The military and diplomatic radio situation in Europe at the end of the novel 1633 is a result of a unique combination of the authors' needs in the story line, the limitations imposed by the authors' choice of town to base Grantville on, and other historical accidents which left us with a wealth of some technologies and a dearth of others.
There are four important elements to the radio background of the 163x series: the environment that the planet and solar system provide due to Eric Flint choosing to start the series in the year 1632, the people of Grantville, the physical resources they have available, and the goals of their government.
From a political perspective, 1632 occurs during the Thirty Years War. From a social perspective, 1632 occurs during the "Early Modern Era." From a biographical perspective, 1632 features players who are still household names, such as Cardinal Richelieu, Galileo, King Charles the First, Oliver Cromwell, etc. It's a fascinating time and a critical point in the development of western culture. When Eric contacted me and asked that I help brief him on the possibilities for radio in 1632, it became quickly clear that from a radio specialist's perspective, Eric could not have chosen a worse time to drop a town into than 1632. Just at the beginning of the period where there are telescopic observations of the heavens, approximately simultaneous with the trial of Galileo, 1632 drops Grantville into the beginning of a time best known to science as the "Maunder Minimum."
At about the same time as Galileo published his description of his construction of the Dutch invention of the telescope, natural philosophers throughout Europe began noting that the sun had imperfections, "spots" on it. This was far easier to watch with a lens, since you could project an image of the sun onto a white sheet, and observe it without destroying your eyes. The novelty led several natural philosophers to begin a program of noting the sunspots on a regular basis. Therefore, we have an excellent European record of the number of sunspots starting with Galileo's first such observation in 1610.
This notion of the imperfection of the sun would have come as no great surprise to the court astronomers of China and Korea. In the court logs of the observations of those staff astronomers, there are sunspot records made with the naked eye going back another millennium and a half. Using those records, we can trace the sunspot number from about 28 bce, using a reasonable relationship between the capabilities of naked-eye astronomers and those using projections and lenses.
For all of this two thousand year period of recorded observations, the number of sunspots on the surface of the sun has varied in an eleven-year cycle. As of this writing, in 2003, we are near the falling side of the peak of the current cycle with sunspots near the historic high of over 200. This extreme activity has resulted in spectacular auroras being seen as far south as 30 degrees north (Oklahoma City). At the other end of the measure, the lows have had sunspot numbers in the low teens to the mid-20s. The "average" low is between 20 and 30.
For reasons that no one understands, starting in about 1610, the number of sunspots plummeted. By 1632, which should have been a peak year, the sunspots were down to the mid-teens, and by 1640, had dropped to zero. (There is an anomalous high data point in 1639.) The 11-year cycle did continue, with peaks as high as 8 or 9 between 1645 and 1700. Then, again for reasons that no one understands, starting in 1710, the numbers went back up, and have continued quite regularly for the last three hundred years. This is not a "lack of observations" artifact, since the court observations in China and Korea correlate quite well with the western records. This is real.
Recent work by observational astronomers using a combination of new techniques by really really smart people on type G2V stars like our sun have figured out a way to measure the sunspot number of a star even though we cannot "image" the star. This work indicates that G2 stars may typically spend as much as 20% of their time in this "quiescent" mode. It could start again tomorrow. No one has any models for why it happens, or what causes it, or why it stopped. It's all quite confusing.
So what, you say? Well, it turns out that the number of sunspots is very highly correlated with the thickness of the upper layers of the ionosphere. There are several "layers" in the upper atmosphere, which get ionized for different reasons. These are labeled, from the inside out: D, E, F and "topside."
It turns out that the number of sunspots is very highly correlated with ability of the atmosphere to reflect radio waves back to the earth. There are several "layers" in the upper atmosphere, which get ionized for different reasons and have different effects on radio waves.
The layers are caused each day by the action of solar X-rays, ultraviolet light, and charged particles streaming out from the sun on the earth's upper atmosphere. This is the same action that splits O2 apart and gets the free oxygen that can combine into O3, to form the "ozone" layer. Ionization increases in the sunlit atmosphere and decreases on the shadowed side. Although the Sun is the largest contributor toward the ionization, cosmic rays make a small contribution. Any atmospheric disturbance affects the distribution of the ionization. These ionization layers form every morning at sunrise, thicken throughout the day, and then begin to fade at sunset. The combination of their chemistry and their electrical properties causes them to absorb and reflect radio waves. The amount of the ionization in each layer controls the absorption and reflection of radio waves. High frequency radio waves are absorbed by the weakly ionized D region.
Further out, the topside and F2 layers are ionized not only by UV light, but also by the action of the solar wind on the outer layers of the earth's atmosphere and its interaction with the earth's magnetic fields. During periods when there are lots of sunspots, the sun puts out a lot of particles, and these ionization layers are quite thick and robust. Without the solar action, during sunspot minima, the F2 layers are thinner and weaker.
The thicker and more robust the outer layers are, the shorter the wavelength they can refract or reflect. During sunspot maxima, the maximum usable frequency (MUF) can get as high as 30 or even 50 MHz (six meters). That is, 30 MHz signals can bounce right off the ionosphere, or be trapped between two upper layers and ducted around the world before breaking out and coming down most anywhere. That's how CB radio "skip" works, when folks listening to the radio in their cars on the highway in Kansas hear the chat between boats working the shrimps in the gulf of Mexico. Normally a CB radio is good for 5 miles, but when the sunspots are high, all bets are off.
During a normal sunspot minimum, when the sunspot count is down around 20 or 30, the MUF stays up around 14 MHz for at least part of the day, and seldom goes below 7 Mhz.
Frequency and wavelength are related. The higher the MUF, the shorter the wavelength and the smaller the antenna that is needed to send and receive radio signals. In general, one wants to use as short a wavelength as possible, because the higher the frequency, the smaller the antenna needed. A 30 MHz transmitter uses a "natural" antenna that is only three meters long. But a 7 MHz transmitter uses a natural antenna that is about twenty meters long.§ Thus, the higher the MUF, the more convenient it is to build radio installations. Most Hams therefore work the 20-meter bands, and the 40-meter bands are not uncommon. But it's the rare Ham who works 80 or 160 meters, since the natural antenna for 80 meters is 40 meters long, and 80 meters long for the lowest common Ham band of 160 meters.
However, remember the missing sunspots? During the Maunder Minimum, during the period that Eric has set the 1632 series in the middle of, the F2 layers of the ionosphere go away to a great extent. Of course, there is always some solar wind, some extreme UV, and some ionization by solar X-rays and cosmic rays. Thus there will be some ionization and some reflection. But for the purposes of the story, the tech team and the authors have decided that the MUF keeps dropping and dropping toward the lowest usable frequency (LUF) until, by the year 1640, to do long-distance communications without relays you would need to be using 2 MHz for much of the day, and can get up to 4 MHz only late at night.
And remember that the D layer absorbs the radio waves, so the low MUF means that you have little if any ability to do long distance communication during the day at all. The absorption of the layers defines a lowest usable frequency, an LUF. It is possible for the LUF to be higher than the MUF. Then, nothing much reflects.