FlightAware Discussions

Comparison of different dongles for 978 MHz UAT reception

Am running dump978-fa 3.7.1 with wiedehopf’s graphs1090
from swapping to FA Orange dongle, tried gain 28

Would have liked to compare results for planes at the same range but different RSSI with the Orange FA . Tried gain 28 for a minute, didn’t see anything and increased gain to 38. Need a UAT or 2 to fly directly overhead and see if the signal degrades from too much gain.Would be nice if the UAT Range could be shrunk to 30 to spread the values. May just need to let it run longer so there is more data to compare. Also might need to adjust the gain.

Can try it if someone can tell me what to type. One my own can probably get something to break and then have to reimage to fix. But would be nice to get real time RSSI, or other indications to increase or decrease gain.

The feed to FA is not gonna work with that outdated version.

You can pull up the RSSI just fine in the current version, just expand the table with the button between map and table.

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Puzzled by results trying to use the orange FA Pro Stick for 978 Starting with RTL-SDR V3 at max gain, switch to Orange prostick due to comments about gain setting for Orange , tried 28 db 38 and 45 db gain. May be difficult to compare results at different times due to low intermittent UAT activity
The RTL-SDR seems to have more range of dBFS -5-26 The Orange Prostick seemed to possibly have more gain, compressed to range above -6

The other possibility was a bad cable. but after swapping cables the original cable seems ok. Is it possible the FA 26" antenna doesn’t like a hot attic, somewhere around 100F

Think a better test setup would be a 2nd pi running dump978-fa to allow comparing signals at the same time with either 2 identical antennas or splitter.

Is there something else I could be missing? Am within the return period for Amazon. Maybe need another sample?

If you have interference, the LNA can be a problem also amplifying the interference and making reception worse.

Therefore in some cases a stick without LNA will perform better, because the analog rtl-sdr front-end isn’t overloaded by the interference.
Lowering gain doesn’t always help in that case.

This doesn’t hold true for the 1090 MHz filtered preamps (rtl-sdr 1090 MHz LNA, uputronics LNA available for 1090 MHz and 978 MHz).
Those generally work well even with interference.

Just use the rtl-sdr v3 for 978 MHz.

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dump1090-localhost-signal_978-14d

This is what I am seeing w/ gain set at 20 (Spider, FA filter, 3" pigtail, FA Orange, Pi3b+) - so I’m pretty sure I have some more adjusting to do now that the rig is fully outside.

That’s probably good that you’re getting a range of dBFS values. After changing back to SDR V3 and no filter.

My ranking of 978 dongles. from worst FA Orange, no filter
Silver Rtl-sdr v3 no filter max gain n best FA orange with 1090 filter

Late to the party here, but basically this - the 1090MHz demodulator is very tolerant of over-amplification because the modulation is basically AM, but the 978MHz demodulator is not because the UAT modulation is FM and over-amplification / clipping tends to mangle the frequency domain.

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Switched from rtl-sdr v3 to FA orange with external filter Fri 1400


Filter seems to have stretched the DbFS range from -3-6 to -5-10. Possibly with no filter the Orange FA is going into overload and no dBFS value is produced?

You really need to turn down the gain when using the FA dongle.
Having the weakest signal around -25 or -30 is perfectly acceptable.

is restart required after gain changes with piaware-config uat-sdr-gain xx ?

I would expect the opposite. AM modulation is sensitive to clipping, FM modulation actually require clipping/hard limiting at demodulation, to eliminate any AM component that would appear after decoding as FM modulation.

https://www.electronics-notes.com/articles/radio/modulation/fm-frequency-demodulation-detection-discrimination.php

yes, restart dump978-fa

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ADS-B uses PPM, which just requires you can detect pulses. As long as the signal is not so strong that it obliterates the difference between a pulse and no pulse it should be decodable. You aren’t actually trying to demodulate anything from the signal, only detect that it exists - the important part is the time domain.

That’s basically the same as FM modulation. You hard limit the FM sinusoidal (to become pulses with 50% fill) and then, instead of counting them, you average the number of pulses in time domain to get the low frequency modulation.

Found a review of FA Prostick and Prostick Plus where they manage to overdrive it with 1090 signal. Also explains why LED is not lighting up (deleted) https://www.rtl-sdr.com/flightaware-prostick-vs-flightaware-prostick-plus-review/ rtl-sdr block diagram

from http://aaronscher.com/wireless_com_SDR/docs/rtl_sdr_block.jpg
Another review where they test the Prostick and detail the preamp specs https://www.rtl-sdr.com/review-flightaware-ads-b-rtl-sdr-lna-positioning/ Prostick Preamp may have 20 dB gain

Keep in mind that we’re talking about a fairly limited SDR here; you can’t do arbitrary stuff in the analog domain, the input the demodulator gets is post-ADC complex baseband, and you have 2 (complex) samples per bit to work with. The phase component of the complex samples becomes very important, and over-amplification mangles that.

(also it’s narrowband FM - IIRC h=0.6 for UAT)

Quick question regarding gain on 1090 versus 978.
I seem to be getting less signal range on 978 as opposed to 1090 MHz. Wondering why this would ocour?
On my test station I am using the blue FlightAware 1090 stick with adsb fiter and lna and get signals from -2 to -24.5 dBFS. On 978 I am using an orange FlightAware USB stick with a light blue Flightaware filter, and a similar antenna tuned for 978 of course. Range seems decent (except today it’s raining). I do note I get similar high end signal readings around -2dBFS but the low side only goes down to 18dBFS. I am running an LNA/ceramic filter up at the antenna on 978 to make up for feedline loss (50’ LMR-195). removing it I still only get signals down to -18, but I get a bit less range.
Playing around with gain settings doesn’t seem to help, current settings seem to be best (16 for 1090 and 24 for 978MHz).
Is this normal?

Are you sure that with a gain of 16 for example you will not get weaker signals?

Personally i would recommend dongles without internal LNA when using an external LNA. That’s mostly my gut feeling but i can imagine the internal LNA becoming saturated and causing problems.