Talk:Near vertical incidence skywave

&mdash; J I P | Talk 09:24, 16 October 2005 (UTC)

The two articles recently deleted from this entry were not spam. I checked them quite thoroughly and found useful HF NVIS antenna design and construction information, historical info regarding use of mobile NVIS antennas during wartime, and no mention of anything commercial whatsoever. The publication mentioned is a non-commercial ham radio magazine, as far as I can tell.

The articles should be restored. Scott Johnson 13:23, 24 April 2007 (UTC)

NVIS as antenna type?
Yesterday I added this article to the "Radio frequency antenna types" category on the basis that NVIS antennas are not infrequently discussed as such in amateur contexts, but today it occurred to me that there is a distinction between the skywave itself and the antenna that makes use of it. Still, such antennas are discussed in the article, even if they are in many cases just a subset of dipoles and (aside from their distance from the groundplane) physically indistinguishable therefrom. Of course, NVIS could be employed by other types of antenna. I'm going to leave the category tag as I think NVIS might be a useful concept to have in a list of antenna types, but wanted to raise the discussion if anyone thought otherwise. --Ninly 01:56, 4 June 2007 (UTC) NVIS Antenna are not a class of it's own, as every horizontely antennas mounted in low height is a low performer in DX and great for local contacts. NVIS decribes the radiaton of an antenna, not an antenna type. Edgar Wollenweber (Germany)09:09, 31 August 2021 (UTC) Wellenweber, your comments are valid as far as Amateur dipole or loop antennas and probably tactical antennas, except for gain antennas such as a quad or yagi, which are horizontal antennas designed to prevent high angle lobes. In Short wave broadcast work, arrays of dipoles fed in phase are used to produce a strong lobe that is directed vertically or nearly so if the transmitter is on the edge of the target area. These can and are referred to as NVIS antennas, although often called "tropical" antennas, as their use is mainly on the "tropical bands" of 90 and 60 meters. These are indeed a "class of it's own". And some amateurs do use in phase horizontal arrays for NVIS. JNRSTANLEY (talk) 12:56, 31 August 2021 (UTC)

Antenna configuration or propogation method, not necessarily antenna type
In my opinion, it is not a new type of antenna. The significance comes from how the antenna is deployed, not built. The key is how far above the ground to deploy the dipole (7ft +/- 3 inches for the 40 meter version). Also, this article only discusses one reflector directly beneath the dipole, but other reference materials show 3 reflectors and 10db of gain, not the 6 shown here. Other articles place two more reflectors on the ground exactly parallel to the antenna but 6 feet to each side of the antenna.

I would also like to see this article referenced in the "40 meters" article as 40 meters is the highest band to benefit from NVIS and is apparently the most popular band for this propogation method. —Preceding unsigned comment added by 68.42.4.138 (talk) 15:39, 10 October 2007 (UTC) You are absolutely right! Every horizontely erected antenna (on the lower bands)is a "Nvis Antenna", as a normal radio amateur is not able to build an antenna in, let me say, in a height of 20 meters. So there is no need to talk of NVIS Antenna. Edgar Wollenweber (Germany)09:04, 31 August 2021 (UTC)

But see my comments on the previous topic. Yes, there is a need to talk of NVIS antennas. As a shortwave broadcast consultant/educator and designer of antennas for NVIS service, I talk of them often. In addition some of the highest gain antennas in the world in the HF range are used for ionospheric research (HAARP, etc) These are clearly NVIS designs. JNRSTANLEY (talk) 13:06, 31 August 2021 (UTC)

NVIS refers more to a comprehensive communication paradigm, not merely antenna
The difference between mere vertical skywave communication and NVIS is in the operational doctrine worked out to provide reliable regional communication during all ionospheric and solar conditions. Automatic Link Establishment and NVIS go together very well for tactical (and/or emergency) communication, although multiple defined calling frequencies across the usable spectrum are sufficient. Pre-arranged robust and efficient methods of accurately forwarding messages are also usually considered to be a necessity.

Remember, skywave communication (even straight-up) was used for a long time before the Germans demonstrated the value of the combined antenna/operator/messaging techniques that became known as NVIS.

For tactical communications (whether military or civilian-emergency), one of the advantages of NVIS methods is that they're very insensitive to antenna altitude. Anything from 18 inches at 100 watts up to 20% of the operating wavelength at 20 watts is "very good", and excessive power creates a fringe-interference dead zone at the border between direct groundwave and skywave-only ranges. For most of the solar cycle, hams can operate 95% of the time by shifting between 80 meters and 40 meters as necessary. Military users will spread their chosen frequencies across the spectrum from 2 to 21 MHz or higher.

While hams often work with cut-to-resonance antennas, this is neither necessary nor necessarily a good idea for emergency NVIS systems. Professionals tend to use the same automatic antenna matching systems used for rapid ALE scanning (and indeed usually combine ALE and NVIS techniques). One reason for this is that changing ground effects (such as flooding rains) can rapidly shift the resonance point of a cut-to-length antenna. Another reason is to enable rapid redeployability. One solution available to ham budgets is to use a twinlead-fed twinlead-constructed folded dipole (either resonant or with an ATU). The maximum power useful for NVIS is 100 watts, so even TV twinlead may be used. For the tuned case, be sure that the antenna is *not* resonant on 80 meters or many tuners will have trouble matching on 40. For the untuned case, switch between antennas resonant on 40 and on 80, or use a crossed-combined element.

Wikipedia is not a howto, and I'm stepping beyond useful background for the article itself, so I'll end it here.

75.140.243.0 (talk) 18:28, 12 November 2008 (UTC) 2A4Fh56OSA (talk) 21:35, 1 January 2013 (UTC)This is a very useful comment - I suggest to include the information on tactical communications and ALE in the article with corresponding references.

Additional external links added on April 2nd, 2008
As I have been performing field-experimentation of NVIS antennas since the early 1990's, I added two external links to related pages on my site at http://www.tactical-link.com .. The links are for "Field Deployment" and "Mobile NVIS" antennas.

Note that the additional links include information on testing done on the amateur radio 40 meter band (see the field-deployed NVIS external link in the article)

Also, the reason for the low height above ground is to limit ground-wave generation so that received skywave signal does not suffer distortion at the receiving station which can occur when receiving both a groundwave and skywave signal.

Trish (talk) 07:28, 2 April 2008 (UTC)Patricia Gibbons wa6ube@arrl.net 66.245.150.200 (talk) 06:55, 2 April 2008 (UTC)Patricia Gibbons wa6ube@arrl.net66.245.150.200 (talk) 06:55, 2 April 2008 (UTC)

More potentially useful external links

 * http://www.athensarc.org/nvis.asp is fairly detailed and fairly accurate. It is from the amateur radio perspective, so is mainly useful as background education before approaching the professional and military literature.
 * http://www.gordon.army.mil/ocos/ac/articles/fiedler/dfNVIS.pdf covers history and mid-1980s problems in usage in the US Army.
 * http://www.athensarc.org/fm2418m.asp is a copy of the US Army FM 24-18 Appendix M. This covers only the propagation part of NVIS.
 * http://www.gordon.army.mil/ocos/ac/articles/fiedler/dfskip.pdf "Skip Zone" is a result of incorrect antenna choice and incorrect training/doctrine.
 * http://www.gordon.army.mil/ocos/ac/articles/fiedler/dfnjarng.pdf initial implementation of NVIS by the NJ ARNG.

2A4Fh56OSA (talk) 20:44, 29 December 2012 (UTC)All links here are dead :( 75.140.243.0 (talk) 18:55, 12 November 2008 (UTC)

References and images
I have added references, more information and added some images, I've just drawn myself :) — Preceding unsigned comment added by 2A4Fh56OSA (talk • contribs) 20:44, 29 December 2012 (UTC)

Broadcast use of NVIS in the tropics
This article fails to mention the use of NVIS in the 120, 90, 75, 60, and 49 meter bands for broadcasting regionally in the tropics. (75 and 49 meters worldwide). This is a very common use of NIVS. I hope to add information about this technology in the next few days. Broadcasters often use higher gain NVIS antennas than those mentioned, so I will include some info on those as well. JNRSTANLEY (talk) 20:11, 17 June 2013 (UTC)

Inconsistent referencing
I intend cleaning up the inconsistent referencing used in this article within a day or two - comments please. Roger (Dodger67) (talk) 15:59, 18 June 2013 (UTC)

Yes, do it. I am quite new to Wikipedia editing so count on you guys. JNRSTANLEY (talk) 18:07, 18 June 2013 (UTC)

Advantages? of using dipoles very close to the ground.
The section on antennas seems to imply that dipoles very close to the ground are about as good (or better) than those at .2 lambda or so. Some of the references (Hawker?) may say this, I can't access them. Yet the military sources clearly indicate that you loose up to 10 or even 15 dB when you go from about .1 down to .01. Also the statement that a single wire under the dipole can add 3-6 dB seems exaggerated, although it might be so for very low dipoles where the losses are already high. Is this also from Hawker? I found one internet article that stated that on 40 meters, 7 feet plus or minus 3 inches was optimum! Can it really be that critical? What happens when someone links to an online or printed article that is itself simply wrong? I suppose one would have to find a more authoritative article stating the opposite. I am pretty sure the vertical pattern is essentially the same from .2 lambda and down, only the losses increase, so there is no QRM or S/N ratio advantage in going lower, even though the noise and signal drop on receive. On transmit it is a dead loss. My concern is that hams might settle for a very low dipole not realizing what the extra loss was. Any comments? JNRSTANLEY (talk) 18:08, 19 June 2013 (UTC)

I have made some changes based on further research into ground wire effects. Comments welcome. JNRSTANLEY (talk) 18:38, 25 June 2013 (UTC)

I have solved the mystery of the 40 meter article at 7 feet plus or minus 3 inches. The author of the original October 1969 73 magazine article used a folded dipole and it showed 50 ohms at this height. Had nothing to do with the gain being max at that height, but the article was then quoted by several others who apparently thought it was. This article made some exaggerated claims about gain. Also, the concerns about ground wave and sky wave producing a fading zone is probably not a concern now, as SSB and digital modes are much less troubled by selective fading than AM was. Thus probably no advantage in lowering the antenna in an attempt to move the fading zone. Only advantage in lower dipole as far as I can tell is stealth and ease of erection. As a compromise in a battlefield or temporary emergency situation has some merit. JNRSTANLEY (talk) 14:51, 28 June 2013 (UTC)

Thanks to 67.180.93.120 for the additional comment on height and the very fine K9CY reference. That reference actually says that the optimum height for NVIS is .22 lambda, which confirms the Dutch study referenced later in the section. However, this is for transmit and the somewhat lower height, .14 lambda, is slightly better for signal to noise on receive, according to the Dutch study. The comment about 3/8 lambda is valid for broadcasting where receive is not an issue and where one accepts a slight dimple in the top of the pattern to get better coverage at further distances. The path length and D layer absorption are less straight up, so the "footprint" is larger and more uniform when one goes a bit above 1/4 wave. However, one must remember that the virtual ground may be a few meters below the actual surface, especially on dry soils. (not an issue in broadcast if extensive ground wires are used). I am all in favor of killing off the widely promulgated idea that lower than .14 lambda is better for NVIS, but at the same time I appreciate that the definition of "optimum" should, IMHO, include the trouble of erecting the antenna, so a lot more effort for a very slight additional gain may not be justified, especially when both receive and transmit are in play. JNRSTANLEY (talk) 18:56, 30 April 2017 (UTC)

NVIS frequencies depend strongly on latitude
The article cites 7 MHz as typical daytime and 3.5 MHz as typical nighttime NVIS frequencies. This is a somewhat misleading statement as NVIS frequencies depend strongly on the geographical location. The cited frequencies may be common for mid-latitude locations (e.g. much of the continental US), but usable frequencies are typically much lower at high latitudes (nighttime MUF may be below 2 MHz) and much higher towards the equator (nighttime MUF often above 7 MHz). — Preceding unsigned comment added by 202.156.85.200 (talk) 15:46, 6 January 2014 (UTC)

I concur with this information and agree that it is useful to include it. I have operated ionosondes on the arctic circle, and have operated both amateur and broadcast transmitters on the Equator. 160 meters, (1.8 to 2.0 MHz) is sometimes necessary at night and especially during the predawn hours for all latitudes but especially north of 45 degrees. I have observed daytime MUFs above 14.3 Mhz on the Equator, and certainly 7 MHz is often usable after dark on the Equator until after midnight. However, I have also seen the MUF fall briefly below 3.3 MHz even on the Equator during the predawn hours in July, the month when the sun is furthest from the earth. The effect of solar activity on the MUFs and absorption could also be included as well as seasonal effects. Thus latitude, SSN, and season (summer/winter/July/January) must all be considered when determining the optimum frequency for NVIS propagation. How much detail do we want to include? JNRSTANLEY (talk) 23:09, 6 January 2014 (UTC)

External links modified
Hello fellow Wikipedians,

I have just added archive links to 1 one external link on Near vertical incidence skywave. Please take a moment to review my edit. If necessary, add after the link to keep me from modifying it. Alternatively, you can add to keep me off the page altogether. I made the following changes:
 * Added archive https://web.archive.org/20151031113702/http://www.emcomm.org/projects/nvis.htm/ to http://www.emcomm.org/projects/nvis.htm

When you have finished reviewing my changes, please set the checked parameter below to true to let others know.

Cheers.—cyberbot II  Talk to my owner :Online 10:14, 14 February 2016 (UTC)

The "Build an NVIS antenna yourself" link seems to go to a Japanese page. JNRSTANLEY (talk) 12:19, 14 February 2016 (UTC)

I have just added two good links to replace a dead one. The Dutch study seems to be as close to a "final answer" as we are likely to got on some issues, especially the height vs gain issue. I think it is time to remove the "insufficient inline citations" warning and I will do so unless someone objects in the next few days. JNRSTANLEY (talk) 15:20, 11 April 2017 (UTC)

Refract vs Reflect
Full disclosure: I'm studying for US FCC Amateur Extra licensing and the topic of NVIS pointed me here. However my high school background in physics somewhat alarms me with the picture described as how signals get "bounced" off the ionosphere - which in my books is reflection and not refraction. Refraction requires a medium change; since the signal remains in the lower atmosphere, there is no refraction taking place, it's pure reflection from the ionosphere boundary. I claim no expertise here but will point to http://www.qsl.net/wb5ude/nvis/ for reference to ensure accuracy of the topic at hand. — Preceding unsigned comment added by 70.57.19.180 (talk) 06:24, 5 May 2018 (UTC)

The wave enters the ionized region at a steep angle which bends it around in a more or less circular path as the density of the ionization increases with height. Below the critical frequency, the peak ionization is sufficient to bend the wave right back on itself, so that if emerges from the bottom of the ionized region and returns to earth. So refraction is technically more accurate. However, a number of sources describe this as reflection which is understandable to about everyone and is close enough to a correct description. Ionosondes calculate a "virtual height" which is the height above ground that is used to signify the equivalent height from which a perfect reflection would happen to produce the same result. I personally find either word acceptable in the case of NVIS. For longer distances refraction is a better choice as it better describes various types of long distance paths which can be quite complex. JNRSTANLEY (talk) 00:54, 6 May 2018 (UTC)

In reading back over the opening paragraph, I was struck by this statement: " If the frequency is too high (that is, above the critical frequency of the ionospheric F layer), refraction fails to occur", and it got me thinking again about the use of "reflection" vs "refraction". They are not really interchangeable. One would not refer waves that pass through the upper E or F layers with some bending, but not enough to return them to earth, as having been reflected, but they are indeed refracted or bent. I would suggest that if we are going to use the term refracted instead of reflected, that the above quoted statement be changed to: " If the frequency is too high (that is, above the critical frequency of the ionospheric F layer), refraction is insufficient to return the signal to earth". Any thoughts, anyone? JNRSTANLEY (talk) 12:09, 16 April 2022 (UTC)

JNRSTANLEY (talk) 23:16, 16 June 2022 (UTC)== Pronunciation of "NVIS" ==

How do you pronunciate the abbreviation NVIS, I have been told it is spoken as "NIVIS" with both letters i in classical pronunciation, like in "mini" and not like in "mine".

Thank you 2001:871:219:56A1:A049:7841:23A3:45F8 (talk) 19:26, 8 March 2022 (UTC)

There is no widely accepted answer to this question to my knowledge. I personally use the whole letters, that is N-V-I-S. and have never heard anyone else do anything but that. But then again, I see it written much more often than I hear it spoken. Something like, VSWR, I suppose. Some say "VISWAR", but S-W-R is perfectly acceptable and the V is not needed. Some might consider "VISWAR" slang, and not appropriate for a formal presentation. And of course, unless the hearer is familiar with it, better to say it all out, "near vertical incidence skywave" JNRSTANLEY (talk) 11:44, 10 March 2022 (UTC)

At a 2020 presentation in Canada, it was stated that it is pronounced “NEH-vis”in Canada and EN-vis elsewhere, but I have never personally heard either one. https://brara.org/documents/presentations/KE4PT-BRARA-version-NVIS_Lord-VE4OV-rev1A.pdf JNRSTANLEY (talk) 23:16, 16 June 2022 (UTC)