          P-ESGR, P-ESIA, P-ESIB, P-ESGF
          mrmac (a) fastest.cc

          Comment


          • #35
            Originally posted by MrMac View Post
            Cell tower interference is not directional anyways.

            /M
            Generally true, but I was asking for ideas on what could be causing a problem in my clearest direction, both in terms of line of sight and freedom from local RF sources.

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            • #36
              The Cell/Mobile signals are in frequency range 800 Mhz to 900 Mhz.

              It will be interesting to see how the following combination behave in the interfering Cell/Mobile Signal range 800-900 Mhz
              • Pro Stick (orange) without filter
              • Pro Stick (orange) with filter
              • Pro Stick Plus (blue) without filter
              • Pro Stick (blue) with filter

              I have now posted scan results for above combinations in the Filter thread:
              https://forum.flightradar24.com/thre...l=1#post100541

              Comment


              • #37
                Originally posted by abcd567 View Post
                The Cell/Mobile signals are in frequency range 800 Mhz to 900 Mhz.
                No, up to 960 MHz for GSM900 downlink. The 920-960 span are the hardest to get rid of and the closest to 1090 MHz.

                /M
                F-ESDF1, F-ESGG1, F-ESGP1, F-ESNK1, F-ESNV2, F-ESNV3 F-ESSL4, F-LFMN3
                P-ESGR, P-ESIA, P-ESIB, P-ESGF
                mrmac (a) fastest.cc

                Comment


                • #38
                  Latest I can find for Australian frequencies (presumably all are the start i.e. low end of band) are:

                  Telstra 3G 850, 2100
                  4G 700, 900, 1800, 2100, 2600

                  Optus & Virgin Mobile
                  3G 900, 1200
                  4G 700, 1800, 2100, 2300, 2600

                  Vodafone
                  3G 850, 900, 2100
                  4G 850, 1800

                  Apart from the 850/900 range, the 1200 could also come into play here.

                  The Pro Stick + is already way ahead of the basic DVBT stick I first used, so I'm hoping that the SAW filter on order will help a fair bit. Failing that its going to have to be a cavity.

                  UPDATE Feb 2nd.

                  Installed the SAW filter and it seems to produce approx 20dB reduction in the GSM signals. Yest to determine any range or coverage benefits. Below is a capture of a rtlpan session, graph is with filter in, the colour bar is filter in at the top and bottom, with no filter in the (noisy) centre.

                  filter.png
                  Last edited by Stealth; 2018-02-02, 05:38. Reason: Update

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