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Sunday, May 10, 2020

Near Vertical Incident Skywave (NVIS) Antennas



HF emergency communications or otherwise, contact stations within the skip zone.

A previous post details Robert VA7FMR’s experiences with a dipole using Hamsticks and a dual bracket. Here is additional information on them and Near Incident Vertical Skywave (NVIS) antennas using Hamsticks. 

SEPAR and many like organizations may be called upon in an emergency to provide ancillary communications for emergency services or primary communications for those emergency service partners who do not have RF communications systems of their own, ESS, The Red Cross and Salvation Army, to name just a few. It is a given that the first stations will be on the higher bands above 50 MHz. Anyone with a Basic License may operate within that spectrum. This should provide reliable communications with modest antennas within the Lower Mainland—no, not just with a handheld and a rubber duckie. Even if many of the local repeaters fail, we should manage to set up a decent network within a few minutes to hours. But what if reliable communications are required for longer distances, for example  to Kamloops or to Seattle, or beyond? VHF and above may not be able to span that distance without gain antennas, placed high with at least 50 Watts of power. HF will be the ‘goto’ bands. We may not need a 60 foot or higher tower to communicate effectively either—this is where NVIS becomes an important emergency communications antenna, especially in the field.

If you recall the antenna theory that you should have picked up in your Basic course, you know that the orientation (horizontal, vertical or somewhere in between) of an antenna will affect its radiation pattern. Much like bouncing a ball off a wall. The greater the angle, the greater the distance the ball bounces away from you. Throw it straight at the wall and it should come pretty close to you on its way back. The same general idea applies to the NVIS antenna. If we cause the RF wave to travel nearly straight up or at a slight angle, it will reflect off the ionospheric layers and come back close to our point of origin. So, if we want to communicate on HF with stations within about 1,500 Kms, we use an antenna that radiates primarily straight up. A DXer on the other hand prefers to talk to stations far away, with a few hops, the farther the better, so DX antennas radiate at angles primarily horizontally to bounce and skip back off the ionosphere for the greatest distance. 



So, the NVIS antenna is one that provides the majority of its radiation at an extremely high angle. That is to say the major lobe is between 75 and 90 degrees to the earth's surface. This will provide excellent omni-directional communication out to a distance of up to about 1,500 Kms with no skip. The maximum frequencies involved will be as low as 1.8 MHz under very poor conditions to as high as 14 MHz under excellent conditions, with the most usable being between 3.5 MHz (80M) and 7.3 MHz (40M).



To summarize, NVIS works for frequencies lower than the vertical incident critical frequency—the highest frequency for which signals transmitted vertically are reflected back down by the ionosphere.  At or below the critical frequency the ionosphere will reflect an incident signal arriving from any angle, including straight up. Because the critical frequency is low, you must usually operate 40, 80 or 160 meters or possibly 30 meters to use NVIS propagation.

Under most conditions you can easily obtain coverage on one of these bands from zero to 350 miles or more with no skip zones. On 75 meters with 100 W and an antenna 15 feet high, contacts with stations over 1000 miles away with excellent signal reports are not uncommon. 

These are the characteristics we look for in an emergency-ready HF antenna for distances up to about 1,000 miles… No skip, easy set-up and take down and reasonably reliable communications.

When I first started looking at the NVIS antenna for "local", primarily emergency communications, the consensus seemed to be that it was a dipole-type antenna, near 1/8th wave at the operating frequency, above the ground. I purchased a set of HamSticks, mounted as a dipole, for this purpose as I was operating from a vacation area surrounded by high mountains.  NVIS antennas are commonly used by the military, as their needs fit these characteristics. There is an excellent, though technical article at https://region6armymars.org/downloads/NVIS-Antenna-Theory-and-Design.pdf

Every horizontal antenna has an NVIS component in its radiation. Similarly, every horizontal antenna has a component that is most useful for DX. Your decision then is to pick the configuration that either favours or optimizes the properties you want.  Reliable local communication on HF dictates NVIS. How then do we determine what NVIS antenna will best suit our needs?  Let’s examine the parameters that have a significant effect in antenna performance. This is information on how to make it work reasonably well, NOT a graduate degree treatise on the theory of NVIS.


Height above ground

The antenna height above ground seems to be the single most controversial subject in discussion of NVIS antennas. Some say anything below 1/4 wave works. Others say anything below 1/8th wave and yet others say ten to fifteen feet works very well. You will note that there is negligible difference in antenna gain between 1/8 wave and 1/4 wave height. There is however a significant difference in the logistics of placing an antenna at 70 some feet in the air versus 35 feet in the air.

Antenna guru L.B. Cebik (W4RNL), writing about NVIS antenna elevation, explains that the height, in the 1/8 to 1/4 wave length above ground, has very little difference in gain. In fact, if you roll in the next parameter, ground (detailed below), height can easily have much less effect than ground.

The Near Vertical Incident Skywave (NVIS) antenna is a half-wave dipole antenna, configured straight or as an inverted vee, mounted not over 1/8th wave above ground (at the highest operating frequency). While 1/8th wave works reasonably well, better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground. A second advantage of lowering the antenna to near 1/20th wavelength is a lowering of the background noise level. At a recent ARRL Section Emergency Test, communication on 75 Meters was started with a dipole at approximately 30 feet. They found communication with some of the other participants to be difficult. A second 1/2 wave dipole was built and mounted at 8 feet off of the ground. The background noise level went from S7 to S3 and communications with stations in the twenty-five and over mile range were greatly enhanced. Simply stated, you want as much of your signal going up as possible and ten to fifteen foot height has shown to function very well. It was also found that a network of stations, all using NVIS antennas experienced much stronger local signals.


Ground

Yet another consideration is the "quality" of the ground below your antenna. By this we mean the conductivity of the ground you are operating above. For any given height (1/4th wave length or less) poor conductivity will attenuate up to 3db more of your signal than high conductivity soil. A documented example is the ARES installation in Longmont, CO at the Emergency Operations Center. That antenna is mounted ten feet above a flat roof. The base for the roof is a grounded steel plate. This antenna consistently performs as well or better than any other in the state. The reason is simple; A full sized resonant dipole antenna mounted ten feet above an excellent ground.

A specific example of how well the Longmont EOC antenna works is one Sunday when testing the antenna, a local ham tried his Yaesu FT-817 running on the internal battery pack. As most know, that configuration produces 2.5 watts PEP maximum output. At that power level he received a signal report from NCS in Colorado Springs (90 miles South) of S9+10db, on 75M just before the net started.

Another example of how the conductivity affects your signals comes from Colorado where they regularly use NVIS antennas on 60M to communicate across the Continental Divide. Doing this on a twice weekly basis for several years now they have established a base-line for comparison. The week of 23 September 2004 they had a slow moving rain storm that put down more than one inch of rain, spread almost evenly over about 36 hours. For those that have thirty to fifty inches of rain per year, that would not be much. In Colorado that is one-fifteenth of their total annual precipitation. After the rain, under less than optimal band conditions, signals were UP 6 to 10db!

The chart by L.B. Cebik's (W4RNL) shows that any NVIS, above excellent ground, out performs an antenna above good ground at optimal height! Hmmm, does that imply that we have found the single most important parameter in NVIS?


Ground wire

Yet another approach is to run a "ground" wire at the surface where the antenna is mounted. A good discussion on this is found at an Australian site by Ralph Holland. He did some research on 160M and found that a ground wire at .02 to .06 wave lengths below the driven element produced the best gain. That translates to about 5 to 15 feet at 75M which would be consistent with the heights seen that have  produced the best NVIS performance. Others claim at least a 6db improvement with this same approach.

Experimenters  also notice an improvement if  you "water" the ground just prior to operation. Pour about one gallon of water on the ground around the ground rod or wire. If it seeps in very quickly, go get another gallon. This has made a noticeable improvement in both transmit and receive signals. 


Counterpoises

The high angle radiation of a dipole (or inverted vee) can be enhanced by adding a counterpoise wire below it, about 5% longer than the main radiating element, to act as a reflector. The optimum height for such a counterpoise is about .15 wavelengths below the main radiating element, but when the antenna is too low to allow for that, a counterpoise laid on the ground below the antenna is still effective.

A knife switch at the center point of the counterpoise can be used to effectively eliminate the counterpoise from the antenna system. This technique is useful for using a dipole for NVIS and longer distances, too. A counterpoise is installed at ground level, or as high as the switch can easily be reached, and a dipole is mounted .15 wavelengths above the counterpoise. When the switch is closed, the vertical gain will increase, and the noise levels will drop. When the switch is open, lower angle gain will increase, improving the antenna's performance for non-NVIS use.


Dual Ham-Stick

This is a portable antenna on a 5-foot mast that does well under ARES/RACES operating conditions. One person can put this up and have it operational in under five minutes! A side advantage of this antenna is its comparatively small size. It is only sixteen feet in length, which makes it much more reasonable for temporary installations.



‘HamStick’ antennas may be paired to make a very usable dipole antenna.
Mounting height will affect  the radiation pattern and therefore propagation.
Above, a typical HamStick and adjustable whip and the dual mount that makes it a dipole with directivity
. 

Inverted Vee

A dipole's close cousin, the inverted vee, is another good NVIS antenna which can be even simpler to support. An inverted vee will work almost as well as a dipole suspended from a slightly lower height than the apex of the inverted vee, so long as the apex angle is kept gentle—about 120 degrees or greater. An inverted vee is often easier to erect than a dipole, since it requires only one support above ground level, in the center.

This design has been successful for the author. It was developed by Dr. Jelinek and is in commercial use by the Armed Forces.


How do I select a frequency for NVIS operation?

The selection of a optimum frequency for NVIS operation depends upon many variables. Among the many variables are time of day, time of year, sunspot activity, type of antenna used, atmospheric noise, and atmospheric absorption. To select a frequency to try, one may use recent experience on the air, trial and error (with some sort of coordination scheme agreed upon in advance), propagation prediction software like VOACAP, near real-time propagation charts (available on the Internet) showing current critical frequency, or even just a good educated guess. Whatever the strategy used for frequency selection, it would probably be best to be prepared with some sort of "Plan B" involving communicating through alternate channels, or following some pre-arranged scheme for trying all available frequency choices in a scheduled pattern of some sort.

http://webclass.org/k5ijb/antennas/NVIS-low-antenna-regional-communications.pdf


A NVIS antenna on a wartime military vehicle.

Finally, this is also an antenna that should be in the ‘kit’ for Field Day or contests. We usually concentrate on working any and all stations however, skip actually works against us when it eliminates many potential contacts up to 1,000 miles or so. The ability to switch to an NVIS antenna will bring in those stations within the skip zone and enhance the score. This strategy has helped us place first in Canada in our 3A Field Day category for several years.

~17/10











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