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Q&D Analysis of Why We Can Use Turnstiles and Eggbeaters for LEO’s - first cut article

Discussion in 'Satellite and Space Communications' started by KD6RF, Aug 29, 2017.

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  1. KD6RF

    KD6RF XML Subscriber QRZ Page

    I probably should have shut off the noisy cable box. It would have made nice pix, but it doesn't interfere with the data we want to measure, or with the audibility. :rolleyes:

    WX-2C wav file ===> 708 MBytes

    The data starts with WX-2C rise at 0 degrees and continues to set at 0 degrees. Date and time of the recorded pass is: 2017-09-10 starting at 23:55:01Z. Approx 145.797 MHz.

    Below is when we can start seeing my transitted 10 Watt signal at around 10 degs.


    AO-7 wav file ===> 972 MBytes

    The data starts with AO-7 above horizon at 10 degrees and continues to 10 degrees before set. Date and time of the recorded pass is: 2017-09-11 starting at 10:28:58Z. Approx 145.953 MHz. - HDSDR_20170911_102858Z_146000kHz_RF.wav

    Below is when we can start seeing my transitted 10 Watt signal at around 17 or so degs.


    You can play these diretly in HDSDR and do any measurements you want.

    You can also hear WB4FWQ calling CQ around 145.804, and compare peak signal levels. This starts bout 1/2 the way through the WX-2C data!!! Note that 1 watt is around the same peak value for Tom's signal.


    Above you can also see the transmitted noise floor - and that I am off to the side a bit.
    Thought about giving him a call, but SSB is kind of a pain w/o doppler correction :mad: But if you see me on and need EM22, give me a quick call.

    HDSDR DL ===>
    Last edited: Sep 12, 2017
  2. WD9EWK

    WD9EWK Ham Member QRZ Page

    This answered my earlier question. Thanks for posting those large recordings.

    You may consider starting with 5W when doing CW, instead of 10W. That would put you on the same level as many who use FT-817s for CW, which seems like a more relevant test of your antennas. Especially for AO-7, which seems to tip over to mode A (too) easily by CW signals that overpower the transponder.

    BTW the other satellite is XW-2C, not WX-2C. XW = XiWang, Chinese for "Hope".

  3. KD6RF

    KD6RF XML Subscriber QRZ Page

    Doh! Must have weather (wx) on the mind. Tnx for the correction re XW-2C.

    Also tnx fior the FT-817 5 W idea. The good news re power level is that we can just take 3 dB off of any measurement where my TX power is 10 W. I have power level and elevation angle embedded in the CW on the recordings.

    Will be doing some low angle passes to fill in the data for the XWings, and for AO-7. Also need to do FO-29.

    One thing that isn't illustrated by the recordings is how attenuated the noise and interference at my location is - one of my claims/ideas is that the ground-mounted omni is going to pick up less low-angle noise than the typical elevated installation. The use of elevated omni's, near to noise sources, may be one of the reasons that they are aren't that popular.

    As they say, "If you can't hear 'em, you can't work 'em..." If a noise source in one's home is parked right square in a back or side lobe of an elevated omni (or yagi), nose pickup will be a problem !!! (There' no doubt that this is one of the reasons we see some folks cranking up the TX power in order to try to make up for a local noise problem on the downlink :eek::eek:)

    For example, above you can see spectral lines from the crummy cable TV box and wiring. With an elevated antenna (even the yagi), those lines of interference are mucho stronger.

    Of course ground mount comes with it's own potential issues, such as blockage from structures, etc. However, the noise attenuation from low angle sources is pretty dramatic, and needs to be documented and measured.
    Last edited: Sep 12, 2017
  4. KA9P

    KA9P Ham Member QRZ Page

    Depends on what you are modeling. If you are modeling a communication" link there are lots of variables an antenna model doesn't cover. Polarization, location of the other station WRT the bird, the capabilities of the other station, path loss.

    You can model a yagi on range, and see a few dB difference, but the other variables swamp that in most case, IMHO.
    WD9EWK likes this.
  5. KD6RF

    KD6RF XML Subscriber QRZ Page

    So it took a while to figure out the most meaningful way to present the results of the XW-2C data. (Note that I also recorded and analyzed another pass on the 18th.)

    I finally settled on presenting a "normalized" graph, where everything is set relative to 0 dB SNR. Where 0 dB SNR is the minimum discernible SNR to be able to copy a CW signal, at 1 Watt uplink power.

    IOW, if we are instead using 5 Watts instead of 1 watt, then add 7 dB. If we are using SSB instead of CW, subtract about 9 dB. If your background noise is better than mine by 4 dB, then your SNR (all other things being equal) then you will have a 4 dB better SNR.

    Stated another way, a better SNR means that the sat is usable a few degrees earlier, a worse SNR means a few degrees later.

    An interesting feature is that the rise in SNR is so fast with these XWing sats, that a few dB here or there only makes a few degrees difference.



    It turned out that the data from the 10th was fairly "clean" and smooth as shown above, with perhaps 1 or 2 fades (fades are not shown). while the data from the 18th was "bumpier" for some reason, and had 2 or 3 fades (also not shown).

    Even so, the SNR's are within a few dB, and elevation angles where the sat becomes usable with the turnstiles is within a few degrees. And as said in the OP, these sats are easily workable with the omni's (ground mounted) above 20 degrees, with the noise floor conditions here.

    AO-7 data (will be posted later) is more like 25 or 28 degrees.

    Of course, the background noise floor at another QTH is the big unknown here! - at any rate we can see that there's a decent margin of around 9 dB assuming SSB and 5 watts. IOW, one would still find omni's usable with a noise floor 9 or so dB worse than what I have here. And more like 18 dB or so if CW is used ;)

    Again, one of the reasons for good noise floor performance of this system is the ground level mounting of the turnstiles at some 40 feet or so away from home noise sources, and those noise sources being greatly attenuated by the antenna's very low gain at low elevation angles. Contrast this idea with an elevated omni or even yagi, where local noise sources may be in a medium gain side or back lobe.

    Naturally, if noise sources at another location don't fit this mold, then performance will vary. More data on that later as I measure the noise floor here with the trunstiles as a function of height.
    Last edited: Sep 21, 2017 at 1:40 AM

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