Has anyone seen pictures of military HUMVEEs with an antenna folded over? Ever wonder why? Do you think it is so they can get into parking garages or under low bridges? Let’s think about that and come back to it later…
Has anyone had the frustration of being able to talk on HF to Southern California but can’t reach a station in Portland, Oregon? Well, the medical services team has had this problem. We had an HF vertical antenna on top of the VA Hospital on Beacon Hill and could not communicate with hospitals in Portland. Anyone want to venture a guess why? To understand, let’s explore HF for a moment…
For HF propagation, there are a few modes of transmission through the atmosphere that are important to understand. Depending on the goal of your communication mission, the type of mode you use will determine whether your communication mission will be successful, or be a failure.
First, there is the traditional “skywave” propagation – skywave is the propagation of electromagnetic waves that are bent ( more technically know as “refracted”) back to the Earth's surface by a layer of the atmosphere called the ionosphere. If you tune your AM radio to 660kHz at night, you can typically hear the station 660News from Calgary, Alberta, Canada here is the Seattle area. If you are lucky and tune to 640kHz, you can also sometimes hear KFI from Los Angeles, California. The radio waves from these stations are bouncing off the ionosphere and landing on your AM radio antenna through skywave propagation. Most long-distance HF radio communication (between 3 and 30 MHz) is a result of skywave propagation. Amateur radio operators take advantage of skywave for long distance or DX communication.
Skywave is an excellent mode for long distance communication, say from several hundred miles and out. A skywave results in the signal going off your antenna at a fairly low angle, hitting the ionosphere, and bouncing back to earth at some distance away. As you can imagine, the angle that the wave hits the atmosphere will directly affect where the signal ends up – remember from basic physics the angle of incidence is equal to the angle of reflection. A good way to think about this is a pool table; if you want to bounce the ball off the side of the table to hit another ball, you need to adjust the angle such that the incident angle equals the reflected angle to hit your mark. The same is true of radio waves.
The second mode of HF propagation is known as a “ground wave”. A ground wave is a wave that travels along the surface of the earth. The distance that ground waves propagate depends on the frequency, which really is dependent on how well the wave is diffracted by the earth’s surface. Most local communication in the 30kHz to 300kHz is via “ground wave” and have been used in such applications as over-the-horizon radar. Ground wave propagation is typically on the order of tens of miles and is also dependent on sun spots, solar flares, and day or night operation.
The problem we now have is if you are trying to communicate in between the ground wave and skywave coverage areas – you are either further out then the ground wave will propagate, or too close such that the incident and reflected wave leaves a “hole” where the signal effectively bounces over you. This is what is known as the “skip zone”. If you were sitting in the skip zone and the skywave was such that the radio wave bounced over you and the ground wave did not reach you, you would not be able to receive the station sending that signal.
So back to our HF vertical on the VA Hospital -- we propagated a signal such that Portland is in the skip zone. Oops!
For many emergency communications activities (as well as military operations), you are not necessarily interested in talking across the continent, or around the world. However, you do need to talk farther then the distance a groundwave signal can reach. So what do we do?
The answer is Near Vertical Incidence Skywave, or NVIS (NE-VIS) for short. NVIS is a electromagnetic wave propagation method that provides usable signals in the range between groundwave and skywave distances, usually in the range of 30 to 400 miles. This easily covers Portland, Vancouver BC, Spokane, and beyond from the Seattle area. NVIS provides effective regional communications.
Although not all radio amateurs have heard the term NVIS, many have used that mode when making nearby contacts on 160 meters or 80 meters at night, or 80 meters or 40 meters during the day. You may have thought these nearby contacts resulted from groundwave propagation, but many such contacts involve no groundwave signal at all and actually used NVIS. You may actually have used NVIS without even knowing it!
A good way to visualize NVIS is to consider a mirror on the ceiling directly above you. You shine a flashlight straight up at the mirror. What happens to the light? It is reflected directly back down at you. This is what we want to do with NVIS – we want to send a radio wave straight up and have it come back down. Since the incident wave is not a perfect beam and the reflection is not perfect, the reflected signal “speads out” to cover an area from 30 to 400 miles.
Can any frequency be used for NVIS propagation? No, there is a limited range of frequencies that will provide NVIS – if the frequency is too high (above what is known as the critical frequency or “f0F2”), there will be no reflection and the radio wave will pass through into space. If the frequency is too low, there will be too much absorption of the signal in the atmosphere. The usable frequencies for NVIS communications are between 1.8 MHz and 10-15 MHz. The most common bands used in amateur radio are 3.5 MHz (75/80 meters) and 7 MHz (40 meters), with some experimental use of 5 MHz (60 meters) and 160 meter frequencies. Always use the highest frequency possible to get away from the critical frequency. Military NVIS communications mostly take place on 2-4 MHz at night and on 5-7 MHz during daylight. The lowest layer of the ionosphere, called the D layer, causes attenuation of low frequencies during the day. This layer disappears at night enabling improved communications at the lower frequencies during the night.
The rule of thumb for NVIS is – higher frequencies during the day, middle bands in afternoon/evening transition, lower at night. For emergency communication use, it is best to have a couple of frequencies in mind in 75/80 meters and 40 meters so you can use the optimal frequency for a given time of day which is below the “critical frequency”.
For HF, you may have heard that “height is best” when it comes to antennas – get your antenna as high off the ground as possible. This is true for long distance HF work, since the height above the ground will give an antenna pattern with low angle of radiation. Well for NVIS, forget this. We actually want to do the opposite – keep the antenna low to maximize the radiation straight up.
An NVIS antenna configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from 1/20th wavelength to 1/8th wavelength above the ground. - hang a dipole at less than ¼ wavelength above the ground. According to US Army studies, 1/8th wavelength is the optimum height. For 80 meters, this is about 30', and for 40 meters it is about 15'. The height is not super-critical, as long as it is below ¼ wavelength. Antennas have been laid on the ground or in the bushes with usable results. The Army even has antennas that are buried two feet under the sand! However, antennas that are very close to (on in) the ground will be less efficient than ones hung at 1/8th wavelength.
Lowering the height also reduces the background noise level. That proximity to the ground forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground wire which is slightly longer (5%) then the antenna, and placed parallel to and directly underneath the antenna (about 0.15 wavelength below); some designs also have three ground wires – one directly under the antenna, and one on each side 0.15 wavelengths away. While the ground wire is not necessary under good to excellent propagation conditions, antenna gain in the 3 dB to 6 dB range are common when the ground wire is used. This gain gives this antenna configuration its unique name- the “cloud burner” or “cloud warmer”. Heights of 5 to 10 feet above ground are not unusual for NVIS setups, and some people use dipoles as low as two feet high with good results (relatively weak signals, but a very low noise floor). I’ve even experimented with running a wire along the ground with success.
You will probably need a good antenna tuner. Antennas that are resonant at ¼ wavelength above the ground will have a higher SWR (standing wave ratio) when hung close to the ground. If ground conductivity is poor (dry sand, for instance), try running a counterpoise wire on the ground directly below the antenna. The counterpoise is actually a good idea with any NVIS installation, since it improves the system's overall efficiency. The counterpoise should be slightly longer (5%) than the antenna. Unbalanced antennas should have a balun installed at the feedpoint to prevent the coaxial cable from radiating.
When both the transmitting station and the receiving station use NVIS configuration for their antennas you can maximize the communications. Why? One potential problem with NVIS operation is "groundwave interference". If two stations are close enough to hear each other's groundwave signal, multipath interference can cause significant distortion. To reduce this effect, both stations should keep their antennas as low as possible, and point the ends of the antennas at each other to minimize groundwave radiation in that direction.
There is also an advantage inherent in the use of NVIS antennas which applies to receiving. The frequencies which are useful for NVIS are the same frequencies which are most susceptible to atmospheric noise. A major source of atmospheric noise is distant thunderstorms. Nearby thunderstorms are the worst, of course, but the noise from all possible sources adds together. Unless there is a nearby thunderstorm, most noise will be the sum of the noise from distant sources which are all propagated to the receiving antenna. Since an antenna optimized for NVIS is listening mostly to signals propagated from relatively nearby areas, and does not favor the reception of signals, static crashes, and other sources of noise and interference from more distant sources, it will not hear as much noise or interference as an antenna optimized for DX operation. The result is a better signal/noise ratio.
Almost any type of wire antenna can be used for NVIS – a random length wire antenna, a dipole, square loop, inverted vee, multi-band dipole, or a hamstick dipole. I’ve seen mobile installations where you run a wire off the back of a pickup truck at about 3 or 4 feet off the ground provide effective NVIS operation. For random wire antennas and inverted Ls, remember to run out a counterpoise wire along the ground to avoid RF burns from your equipment! Place the counterpoise on the ground directly below the radiating element. Avoid stringing long-wire antennas with any significant vertical radiating sections to keep ground-wave propagation to a minimum.
Other proven NVIS antennas include the Shirley dipole and the Patterson Loop (used by the military), but these are more complex to build and install. Some Amateurs have experimented with horizontal fiberglass loaded-whip dipoles (such as those from Hamstick, Iron Horse, and Valor), although they are far less efficient than full-size dipoles. In an electrically noisy environment, they do not have enough gain to work well.
So which frequency do I choose? The selection of the optimum frequency for NVIS operation depends upon many variables, including time of day, time of year, sunspot activity, and 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 – this is a popular method used in Automatic Link Establishment systems), you can use propagation prediction software, or probably the easiest is to use near real-time propagation charts which are available on the Internet and show the current critical frequency. A good site is from the Australian Government IPS Radio and Space Services website, and provides the optimum NVIS Frequency Map Based upon hourly ionosphere soundings. See http://www.ips.gov.au/HF_Systems. Look for the f0F2 plot.
High power is not required for NVIS either. We are not trying to talk around the world, just out to a few hundred miles. I’ve used an FT817 on an alkaline battery pack at low power with success.
Among the many advantages of NVIS are:
- NVIS covers the area which is normally in the skip zone, that is, which is normally too far away to receive groundwave signals, but not yet far enough away to receive skywaves reflected from the ionosphere.
- NVIS requires no infrastructure such as repeaters or satellites. Two stations employing NVIS techniques can establish reliable communications without the support of any third party.
- Pure NVIS propagation is relatively free from fading.
- Antennas optimized for NVIS are usually low. Simple dipoles work very well. A good NVIS antenna can be erected easily, in a short amount of time, by a small team (or just one person).
- Low areas and valleys are no problem for NVIS propagation.
- The path to and from the ionosphere is short and direct, resulting in lower path losses due to factors such as absorption by the D layer.
- NVIS techniques can dramatically reduce noise and interference, resulting in an improved signal/noise ratio.
- With its improved signal/noise ratio and low path loss, NVIS works well with low power.
- Only good for distances <>
- For best results, both stations should be optimized for NVIS operation. If one station's antenna emphasizes groundwave propagation, while another's emphasizes NVIS propagation, the results may be poor. Some stations do have antennas which are good for NVIS (such as relatively low dipoles) but many do not.
- NVIS doesn't work on all HF frequencies. Care must be exercised to pick an appropriate frequency, and the frequencies which are best for NVIS are the frequencies where atmospheric noise is a problem, antenna lengths are long, and bandwidths are relatively small for digital transmissions.
- Due to differences between daytime and nighttime propagation, a minimum of two different frequencies must be used to ensure reliable around-the-clock communications.
- So let’s end with answering the initial question – why do the HUMVEEs have the antennas folded over? Because they are operating NVIS!
Yahoo! Group NVIS Discussion: http://groups.yahoo.com/group/nvis/