ad: vanity

857D for 2m weak signal work / digi modes

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KO4LZ, Jun 26, 2020.

ad: L-HROutlet
ad: l-rl
ad: L-MFJ
ad: Left-3
ad: Left-2
ad: abrind-2
ad: Subscribe
  1. KN4ZKT

    KN4ZKT Ham Member QRZ Page

    Sorry if I'm not clear. When I mean an opening I'm talking about tropospheric ducting. There was a pretty good opening yesterday in the Southeast.

    When I say dead band conditions I mean no special enhancement, like ducting, on 2m. You don't need any opening to make contacts on 2m, but it helps if your antenna isn't in the clear or very large.

    I presume some of your contacts were via meteor scatter? A 1,000 mile contact via troposcatter would be impressive. I presume these were digital contacts?

    I'd highly recommend reading this article, very informative: https://www.qsl.net/oz1rh/troposcatter99/troposcatter99.htm
     
    Last edited: Jul 6, 2020
  2. KO4LZ

    KO4LZ XML Subscriber QRZ Page

    Nope, all of the ones I mentioned were FT8.
     
  3. KN4ZKT

    KN4ZKT Ham Member QRZ Page

    You made a 1,000 mile FT8 contact on 2m? with who?
     
  4. KO4LZ

    KO4LZ XML Subscriber QRZ Page

    KE0SIS_KO4LZ_2m_FT8.png
     

    Attached Files:

    PY2RAF likes this.
  5. N4OGW

    N4OGW Ham Member QRZ Page

    I have also wondered about noise blankers and WSJTX since I often get power line noise on 6m and 2m.

    I have tested WSJTX FT8 decode on HF (1.8 MHz) using two K3s running separate WSJTX instances. Radio 1 transmits at very low power into a dummy load and radio 2 receives. I was able to make weak enough signals to see the minimum reported SNR of WSJTX. Probably you could do something similar at 2m. I don't have two 2m ssb radios to try it, although I could try it on 6m.

    If the receiving radio was on an actual antenna (I just used a dummy load on both radios) where you get noise you could test the effect of the noise blanker. The problem might be making a weak enough transmit signal in that case.

    Tor
    N4OGW
     
    KO4LZ likes this.
  6. N4OGW

    N4OGW Ham Member QRZ Page

    Looks like you caught the 2m Es opening yesterday- nice! I was calling cq and listening but didn't hear or work anyone. Given where the center was, I might have had propagation to the center of Lake Superior :)

    Tor N4OGW
     
    KO4LZ likes this.
  7. KN4ZKT

    KN4ZKT Ham Member QRZ Page

    Nice work. Good job catching the e-skip opening.
     
    WD4ELG and KO4LZ like this.
  8. KO4LZ

    KO4LZ XML Subscriber QRZ Page

    I do have two 2m SSB radios, a box full of fixed and variable attenuators, splitters/combiners, and a vector signal generator capable of creating any arbitrary level and type of noise (AWGN and/or impulse noise). Radio 1 would be set up to send FT8, signal, then attenuated to the desired level and combined with the noise output of the signal generator before being fed into Radio 2.

    As test scenarios go, this one is pretty easy to setup and would allow a direct and controlled comparison of decode performance by signal level, with or without the presence of Gaussian (for testing NR) and/or impulse noise (for testing NB) at any arbitrary level for all three components. One could also study the effects of audio settings, etc. If I ever have a week where I'm not setting up RF test scenarios for work, I'll see if I can get this going and write up the results. :)
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Obviously need to define mode and BW for this to have any real meaning. The "12 dB SINAD" measurement on FM using a fairly wide BW doesn't relate well to weak-signal work which is all linear detection and much narrower bandwidths.

    For the digi modes, "MDS" (minimum detectable signal) is probably the only thing that truly matters, and thanks to Joe Taylor's fine work, digital signals can be decoded below the "ear copy" MDS.

    I have an old FT-736R (c. 1987), a 10 year-old FT-857D (mobile use only) and a pretty new IC-9700. Using a simple coaxial switch and a single antenna, I compare their receivers in real-life (not laboratory) situations by tuning in a weak but steady beacon signal (like N6NB/B which is 65 miles away, blocked path, QRP, solar powered) and found:

    FT-736R: Rock solid copy, easily readable (CW), 500 Hz BW (no preamp available, the front end is always engaged)
    IC-9700: Rock solid copy, not quite as readable as with the 736R but good enough (estimated ~2 dB more noise than the 736R), 500 Hz BW, preamps on
    FT-857D: Marginal copy, detectable but not easily readable, 500 Hz BW

    Not seriously scientific, but repeatable and demonstrable.

    Easily noted difference, though: FT-736R, attach antenna pointed in any direction and noise level audibly increases with S meter going from "0" to about "S1."
    IC-9700, attach same antenna, noise level increases very slightly but sorta detectably, S meter doesn't change.
    FT-857D, attach same antenna,noise level doesn't change, S meter doesn't change.

    So, that's a really easy test!:)
     
    WD4ELG and KO4LZ like this.
  10. KO4LZ

    KO4LZ XML Subscriber QRZ Page

    @WB2WIK - First and foremost, many thanks for running those tests and sharing your results / observations: they are very helpful and much appreciated

    He says, and then runs an experiment that involves copying signals by ear :)

    The vast majority (all?) of receiver manufacturers specify sensitivity in terms of 12dB SINAD, not MDS. It's worth noting that the ARRL lab also uses test equipment when testing transceivers and specifies sensitivity in terms of SINAD.

    I completely agree with your observations and ad hoc test methodology -- it's an excellent approach that, as you say, takes very little effort / equipment and produces useful information. For 99% of hams, your method is the one that I would recommend wholeheartedly.

    On the other hand, having access to equipment that can very precisely measure SINAD (I'll re-run the test for CW/SSB) or many other RF parameters, I will still claim that this is a better approach, albeit one that few amateurs have access to .. hence the reason I posted the results.

    Again, thanks for all of the effort and helpful information: it's contributions like yours that make QRZ such a great place :)

    73, Paul, KO4LZ
     
    Last edited: Jul 7, 2020

Share This Page