Monday, January 19, 2015

FLARM filter 868 MHz

Another growing interesting activity beside the ADS-B is also the FLARM. FLARM is collision avoidance system for general aviation but targeting the light aircraft's and UAVs. The frequency where the FLARM is operating cover the range from 868.0 up to 868.6 MHz. Very close to GSM bands and using only 10mW of ERP is not easy to detect, so the receivers with high LNA and narrow filters are required generally.

The FLARM filter I build will not protect the receiver from the nearby GSM signal but can help a lot filtering the rest strong signals coming from the radio and TV broadcast towers. The filter is built around the same PCB used for the ADS-B and H-line filter not including the Bias-T parts. If the Bias-T option is required, the user should add a few missing parts to have this option available too.

The filter response is not so sharp and has the same characteristics as the ADS-B filter using the LTCC technology. The measured insertion losses on 868 MHz are not so high, measured 2.0dB with the descent rejection of the unwanted frequencies. The real measurements match very closely to the designed ones and on the next diagram the red curve represent the shape and the characteristic of the 868 MHz FLARM bandpass filter.

The filter is designed to pass the 868 MHz with the lowest insertion losses and at the same time to attenuate all other frequencies. he amount of attenuation is represented by the yellow bars. The longer bar - the higher attenuation. Vertical scale is 10dB/div and horizontal 300MHz/div.

Quickly going through the diagram, one may notice attenuation on the radio WFM frequency 100 MHz of 62dB, GSM frequencies 1870 MHz attenuated 41.7dB, LPD at 446MHz attenuated 47dB, Wi-Fi attenuated for 35.7dB. etc. TV broadcasting band around 550MHz is attenuated 23.5dB and there is also some attenuation on the radar surveillance primary and secondary frequencies.

FLARM frequency range is very close to the 800MHz and 900MHz UMTS bands and it is very difficult to filter out this signals using such a cheap and simple filters. The UMTS bands 5 (824-849MHz and 869-894Mhz) and 6 (830-840MHz and 875-885MHz) are filled with the various mobile operators and a network of powerful transmitters so the miracles are not possible. On the other side, using this filters you will be able to filter all other signals that may cause you a headache when receiving a weak FLARM signals from the aircraft's.


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