FlightAware Discussions

Microwave towers and filters

I seem to have lots of trouble with the blue flightaware SDR with built-in filter experiencing interference at an otherwise very nice mountain top microwave tower site. Would an extra filter help, such as the one Amazon offers? Or should I use the orange flightaware SDR with external filter instead? I tried various gain settings already, and it worked a little bit at a very low gain setting. It worked fine at a different testing location. Any other suggestions?

You need good filtering. You can try if the FA filter helps, no need to change the dongle in that case.
The filter in the blue dongle is behind the LNA (amplifier) which can get overloaded.
With the FA filter in front it helps not overloading the LNA before the signal even gets to the filter thats built into the blue dongle.

But with such filtering needs i would go with the rtl-sdr LNA: https://www.rtl-sdr.com/new-product-rtl-sdr-blog-1090-mhz-ads-b-lna/
It has very good filtering.
Dongles with bias-t to go with this external LNA: https://github.com/wiedehopf/adsb-wiki/wiki/Useful-items-for-purchase#highly-recommended-combination-of-lna--dongle

Both dongles with those bias-t also have a metal housing which is useful in your case with strong interference.

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At a tower site with lots of strong RF emitters you may need to take several measures. They include

  1. You may need to shield the receiver system inside a metallic enclosure.
  2. A narrow band and highly selective filter between the antenna and the receiver may be required to reject the strong nearby transmitters.
  3. Shielding and/or filters on the DC power leads into the receiver enclosure.
  4. If you are using Ethernet to connect the receiver that cable and entry into the receiver enclosure may need filtering.
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Thank you, this is very good information.

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Thanks, very good info as well.

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Do I Need A Filter?

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GSM band ƒ (MHz) Uplink (MHz) (mobile to base) Downlink (MHz) (base to mobile) Regional deployments
GSM-850 850 824.2 – 848.8 869.2 – 893.8 North America (Canada & USA) Caribbean, Latin America
E-GSM-900 900 880.0 – 915.0 925.0 – 960.0 Europe, Asia-Pacific, Middle East and Africa

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The Uplinks (signals transmitted by phones) are weak.

The Downlink (signals transmitted by Cell/Mobile Towers) are very strong and cause problem to our receivers.

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