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Thread: Start of my RPi journey

  1. #81
    Flight attendant
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    Quote Originally Posted by wiedehopf View Post
    Get yourself some filtering and amplification: https://www.rtl-sdr.com/new-product-...mhz-ads-b-lna/

    Then you can use your old coax (with the amplifier close to the antenna).
    The filtering is probably what you need the most with a discone antenna.
    So, the filter in the flightaware stick not good enough...or in the wrong place?

  2. #82
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    The filter only comes after the internal LNA, which is often too late.
    With a discone i suspect the internal LNA will get overloaded and produce noise on all frequencies.

    But obviously it's also good to help with your coax to have some amplification near the antenna.
    Not sure if you can access the coax after maybe 2 to 5 m of length and put the LNA there without going onto the roof?

    You can check the situation with the noise like this: https://forum.flightradar24.com/thre...ignals-In-Area

    Even if the noise is not a problem, it will help greatly to compensate the coax loss even if you can only mount it half way between receiver and antenna.
    (Optimal is closer to the antenna, but i'm sure you are aware if you have done radio stuff before )
    Last edited by wiedehopf; 2019-10-17 at 08:01.

  3. #83
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    Hi wiedehopf, yes I can get to the coax, approx. 3 metres directly underneath the present mast in the roof cavity, so my wife will be happy i am not on the roof...At present, this RG6 feeds my airbands scanner. I simply removed the BNC fitting from the scanner input and connected it to the Flightaware dongle. I had a look at the link...Do i need a filter etc... People seem to put them, in front of the dongle, or near the mast or....Maybe after all this manoeuvring to use my discone, I should get/make a better antenna. If i want to fit a filter on the coax in the roof cavity, I will use F connectors.

    My immediate plan is to create the four leg spider (got materials today) and try it in my office to see what reception it gets. I suspect though, I may end up on the roof and fit it up there next to my discone. Bring its separate RG6 into the roof cavity. Then "chop" the antenna cable that connects to my scanner or RPi, so that I can use this lead for both scanner and ADS-B antenna by swapping over connections in the roof cavity. I will use F connectors.

    When I looked at the graph/stats for the signal, (see my post page 8) it seems very tight compared to others that span the entire graph. Maybe because of the Discone (https://www.diamondantenna.net/d130j.html) with such a wide band and very low gain!) Would providing a different whip length (68mm) help the ADS-B signal on the discone? I have a spare set of the radial arms which I could also cut down to suit, turning my scanner discone into a ADS-B antenna?
    Last edited by POR911; 2019-10-18 at 04:40. Reason: Later thought

  4. #84
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    Yeah making the discone more like a spider antenna will probably help.

    The signal spread in the graphs can be low because of noise for example.
    Or there may just not be enough signal left at the end of the coax.

    In either case the amplifier/filter i linked should fix both problems.

    Yeah a self-built antenna without it being on the roof can also get good range.
    Might be that the coax you are using is just killing too much of the signal.
    Building your spider and trying it in the office will be a good test for that.

  5. #85
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    The RPi is working great, and I am now feeding Fr24 following very helpful advice on this forum. I received my welcome email from FR24 with my sharing key and radar code. I am not sure how the free business subscription kicks in. FR24 provided a 'free business subscription' link, (https://www.flightradar24.com/premiu...p?account=free) but it seems to take me to the usual subscription page.

    I was also automatically resubscribed to the Gold subscription in May 2019. Is the balance refunded or frozen?

    Because I will be changing antenna's, that will no doubt cause disruptions to the feed, what leeway is given for feeder being down before you get chopped? maybe always keep a basic antenna connected, while setting up "improved" antenna, and do a quick swap over!!

    Over the next month or so, I will be experimenting with different antenna's and setups. I will report back here with results. First tests will be:
    1. Using present Discone. It see's most a/c that FR24 indicates, though usually only within 120kms. I will use it for a few weeks.
    2. Using present discone setup, but changing its single 900mm whip, 8 x 280mm horizontal ground plane and 8 x @ 45 degree 820mm radial elements to ADS-B length. Is there a benefit of using full 275mm wave length or 1/2 or 1/4? Certainly, 1/4 is easier to fabricate.
    3. As my discone will become an 8 leg spider, I don't see the benefit of building another?
    4. Using some sort of amplifier/filter close to the antenna mast.
    Last edited by POR911; 2019-10-19 at 01:49. Reason: later thought

  6. #86
    Flight attendant magnum's Avatar
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    For me the upgrade of my basic account to Premium took a few hours.

    You will be notified once you stop feeding. After some days without action from your end they will lower your account.

    I had my device down for maintanance and reinstall for half a day without any issues regarding the account

  7. #87
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    1/4 wave is the closest to 50 Ohm impedance.

    Full or half wave ground plane antennas would need impedance matching to work well.

    From practical experience: You'll need a whip a bit shorter than the theoretical 1/4 wavelength (69 mm)
    64 to 67 mm whip length has been the sweet spot for me when experimenting with 1/4 ground plane antennas.

    Oh to make your discone a spider antenna, you'll have to get rid of the horizontal elements, the disc.
    You only want the whip and the slanted ground plane.

    Otherwise it stays a discone and you should follow the numbers for discones.
    They usually have a lowest and highest frequency given, 1090 MHz should obviously then be somewhere in the middle of that range?
    Last edited by wiedehopf; Yesterday at 08:52.

  8. #88
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    Thanks magnum, I contacted FR support and they said because it was the week end, it can take several hours or more to swap over to business account, due to manually being applied. I was to contact them next week if nothing happens. They also said because I had a current Gold subscription, it was important that a I cancelled it, otherwise, regardless of the free business subscription, the gold subscription would be renewed automatically. Also, even though cancelled, the gold subscription, having no pro rata cancellation, means it does not take effect until your renewal date...so I will have both Gold and a business account current until 2020.

    Thanks weidehopf, I am sure you know all this, very interesting to me...http://cablesondemandblog.com/wordpr...coaxial-cable/ So, it is saying, if you can afford it, and have space, use 50 Ohm. Do not mix impedance if at all possible. When I looked at my BNC fittings, they are all 50 Ohm, my RG6 is 75 Ohm? So does this mean err on the 50 Ohm side, as less resistance etc?

    Apparently, my discone has a 'loaded coil' (what ever this is) under the top whip? I thought I would remove the loaded coil barrel and make some fitting for the 1/4 wave length whip. I will start at 69, and cut it back 1 mm at a time and see if it improves after a days use. OK, ditch the horizontal elements. Create the 45 degree ground planes at 69mm. OK, next week or so job.

    Is it the accepted thing that full size wavelength is not the way to go for ADS-B antenna's, for amateurs playing without proper tuning equipment...and of course lots of knowledge and experience.

  9. #89
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    It's not resistance, it's impedance.
    Going from one impedance to a different impedance creates some attenuation.

    It's very common to use 1/4 ground plane antennas or 1/2 wave dipole antennas. (which basically are both 1/2 wave antennas)

    Using a 5/4 wave for example requires impedance matching to be as good or better as a half-wave dipole: https://en.wikipedia.org/wiki/Dipole...arious_lengths

    Going from 75 to 50 Ohm and back isn't that bad, and many have used RG6 coax successfully for receiving ADS-B.

    If you want best reception, just get the filtered LNA i mentioned very close to the antenna and the coax shouldn't be a problem.

    To be honest i'd just build the quick spider on the end of some RG-6 to avoid all connectors:
    https://forum.flightradar24.com/thre...g-No-Connector

    That way you can also leave you discone intact.

  10. #90
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    Hi weidehopf, I agree. Easier and better to buy/create a ADS-B antenna built for purpose, ideally with a some sort of filter and amp close to the antenna and leave the discone in working condition. (My scanner breathes a sigh of relief) I was up in the roof cavity yesterday and see that I have already have a PAL connection between my discone coax and lead in coax to the Scanner/RPi. So, not too difficult to run a new RG6 from a 4 leg spider, and connect to the discone riser mast. As the LNA wants 5v power, I need to work out the safest way to do this, as leaving a 240v - 5v power supply transformer unattended in a roof cavity not my ideal choice. Will this do the job next to the RPi, https://www.ebay.com/itm/172460255780

    I had also asked on an air band forum about using my discone for ADS-B. Their reply was, don't bother trying to use the discone for optimum 1090MHz because its top hub that holds the elements is already longer than the 1/4 wave. They agreed with you, "Ideally you have a high gain antenna, then a low loss filter and preamp right at the antenna to set the noise figure of your system and the preamp can then drive a long run of coax without further degradation." While FR24 business feed account being sorted, I will obtain the https://www.rtl-sdr.com/new-product-...mhz-ads-b-lna/ and build the 4 leg spider.

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