Admin Note: This is the second part of the article submitted by Keypounder. Topics covered include the layers of the ionosphere and frequency selection characteristics.
We’ve mentioned the ionosphere, but understanding the basics of ionospheric composition and characteristics is very important to proper NVIS operation, especially when signal security and DF are considered.
The D layer of the ionosphere is the lowest layer, from about 50 to 90 kilometers up, and it functions largely as a signal absorbing layer. The lower the frequency of the signal passing through it, the more the D layer attenuates that signal. This is a problem for ham radio operators trying to work DX on 160 or 80 meters, and to some extent on 40, because the D layer attenuates a signal each time it passes through, and the lower the angle of incidence, the more of the D layer the radio wave must traverse, further increasing the loss. For example, if the loss on a single pass straight up through the D layer is 5 db on a given day and frequency, then the total loss due to the D layer is 10 db to get from transmitting antenna back to receiving antenna. This excludes the loss due to the reflection itself and losses in transmission line, antenna, etc.
If your transmission path from transmitter to receiver is at a 45 degree angle from the vertical rather than straight up, the loss due to the D Layer increases to 14 db (1/sine of 45 degrees =1.414 time the original loss of 10 db is ~14 db) If the angle increases to 60 degrees, the loss is 20 db. For EACH hop. In addition, for low dipoles, the signal sent out is less at lower angles, as will be demonstrated shortly. For amateur operators trying to make daytime contacts far away via multi-hop skywave, this attenuation is an issue, but for NVIS operation it can be an advantage.
The D layer owes its existence to the presence of sunlight; it has been demonstrated that the D layer can disappear within a minute during a complete eclipse, for example. As the sun rises each day, the light from the sun progressively illuminates the D layer and gradually starts to attenuate radio signals that pass through it even before the sun can be seen from ground level.
Anyone who has worked the low bands at sunrise can hear the noise dropping as the sunrise line approaches, and has heard distant stations fade away as the D layer gets recharged by direct sunlight. The same thing happens in reverse after the sun sets; as the D layer fades away, contacts on the low bands can be made farther to the west. That is why ham operators talk about the lower HF bands “going long” in the evening as the sunset line moves west and D layer attenuation drops enough to make multi-hop long distance contacts possible. As far as NVIS is concerned, during the daytime, the attenuation of the D layer reduces atmospheric noise, which also bounces off the ionosphere, and reduces the area that the transmitted signal can be heard, both advantages for NVIS communication as long as both the sending and receiving stations are within about 300 miles of one another.
The F layers of the ionosphere are above the D and E layers, and are largely what makes HF skywave communication possible. They are ionized by solar emissions of various sorts, and unlike the D layer, whose existence is solely due to direct sunlight, F layer ionization lasts for a fairly long time. During the day, the F layer separates into two levels, the F1 and F2 layers, each with different characteristics, but after nightfall the F layer gradually decreases in height and coalesces into one layer closer to the Earth.
During the day, when it is exposed to direct solar radiation, F layer ionization is most intense and at its highest height, and the ability of these two layers, (F1 and F2) to reflect radio waves is best. During the daytime, depending on solar output , the F layers can directly reflect radio waves as high in frequency as 10 mhz or possibly higher, especially at the equator. During solar minimums, such as we are starting to see now, the F layer may not be able to reflect 7 mHz signals even at local solar noon, again, depending on solar flux, location and time of day.
Here in the mid-Atlantic as of July 2016, the FoF2 ranges from a low of around 2 mHz to a high of just a tad over 6MHz. It varies quite a bit, depending on the K index, the solar flux and other factors, but the graph below is reasonably representative of mid-latitude northern hemisphere locations. This is a copy of an ionogram from Wallops Island Virginia, showing a graph of the critical frequency (FoF2- the highest frequency that will reflect a vertical signal back to Earth) versus time of day (UTC or Zulu)-
Sunset is about 8:30 PM local time, or around 0030Z – notice that the FoF2 starts to decline after sunset, and bottoms out just before sunrise, which is around 0600 local time or about 1000Z. You will also notice that there is considerable variation in the FoF2 over just a few days, and that there can be anomalously high or low readings throughout the day. Here is a link to a NOAA/NGDC website that has a listing of all the ionosondes and access to their data if they are active:
Here is another site that presents these data in a simpler format:
Ionosonde data, as well as solar flux and geomagnetic data are important to any NVIS operator, but in a grid down situation, they may not be readily available. Government and military radio operators can be expected to have access to whatever data can be collected; they also have access to a wider selection of frequencies on which to operate. In any case, it is worth the time for any radio operator to become acquainted with the general trends for his area, over the different seasons and during different levels of solar flux, so that if the ionosonde date are not available the operator will have a good idea of what can be expected. For the average amateur operator with unmodified equipment, the choice of frequency band will be somewhat limited; you can pick 160, 80 or 40 (and possibly 60 meters, although that band as presently constituted has significant drawbacks.) And that is a good segue to our next topic-
Frequency Selection and Time of Day
For most amateur operators, who are generally not concerned with being DFed, it makes sense to use the highest frequency that you can, in order to minimize loss from the D layer and maximize coverage area, so that you can talk to as many other amateurs as possible. In a grid down situation, where COMSEC is important, you may well decide to use the LOWEST frequency you can, and operate during daylight hours to maximize the loss due to the D layer and reduce the probability of interception and DF location. As you can see from the above ionogram, just before sunrise the only amateur band useful for NVIS in the mid-Atlantic US would be 160 meters. Keep in mind, however, because of the lack of the D layer, even 160 meter NVIS transmissions can bounce all over the dark half of the globe. My 60′ high inverted vee has been heard in Australia, not to mention all over the US, at NIGHT, and just before sunrise, when it is dark all the way from the eastern seaboard of the US to the land of the kangeroos.
During the day, even with the D layer, with 100 watts and an NVIS antenna I can usually put a readable SSB signal out to perhaps 300 miles on 160, and maybe 500 or so miles on 80 meters. On 40 meters in daytime, I can easily be heard on sideband up and down the East Coast, and into the Midwest. For NVIS operation, keep in mind that the D layer is gone shortly after sundown in your immediate area, and HAS BEEN GONE to the east, so if you are in the US your signal can carry easily to Europe.
(True story, a friend of mine had an 80 meter inverted vee dipole up in the tallest tree he could get to, about 40 or 45 feet up. During a windstorm this past winter, the branch he had the antenna suspended from broke and his 80 meter dipole ended up mostly on the ground. He was annoyed that that his antenna was down, as he had been looking forward to 80 meter operation during the winter quiet, and I told him to stretch the dipole along the wooden fence he had along the back and try it anyway. He was very skeptical and doubted that a 5′ high antenna would do anything, but later that week he told me that he had worked England and several other European countries using his 5′ high dipole (and about 400 watts!) at night on 80 meter SSB.)
Not only may YOUR signal be heard where you do not want it to be heard, but other folks’ signals can be heard, too. Apart from being overheard in the wintertime, interference, accidental or deliberate, can be a problem, especially at night. Night time atmospheric noise, especially during the summer, can cover up weak, or even not so weak, signals.
For example, a strong NVIS signal (using a 100 watt transmitter and a good horizontal dipole at 30 feet up) during the day on 80 meters between two stations 100 miles apart would be s9 to as much as 10 db or 20 db over S9 depending on the ground and the geomagnetic and solar weather conditions. The ambient noise level might be anywhere from S1 to about s3 or s4, (unless there were thunderstorms within 3 or 4 hundred miles) and therefore the NVIS SSB signal would be 6 S units, or about 35 db, above the noise level and clearly audible. In fact, NVIS signals between the same two stations with 5 watts transmit would still be about S7, again, clearly audible as long as there was no close thunderstorm activity.
At night, the signal strength would be about the same as long as the FoF2 was above 3.5 Mhz, which it would be likely to be in the early evening, since the F layers (remember them?) would be likely to still be in good shape and able to reflect that frequency straight back down to Earth. However, the D layer does not exist in darkness, so there would be no reduction of noise and every man made noise source along with every thunderstorm in your hemisphere spanning the globe all the way from the sunset line back east to the sunrise line would be clearly audible. It is not unusual for nighttime steady state noise levels in the summertime to be over S9 on the low bands, with occasional static crashes even higher, perhaps as high as 30 db over S9. Your 100 watt NVIS signal would be buried in this noise. During the winter, when thunderstorms are much less common, this situation improves significantly, and noise is less of a problem, but your signals can be propagated all over the dark side of the globe. This is why ham operators typically try to operate on the low bands for long haul communication more during the winter.
With 160 meters excepted, (160 can propagate in odd ways and reflect off the ionosphere at odd angles, a potential advantage with respect to avoiding being DFed ) radio signals that bounce off the ionosphere at low angles of reflection tend to reflect in straight lines, so even at hundreds of miles or even a thousand miles away, you can be DFed within ~3 degrees or less even at that distance. Keep in mind that between a quarter and a third of all U-boats sunk during the Second World War were lost to a direction finding antenna invented in 1919 (do an Ixquick search on ‘Adcock antenna,’ ‘goniometer’ and ‘Huff-Duff’.) U Boats transmitted their messages in less than 20 seconds, btw. Then consider that modern DF equipment is MUCH faster than a manually rotated knob.
So give some thought to where you want your signal to go. Choose a time of day that is least likely to cause you problems, either with regard to being heard by unfriendly ears, or with regard to noise.