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Cloud Warming in style or what is NVIS?

Discussion in 'Amateur Radio News' started by VK6FLAB, Sep 15, 2018.

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  1. VK6FLAB

    VK6FLAB Ham Member QRZ Page

    Foundations of Amateur Radio

    Cloud Warming in style or what is NVIS?

    The term NVIS, or Near Vertical Incidence Skywave is in my short experience as an amateur heaped with scorn and ridicule. Terms like cloud-warmer come to mind when people discuss the principles associated with NVIS, but that does happen in the context of where I live, that is, one of the most isolated cities on the planet, Perth in Western Australia.

    NVIS has several advantages over other forms of HF communication, it can be done with low power, there is little or no signal fading, simple antennas work well, it has low path loss, better signal to noise ratios and if you're in a valley, you can still use it.

    So what exactly is NVIS?

    In the past I've talked about long distance HF communication. Your radio signal bounces off the ionosphere, bounces back to earth and so-on. Like skipping a stone on a pond, the angle at which your signal hits the ionosphere determines what happens next. In general, shallow is good, steep is bad, much like the plop you hear when you don't hit the pond just right, a radio signal can go through the ionosphere, never to be heard again.

    NVIS is about hitting the ionosphere at a steep angle, in such a way that it reflects back to earth. Without going into detail, generally you can use 40m during the day and 80m at night with some variation depending on the solar cycle and whom you want to talk to.

    NVIS gives you communications less than 1000 km away, plenty to talk to everyone in your city and surrounding area. In the case of an emergency that's also likely enough to get out of any emergency affected area, so plenty of excuses to set up and try for yourself.

    I can start talking about angles, maximum usable frequencies and so-on, but I won't. These all relate to specific circumstances, depend on what antenna you're using, what the ground conductivity below you is and as is typical in our hobby, many other variables.

    What I can say is that NVIS to NVIS station works best, so if you're going to test this with a friend, it will help if you both set up a similar station while you learn the variation associated with this kind of communications.

    Now I did mention up to 1000 km, that isn't enough to leave Western Australia, Perth to the border is about 1500 km, but if you live in the Netherlands, you can get to 15 or so countries. Depending on where you are, NVIS will give you different outcomes and what I'm talking about affects each station differently.

    For me, the attraction of NVIS is solid communications on 40m and 80m, something that has eluded me so far. It also allows for a low simple wire antenna, an inverted vee dipole, two bits of wire strung up on a pole, 6m in the middle 2.5m at the end will get me up and running. Perfect for a field-day, excellent for a local contest and brilliant if you're only using low power as a beginner.

    Because the antenna is close to the ground, it's pretty much omni-directional. If you set-up an antenna for 40m and then cross that with an 80m antenna and feed them both from the same point, you'll have a configuration that will operate well for 24 hours without needing to move antennas in the dark.

    I have no illusions that an NVIS antenna will help me make contact between Perth and Japan, but then it's not intended for that. I've spoken in the past about finding the right tool for the job. NVIS is a tool, it has a job and it's very good at doing that. It's not for everyone, all the time, but it's a tool that you as an ambitious amateur should know about.

    I'm Onno VK6FLAB

    NVIS Pattern-2.JPG
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    K5NSN, W7GST, N8VWY and 3 others like this.
  2. K4KWH

    K4KWH Ham Member QRZ Page

    Many hams have never heard of the term. They, like CBers, think the higher the better, the more power the better. But, as you say, there good uses for NVIS. In specific situations, one may not want such long distances, but rather to cover out to a few hundred miles or less. Or to cover a certain direction at less distance. It has a lot of advantages that don't jibe with the prevailing attitude of "more".;)
    K4VW, W7GST, KC3FOI and 2 others like this.
  3. AG5CK

    AG5CK Ham Member QRZ Page

    The cbers probably think higher is better because they aren't using NVIS on 27 MHz. You won't hear many people using low dipoles on vhf either.
    K8AI, KF7PCL, KG5CJA and 2 others like this.
  4. K4DL

    K4DL Ham Member QRZ Page

    I was fortunate to learn about NIVS while dealing with Army set ups. I have been using the concept for over 25 years now.
    DL6BCX, HK3RLG, W6PXL and 7 others like this.
  5. KQ6XA

    KQ6XA Ham Member QRZ Page

    NVIS: Frequency choice is way way more important than antenna height.

    Often, just moving 250 kHz can make a huge difference in NVIS signal.
    Commercial and Governmental stations that intentionally utilize NVIS may have a variety of channels to choose from throughout the 2 MHz~13 MHz range.

    For hams to take advantage of the NVIS at this point in the solar cycle, operators need frequency agile capability mainly on the following bands:
    160 metres 1.8~2 MHz (NVIS Top Band late night when 80m doesn't work)
    80 metres 3.5~3.8 MHz (NVIS evening and some daytime)
    75 metres 3.8~4.0 MHz (NVIS evening and daytime)
    60 metres 5.3 MHz (Primary NVIS mid-latitude daytime on the Rock Band)
    40 metres 7.0~7.3 Mhz (NVIS mid-latitude daytime)
    30 metres 10.1 MHz. (NVIS daytime mainly in equatorial latitudes)
    Recently, the only daytime NVIS ham bands here in California have been 60 metres and 75 metres... either one or the other... rarely both at the same time.

    Here's a snapshot of the ionogram right now, at about 4pm California time.
    Look at the foF2, it is 4.788 MHz.
    For all practical purposes, we consider the foF2 as "the maximum possible NVIS frequency".
    That means that 75 metres is probably the best NVIS frequency here at this moment.
    If I want to work NVIS within 100 miles of me, I need to be on 75 metres.
    Further than 100 miles, out to 400 miles, I need to be on 60 metres.
    The foF2 may be different at another location.
    This is the ionosonde that is nearest my QTH.


    Check out the real-time Ionogram for your location:
    Last edited: Sep 18, 2018
    K7ART, K4VW, W7GST and 3 others like this.
  6. NT7C

    NT7C Ham Member QRZ Page

    I was part of 'Operation Dewey Canyon II' stationed in Khe Sanh, RVN in the Army. I ran a comm center for local VHF and HF to I Corps headquarters in Da Nang, RVN (a distance of approximately 90 - 100 air miles - 230 km by crooked roads.) We ran 24/7 0n 4010 kHz SSB voice and RTTY. We used a half-wave dipole at 12 feet (classic NVIS configuration). Call it a 'cloud warmer' if you want, it worked every time we had traffic, day or night.
    HK3RLG, W7GST, K8CGS and 7 others like this.
  7. K5CO

    K5CO Ham Member QRZ Page

    I'd disagree; most hams that I know are familiar with NVIS and some use it quite a lot. Perhaps the no-coders missed out, but I think any of the"old timers" (CW users) know a good deal about NVIS in use by Hams as well in WWII.
    KK4WIG likes this.
  8. KQ0J

    KQ0J XML Subscriber QRZ Page

    NVIS = I put up a crap antenna but now its called something cool so I will go with it.
    K5CQA, K8TE, AA4MB and 5 others like this.
  9. KL7AJ

    KL7AJ Ham Member QRZ Page

    My favorite NVIS antenna.
    KQ6XA likes this.
  10. KQ6XA

    KQ6XA Ham Member QRZ Page

    That was in 1971, at the peak of Solar Cycle 20, which was a great time to work HF and NVIS.
    Also, Viet Nam is near the magnetic equator, which has a much better NVIS frequency range than we do here in mid-latitude CONUS.
    4 MHz remains a good frequency at certain times of the day for NVIS, here at the bottom of the solar cycle, but 4 MHz certainly is not good for NVIS 24/7.

    The good olde daze :)

    Last edited: Sep 18, 2018
    NI0P likes this.

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