2m Tropo - But from which direction?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by MW1CFN, Oct 5, 2015.

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  1. MW1CFN

    MW1CFN Ham Member QRZ Page

    At the end of last week, we had a huge high pressure system over northern Europe, bringing outstanding tropo conditions on 2m.

    I'm not at all experienced in 2m DX, but I found myself involved in an interesting experiment, raising a question of where in the sky the incoming signals appeared to be coming from.

    I happened to be rebuilding my 5 element stick-'n'-wire quad, which had seen superb service until the XYL drove into it some time ago (tsch!), and had it on a stand for testing as the tropo was running. I switched between it and my SlimJim, to find the signals were essentially identical, no matter where the signals originated (England, Ireland, NI, Scotland, or Wales), or which direction the beam was pointed at.

    Having now confirmed the beam has been rebuilt correctly and functioning as well as it always did, the phenomenon clearly wasn't a fault with it or its feeder.

    I wonder if anyone who has spent years on tropo work could enlighten me - and, no doubt, many others - how these signals were propagating in order to have no discernible directionality?

    Thanks
    J.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I've been working 2m tropo for 49 years and have never, ever seen the phenomenon you describe.

    It's always been highly directional, and you aim the antenna right at the other station and not anywhere else. That's where they peak.

    Signals are greatly enhanced in strength during a temperature inversion, and more so during a duct, but their source direction doesn't change.
     
    VK2KRR likes this.
  3. N9DG

    N9DG Ham Member QRZ Page

    Was the stand that the antenna was on close to the ground? Close enough that it made the antenna lose any directivity, perhaps?

    I have been in the middle of tropo blobs before where the distances reachable weren't any more than my normal range, though everyone out to those normal distances was louder than normal. In those cases I could pretty much work the people I normally do out to those distances no matter which way I was pointed, but the antenna did still have obvious directivity.
     
  4. KK4YWN

    KK4YWN Ham Member QRZ Page

    my tropo experiences are the same as others: i spend a lot of time twiddling the rotator dial.

    i'm glad to see the vhf forum picking up lately. thanks for posting.
     
  5. MW1CFN

    MW1CFN Ham Member QRZ Page

    Wow! I was thus either mistaken in my assessment, or, more excitingly, there is a lower than 1 in 49 years chance per typical operator of experiencing this (provided the situation between the EU and US is comparable.)

    I had an inkling, ahead of any thought, that it might be due to high angle radiation, this being the only sensible explanation for 'overcoming' the beam's gain. According to MMANAGAL, the SlimJim has a pattern of about -3 to -9dBi in the 60 to 90 degree elevation range. The 5 element beam seems to have a similar but somewhat less even gain pattern of -8 to -12dBi in the 70 to 90 degree elevation range (there is a conflict between the plot axis and numerical output in MMANAGAL.) So, even allowing for the oddities of modelling, not much practical difference.

    So, whilst the possibility of operator error must remain, the initial working hypothesis is that the signals were arriving from high-to-very-high elevation angles. I can only explain the non-directionality by means of some kind of 'dome' reflecting layer, akin to sound reflecting domes in concert halls. Might my location relative to the signals be relevant? I am in the geographic centre of a circle encompassing the stations heard; it would be interesting to know if Irish stations, on a geographical fringe, for example, experienced anything similar. Like the best research, more work is required, so I will cross all appendages for more big high pressure systems soon!

    Yes, all highly speculative, and I agree that other tropo events here have been directional. That's why it was as much of a head-scratcher to me as anyone else!

    Thanks,
    J.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I assume you were using FM based on the SlimJim discussion, so you were likely using vertical polarization even for the beam.

    A fairly low vertically polarized beam will never be as directional as the same beam up several wavelengths above earth, so this might play into it a bit; but also "S" meters on FM receivers are notoriously worthless, so if you were looking at a meter to determine signal strength that's not a good way to judge much of anything on FM. Better to use "weak" signals and just listen to them, peaking for minimum noise.

    Every FM rig I have, or have ever owned in the past, has a meter that indicates S1-S2 or so on a 0.2-0.3 uV signal, and then "pegs" (full scale) at less than 30 uV. Some peg at 10 uV. This is only a 30 dB change from "no scale" to "full scale," and 30 dB isn't much. Receivers with linear detectors and AGC systems that really work will be closer to S1 = 0.2 uV and "pinning" = 10,000 uV or slightly more. That's more like a 90 dB change, which is more realistic and also more meaningful.
     
  7. MW1CFN

    MW1CFN Ham Member QRZ Page

    Yep, all correct! We're 300ft up on a clear ridge where extra height is less of an issue than other sites. Ground gain computations with the ARRL software yield high total system gain even at 3-4 metres above ground.

    Anyhow, will think some more and see if it ever recurs!

    J.
     
  8. K0RGR

    K0RGR Premium Subscriber QRZ Page

    The closest I can come to something like that is using an antenna that's cross-polarized with the signal being received, but even then, you should see some directivity.

    Years ago, I had circular antennas on an Az-El mount, and I tried elevating the
    antenna to see if it helped on distant weak signals. Sometimes, it did, but I was
    in California, surrounded by mountains on three sides, so raising it probably
    improved knife-edge diffraction or reflections from other mountains.

    Is the quad low to the ground surrounded by houses with metal roofs ?
     

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