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Thread: Very Dim Newbie v What Am I Seeing

  1. #41
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    This is all very interesting. I just had another look at the noise in my area and I can see exactly what you mean. I see noise 870-889MHz (GSM 850) and 944-958 (EGSM 900). This coincides (obviously) with the tower downlink frequencies in the post you made previously with the GSM tables.

    I'm quite surprised I'm seeing the GSM850 band as I thought Australia (Oceana) was EGSM900.

    I've been posting that I have the Flight Aware Pro dongle but I actually have the Pro Plus. That wasn't deliberate, just didn't get right what I'd bought. I can see the Pro Plus has a filter but I can understand it is too broad and that the dark blue filter is the one to go for. I assume the filter can go anywhere prior to the dongle. Ideally I wouldn't want it to attach to the antenna as my coax at the moment leads all the way, unbroken, to the dongle. I would prefer to simply attach the coax to the filter and then screw the filter directly to the SMA fitting on the dongle.

    I've just seen in my Googling of what RSSI means (I saw it on my Dump1090 map) that a gain of -10 basically turns on auto gain. A great setting for the likes of me

  2. #42
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    Filtering and maximum theoretical ranges are really easy concepts to understand but this whole gain thing just won't sort itself out in my head.

    I've Googled the concept and understand the theory of 0dB. What I don't understand is why we don't just crank up the gain to max (turn up the volume) to see aircraft as far away as possible. I get that -10 sets the auto-gain going and is great for those who can't get their heads around these things (yes, I know I'm looking in the mirror) but why don't I just put a positive number in the settings to try and see stuff further away?

    I've attached some of the graphs since I set the gain to -10 at 0045 UTC. I haven't seen anything near the 200nm mark yet but that's probably a function of airline schedules more than what my set up is seeing but I was quite surprised by the CPU use increase (presumably as it cranks away to figure out the best gain to use) and the noise level increase.

    Anyway, I must get on with the garden or she'll murder me when she gets home
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  3. #43
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    Your scan was done using Pro Stic Plus with builtin filter, right? And still the GSM-850 & EGSM-950 reached the receiver. This means the builtin filter is not sufficient, and you need external filter.


    Arrangement of Dongle, Filter Coax and Aantenna:

    For Beginners - How-to Connect FlightAware Antenna, Filter and Pro Stick - Connectors and Cables Required

  4. #44
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    As far as I know, currently active sub 1GHz 3G/4G bands in Australia are still:
    850MHz – Telstra, Vodaphone
    900MHz – Optus, Vodafone

  5. #45
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    Your scan was done using Pro Stic Plus with builtin filter, right? And still the GSM-850 & EGSM-950 reached the receiver. This means the builtin filter is not sufficient, and you need external filter.
    Excellent point (why didn't I think of that)

    I like gadgets, so it's off to Amazon now for a dark blue filter.

  6. #46
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    In my opinion the filter will likely reduce range in your case.
    The noise isn't overly strong and no one can say for certain if it's overloading anything.

    (Overload problems are more likely with the Flightaware Pro Stick Plus because it has a builtin LNA that is sensitive to overloading.)

    Just get the rtl-sdr LNA and be done with it. It shouldn't be much more expensive and especially if you just have a standard dongle right now, the improvement will be much bigger than just buying one of those filters.
    The problem with the filters mentioned is that they reduce the signal as well.

    https://rtl-sdr.com/new-product-rtl-...mhz-ads-b-lna/

    It's a little more tinkering involved to get either a bias-t and supply 5V that way.
    Or you get their rtl-sdr blog v3 dongle as well and then you do some fiddling in the software to activate the internal bias-t. (guides available)

    Edit: Ok, just properly read the thread and saw that you are using the ProStick Plus.
    I'll still recommend the LNA, the performance per dollar you gain is just much better in my opinion.
    (Not sure how much the filter is in Australia though)

    Edit2:
    Also -10 is the highest gain setting. It's called automatic gain control, but that was designed for DVB-T signals, the original purpose of the rtl-sdr chipsets.
    ADS-B is too intermittent a signal, so AGC does not work and just switches the gain to maximum.
    Last edited by wiedehopf; 2019-06-11 at 07:36.

  7. #47
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    Glad you spotted I am using the ProStick Plus

    I will get the LNA as you suggest, because that is another gadget as well. I'm like a jackdaw and like collecting shiny things

    A gain of -10 is the highest setting. This is what I don't understand; why is a negative setting higher than a positive setting?

    I've had the gain set at -10 for about 7:45 now. The messages > -3dBFS is 11.6%. Max range in the last 6 hours has been 217.5nm (220nm within 24 hours i.e. when gain was not adjusted). The ADSB signal level looks like the attached graph. Going by what you said before I think I may have to turn the gain down a bit. Does this mean I now have to put --gain 49.6 or would your advice to be to get rid of the --gain instruction altogether?

    Sorry to keep asking silly questions. I have tried to Google the whole gain thing but I'm still puzzled why a -ve number gives a higher gain
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  8. #48
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    Because its DBFS

    '0' on the scale is maximum. -50 the other end
    https://en.wikipedia.org/wiki/DBFS

    -10 is just the trigger to make the app go 'auto gain' while other values are valid fixed gain levels on the scale.

    As '-' is a command trigger for most things linux, the values need to be entered in + increments.

    Per abcd
    The dongle gain can only be set to a value between 0 and 49.6
    The dongle does NOT accept any negative (-) value of gain.
    The gain value -10 is a special number. It does NOT sets gain to -10 . It SWITCHES dongle into AGC mode (Automatic Gain Control mode). In this mode the dongl dynamically adjusts gain according to received signal strength.
    Posts not to be taken as official support representation - Just a helpful uploader who tinkers

  9. #49
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    --gain 49.6 is the default setting with dump1090-mutability.
    So you could remove it altogether but i would have it in there explicitly, then you don't need to look up default values.

    The negative number is just a programming thing.
    It's not a normal gain value, in other words a special value.

    Special values are often represented by negative numbers if those are not in the range of values.

    So it's just a label, not a number.

    So in the logic of the driver it checks first if the gain is set to -10 and accordingly enables AGC for you.
    Otherwise it will set the gain you entered.

    Hope that makes some sense.

    And due to AGC being made for other signals than ADS-B, the automatic is dumb and just sets to the maximum possible.
    This maximum possible is due to bad driver programming not available as a discrete value (technically it is possible).

    If you use the LNA, you don't need the filter. In fact using the dark blue filter in front of the LNA would reduce LNA performance.
    But if you like trinkets anyway, go right ahead.

    In that case i'd just order the rtl-sdr v3 stick and use that with the rtl-sdr LNA. (https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/)
    Then you can have a second station with the Flightaware Pro Stick Plus and the dark blue filter.

    Or you can test which combination you like better

  10. #50
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    Thanks guys. Now the -10 makes sense. No wonder I couldn't figure it out when trying to increase gain!! However, nothing is wasted and I now know more than I did this morning about isotropic things, gain and (sort of) DBFS

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