NVIS antenna, NVIS distance, but not NVIS

Discussion in 'General Technical Questions and Answers' started by KK5JY, Jul 14, 2018.

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  1. KK5JY

    KK5JY Ham Member QRZ Page

    Over the last few days I have been copying a lot of stations on 40m and 30m that I don't think I should be able to hear. Many of them are very strong.

    The distances involved are between roughly 60 and 300 statute miles, well within the "skip zone." Distribution within that range appears to be rather uniform. Stations at 100 miles appear to do as well as those at 200 miles.

    When I see stations coming in from such distances, I check the ionograms closest to me. None of them have (yet) shown foF2 support for NVIS anywhere near 40m or 30m, and none of them show foE support, either. These frequencies are far too high for ground wave over the distances seen. So I can't think of any propagation modes that can explain this.

    I have written to a few of the stronger stations to inquire about transmit antennas they are using, and almost universally, they are using antennas you would expect with NVIS -- low dipoles. One particularly strong station, at about 150 miles, was using a 40m antenna at 66', which is a little high for solid NVIS coverage on that band.

    If this was just one or two stations for an hour on one day, I'd chalk it up to "sporadic something-or-other" and move on. But this has been going on for days, for most of the day, and I suspect if I watch it for a while, it will continue.

    An interesting side-note -- I'm not seeing this on 20m. When I look at the map of the stations I have heard on 20m (so far), there's a very satisfying "skip zone" hole around my location. The unexplained propagation seems to be specific to 30m and 40m.

    Any idea how these signals are getting to me? When I look at all my texts on propagation, they tell me nothing that could explain this.
  2. K5URU

    K5URU Ham Member QRZ Page

    The Austin ionosonde's foF2 trend chart hasn't received updates in a few days, but within the prior week, the foF2 here has been peaking well above 7 MHz fairly often during daylight hours, and even above 10 MHz a few times. Of course, that's straight up, so it would seem that some NVIS activity would be reasonable on those bands. The ionograms only run once every 5 minutes, and I don't know what threshold they use to determine which frequency gets the high mark in each sweep, but would this be enough to explain your observations?

    Unscientifically, from my own anecdotal observations, I feel like my memory and experiences tell me that actual amateur radio communications are often possible at slightly higher frequencies than the ionograms indicate. Maybe we should think of ionograms more as a floor of confirmed results, rather than a limit on possible results. Does that make sense?

  3. W6RZ

    W6RZ Premium Subscriber QRZ Page

    Those large upward excursions of foF2 aren't real. They're caused by the plotting software getting fooled by sporadic E returns.
    KK5JY likes this.
  4. K5URU

    K5URU Ham Member QRZ Page

    I misspoke. I understand that the F2 layer isn't supporting vertical returns at those frequencies at this point in the solar cycle. I should have repeated the phrase. "... the foF2 trend chart here has been peaking..."
    Regardless of which layer is doing the work, the point is that something up there above Austin is reflecting the test signal back down to the receiver, and this result is visible in the foF2 trend chart. KK5JY hadn't seen any indication of NVIS-type reflections in his area, so I wanted to bring this to his attention.
  5. AI3V

    AI3V Ham Member QRZ Page

    You are confusing the muf and fot of a vertical incedent ionagram, with the muf and fot for stations hundreds of miles away.

  6. K5URU

    K5URU Ham Member QRZ Page

    I'm not sure if you're replying to me, but if you are...
    I don't think I'm confused, but maybe I am. :confused: Perhaps I didn't explain my thoughts well. I'll try again with more detail. Let me know if I'm missing something.

    The generic term Maximum Usable Frequency (MUF) is an estimated value derived from the Critical Frequency of the F2 layer (foF2), as measured at a vertical incidence. Because the MUF depends on the angle of incidence, and the angle of incidence depends on the distance between stations, MUF is a meaningless term unless a distance is specified. Usually, when people refer to the MUF without specifying the distance, they mean the MUF for a 3000 km distance between stations. As the distance between stations is reduced, the MUF decreases as the incidence becomes more and more vertical. At a distance of zero, the wave would be at a vertical incidence. Again, this is how the Critical Frequency is measured in the first place! foF2 is MUF(0 km).

    Because NVIS literally refers to a Near-Vertical Incidence Skywave, the Critical Frequency (foF2, which is actually the measured MUF of a true Vertical Incidence Skywave) is usually an excellent tool for evaluating the possibility of NVIS communications up to several hundred miles away. The MUF(400 km) is usually only a few hundred kHz higher than foF2. So at a glance, the foF2 can tell you if there is no chance, some chance, or a good probability of a usable NVIS path over a few hundred miles. For example, if the foF2 is over 7.3 MHz, you can expect great results with 40 m NVIS at a few hundred miles.

    Even when Sporadic E fools the ionosonde, the incorrect foF2 data can still be useful to some extent. Sometimes, the E layer is so strong that it blocks or interferes with returns from the F2 layer. In those cases, one of two things happens. One possibility is that the software cannot solve for foF2, in which case, it will show "foF2: N/A" on the ionogram and will not plot a point on the foF2 trend chart for that time period. The other possibility is that the interference fools the software into incorrectly identifying the foF2, in which case the false result is presented and plotted. However, if you examine the ionograms which include this erroneous foF2 value, you'll see that because the software filters out the low-elevation E-layer, that incorrect foF2 value is actually more accurately described as the Critical Frequency of the echo of the E layer! The Critical Frequency of the true E layer is even higher than that, and both are usually higher than the frequency the F2 layer can support (otherwise, the true foF2 would be calculated accurately). My point is that if you know how to interpret the foF2 data, you can use even obviously incorrect information to evaluate how and when NVIS might be possible at higher frequencies than normal, regardless of which layer provides the actual path.
    Last edited: Jul 14, 2018
  7. N1OOQ

    N1OOQ Ham Member QRZ Page

    Once again, the moral is, get on and call CQ. Stop doing what automated data collectors tell you to do... They're not perfect.
    WE4E and W4IOA like this.
  8. K2XT

    K2XT Subscriber QRZ Page

    My understanding is, he is experimenting, learning, applying what he knows, wants to learn more, and is progressing in his knowledge amazingly well, using ham radio for what it was intended.
    In the process I have benefitted, because I don't have a clue what the charts mean but there is enough here to enable me to learn too. Why would he throw all that knowledge to the wind and just call CQ?
  9. AI3V

    AI3V Ham Member QRZ Page

    If you had 2 cars, a red one and a blue one, and the red one was out if gas, would you put the gas in the blue one?

    And then lecture how you know all about octaine?

    You want to talk 200 miles? Want to know what the ionesphere is doing on THIS PARTICULAR PATH?

    Or do you want to estimate the theoretical gas consumption of your blue car?



    Who used to use a ionosounder on a aircraft carrier to try and keep the admirals radio talking to base.
  10. AI3V

    AI3V Ham Member QRZ Page

    What I am trying to say,

    There is very little in common with different skywave paths,

    We had a receiver that would follow scans from every fleet relay site, and provide the classic ionagram.

    Every path was different and constantly changing.

    The ionesphere is anything but a smooth homogeneous place, a glance at the visable Aurora proves that.


    P.s. why these research stations don't share there transmit schedule (synchronized to gps thank you please :) ) so that in addition to vertical incedent scans, we can have a network of oblique scans......

    K5URU likes this.

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