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In "Marine Radio in the 1632 Universe" (Grantville Gazette 52) and "1636: Marine Radio in the Mediterranean" (Gazette 66) we explored the possibilities for communication across salt water. We also considered, briefly, a few overland paths of special interest to the Navy and commercial shipping interests.
Here, we'll turn the focus to communication across land. As before, we'll concentrate on reliable Morse code message-handling at commercial speeds and not other radio services such as broadcasting or navigation.
In the previous articles, there were certain routes of particular interest, for which we could calculate power requirements. It's much less certain where military units will operate in the coming land campaigns, so instead we'll estimate the distances achievable with the power levels and antennas most likely to be available. Where to apply those capabilities must be left to authors and their topographic maps.
Due to the complexity of the subject, this will be a simplified treatment of some representative cases. It would be impossible in a brief article to give thorough coverage to the motley menagerie of physical effects by which a radio wave can propagate across land. Not only are there entire books on the subject, but a thorough engineering analysis of any communication route requires topographic maps, ground conductivity maps, and local atmospheric data which neither we nor our fictional characters have.
Beyond those limitations, canon decrees a decades-long hiatus in the high frequency ionospheric skip by which hundred-watt ham stations in our own era are accustomed to reach halfway around the world. That leaves our down-time friends with a remaining menu of propagation modes for which there is little published performance data in the high frequency region. It's possible to extrapolate from the handbook charts, but the uncertainties will be larger, and some useful physical effects may be overlooked altogether.
Fortunately, our purpose here is not to achieve the accuracy and certainty which professional communication systems engineers are called on to accomplish in the real world. That takes shelves of reference books, adequate time to collect and analyze field survey data, and years of experience. Our objective is to offer reasonable guidelines for plausibility in science fiction.
What we can do, then, is examine the major workhorses among the many land propagation modes and run the numbers for some representative cases. Those results can suggest when our characters could plausibly get a message through, when they couldn't, and when communication could become marginal and intermittent.
Overview, for the non-technically inclined reader
Grantville Gazette readers and authors come from a wide variety of backgrounds. A few preliminary remarks may be helpful to orient those whose first language isn't tech talk.
First, the folks who are faced with setting up radio communication, whether in the real world or in our fictional universe, have a variety of goals that revolve around what reliable range is achievable with what means and at what cost. The tradeoffs get tighter if the station must be mobile; limitations on equipment size, weight, and antenna height affect range. And, all of this is a moving target. The bounds of what is technically and economically feasible will expand, rapidly at times, as the electronics industry and the national economy mature.
Second, radio waves can travel from place to place by several different physical mechanisms, called "propagation modes" in tech jargon. They often occur in combination along different parts of a single geographic path. Each mode has its own quirks. The details of how a signal becomes weaker as it travels further from the transmitter determine what range is possible using a particular frequency, transmitter power, antenna design, and station location. We'll examine three major propagation modes: ground wave, free space, and sky wave. We'll also look at diffraction and reflection. Whether to think of the latter two as separate modes is as much a matter of semantics as anything else. They're separate physical effects, but in practice they generally show up as part of a path that's otherwise free-space.
Third, the variables that radio specialists juggle are station location, transmitter power, frequency, antenna design, the height of the antenna's supporting structure, and the surrounding terrain. Location can be a compromise between where the communication is actually needed, and where it's possible to get a signal out past terrain obstacles. Power and frequency both depend partly on the transmitter technology (tubes, electromechanical alternators, spark gaps).
The very longest ranges occur with night-time sky wave, largely limited by our period's quiet sun to frequencies below 700 KHz (wavelengths greater than 428 meters). Consequently, maximum performance requires very tall and expensive antennas, and high power to overcome the strong natural noise at such low frequencies.
Conversely, mobile operations favor the smaller antennas that go with higher frequencies, and operate mostly by ground wave and diffraction-boosted free space modes. Ground wave ranges decrease with increasing frequency, but not in a linear fashion. Free space ranges depend almost entirely on antenna height above surrounding terrain, and diffraction is governed by bend angle over terrain obstacles.
Where we stand
By 1636, Grantville's electronics industry is no longer strait-jacketed by the dwindling legacy of up-time parts. In the last year and a half, it has crossed the threshold of sustainability. It's now manufacturing all the components for a simple but practical tube-based radio communication station. Production is still limited, but growing all the time.
The main focus here will be on the performance achievable with that equipment. However, we'll also touch on the fairly numerous fractional-watt "tuna can" transceivers made earlier from salvaged up-time transistors.
Calculations will lean toward the conservative side. The criterion throughout is a reliable and predictable communication service for military and commercial needs, when conditions are at the unfavorable end of their natural range of variation. At other times, signals are likely to be stronger and easier to copy.
Supporting technical information
The Terminology section of the original article in the series "Marine Radio in the 1632 Universe" contains a good deal of background information, which readers may find helpful to review. Two of the definitions are ubiquitous in propagation and antenna calculations, and worth repeating here:
Decibels or dB: A logarithmic way to express a power gain or loss ratio P2/P1
The dB form of expression is very convenient. Gains and losses expressed in logarithmic form can be added up algebraically, instead of multiplying very large and small numbers. Gains are positive, losses are negative. For example, an increase in power by a factor of 10 is +10 dB, while a decrease by a factor of 1000 is -30 dB.
Absolute power levels can be expressed as dB relative to some stated reference level, such as one milliwatt or the thermodynamic noise floor of a reference antenna.
dBm: decibels relative to 1 milliwatt
1 W=+30 dBm
Fixed versus mobile stations
One very convenient way to classify radio stations and networks is by mobility.
1636 is a little early for the industry to achieve the miniaturization and the high frequencies best suited to mobile-in-motion operation.
In the context of 1636 logistics, a reasonable definition of a "mobile" land station is one that can be transported in any vehicle up to a horse-drawn heavy freight wagon or a river barge, and set up in the field in half a day or less. "Fixed" stations would be everything else.
Mobility has a major impact on the practical size of a station's equipment and the amount of radio frequency power it can generate—and indirectly, on the frequency bands and propagation modes it can use most effectively. The lower the frequency, the longer the wavelength, and the larger an antenna must be if it is to deliver optimum results.
There are degrees of mobility. For a wagon-mobile station, the height of a tall tree is a practical limit for an antenna structure, whether actual trees or guyed poles are used to support the antenna. Sustained operation at up to fifty watts would be reasonably manageable for this kind of station. Anything more than that would present some difficulties.
Five watts and a wire antenna would be more reasonable for a station that must be transported in a mounted scout's saddle bags.
A likely practical limit for a major fixed station in this period would be a single guyed tower 150 meters high, with steam or water power to run the transmitter. Depending on the transmitter technology and prime power source, a kilowatt or more would be possible.
Signal types and technologies
We can also classify communication stations according to the type of signal they can generate and receive. That, in turn, depends on the transmitter and receiver technology.
Tubes, transistors, and electromechanical alternators generate a fairly pure continuous sine wave, a "CW" signal. This concentrates the power into the minimum bandwidth necessary to contain the on-off keying of a Morse code signal—on the order of 100 Hz wide. Since the amount of natural noise that gets through the receiver is proportional to the bandwidth of the receiving filter, a narrow signal helps in maximizing the signal-to-noise ratio.
The CW signal has no modulation other than the keying. It must interact with a tube or transistor oscillator in the receiver to generate an audible tone. Again, this helps maximize the signal-to-noise ratio by not wasting power on a steady carrier wave that contains no information. On the other hand, it also means that Grantville-made components are required in the receiver as well as the transmitter.
Large fixed CW stations would start to appear toward the end of 1635. They would grow over the next few years into the backbone of Europe's new communication infrastructure. Once that backbone is up and running, a mobile unit (or one station in a mobile net) would only need to set up where one of these big stations can hear it. From there, it could dispatch a message anywhere the net reaches. Think of the fixed stations as the late 1630s information superhighway.
Spark stations could be built nearly anywhere in Europe using down-time skills and materials, and they could be built long before Grantville learns how to make tubes. Rick Boatright has suggested that enterprising down-timers will get busy bringing up local spark nets and relay arrangements as soon as the cheat sheets appear.
Unfortunately, a spark transmitter's output is a train of poorly-shaped short bursts of radio frequency power that repeat at an audio rate. This results in a low average power output and poor frequency control, spreading its limited power across a wide bandwidth.
Complementing the spark transmitter, a crystal set doesn't require Grantville's manufacturing facilities, either. It can receive the burst-modulated spark signal, but it has both wide bandwidth and no amplification. It lets a lot of atmospheric noise through, and it's not very sensitive.
Consequently, spark stations make much less effective use of their power than CW stations. They're far from useless, but their effective range is nothing like that of CW stations of similar power consumption and antenna design. Worse, far fewer of them can operate in a given frequency band without mutual interference, because of their broad signals.
Most of the calculations that follow will be for CW, which is much easier to describe mathematically as well as much more effective. We'll get to spark, though.
Suitable frequency bands for land communication
For a given communication need, the choice of band depends on a variety of considerations. For any propagation mode, some bands work better than others, or reach further than others, or require less power than others, or are easier to build equipment for than others.
By 1636, we can expect a first-generation family of simple tubes that deliver reasonable efficiencies at frequencies up to perhaps 15 MHz, at power levels from under a watt to a few hundred watts. That isn't everything the communication services would like to have, but it's enough to accomplish quite a lot. It will be a couple more years before the industry can master the design, materials science, and manufacturing of the more complex and expensive tubes that will open up the higher frequencies.
Electromechanical alternators top out at around 600 kHz, but can reach tens of kilowatts.
On the other hand, 500 kHz is about as low in the spectrum as we can expect the early builders to construct full-size transmitting antennas, even at the largest fixed stations. A standard quarter-wave vertical antenna for that frequency requires a 150-meter tower centered on a radial-wire ground plane 300 meters across. (The radial wires need not impede farming or grazing if they're buried or elevated.) Such an antenna could be externally tuned down to 400 kHz or so and still perform fairly well.
To get a feel for the size of this kind of structure at such a low frequency, look at this picture from the Wikipedia article on antennas: https://en.wikipedia.org/wiki/File:Sendemast_Hirschlanden.jpg. Even this example is slightly shortened from optimum height, with a small capacitive top hat.
Below that frequency we'd have to accept the engineering and cost tradeoffs of shortened antennas, which are both more expensive and less efficient. This picture from the Wikipedia article on T antennas is probably at about the maximum height that could be built with wood lattice towers: https://en.wikipedia.org/wiki/File:Antenna_of_WOR-AM.jpg.
Many low-frequency antennas are a lot more complicated and expensive than that. See this example: https://en.wikipedia.org/wiki/File:Grimetonmasterna.jpg. They're technically possible, of course, but not likely to happen this early.
The cost and real estate of huge antennas isn't the only obstacle to the early use of the favorable propagation characteristics at low frequencies, either. The atmospheric noise rises very rapidly below 500 kHz, requiring much more power to be heard at the greatest potentially possible distances. It's doubtful that such super-powered transmitters would be feasible or affordable this early.
Bottom line: in this period, the most useful frequencies lie between about 400 kHz and 15 MHz.
Propagation across land often doesn't lend itself to straightforward rules and calculations, because land isn't a uniform medium. It's not flat, the ground conductivity varies from place to place, and some locations are covered by lakes and swamps instead of low-conductivity dirt and rock.
Multipath effects are common. Signals can arrive at a receiver by multiple propagation modes, and along multiple terrain paths by the same propagation mode. They can add in phase, enhancing the signal strength by 3 to 6 dB, or add out of phase, causing deep cancellations of 20 dB or so. As the temperature and humidity distribution of the atmosphere changes, the arriving signals can drift in and out of phase, sometimes as rapidly as a couple of times a second.
Different parts of a single path often involve different propagation modes, making calculations complicated even where the detailed data exists to estimate path losses. This article will focus on conservative estimates for several fairly simple but common types of land paths.
As before, we'll concentrate on propagation modes that can provide reliable day-in, day-out service at commercial Morse code speeds. Exotic modes that provide only sporadic openings are of interest to hams, but usually not to military services and businesses, unless an author wants to use a freak band opening as a plot device. (There are ways that can happen, especially in summer.) We'll also leave out of the discussion potentially useful modes that would require hardware not yet available.
With the tubes and other radio parts expected to be in at least limited production by 1636, the USE and its partners could reasonably expect to exploit (or wrestle with) the following modes for land communication:
- Ground wave
- Free space propagation
- Sky wave
Ground wave mode
Ground wave is an interaction between a radio wave and the electrical conductivity of the earth. The traveling wave induces currents just below the surface, which cause it to deflect downward toward the surface so that it follows the curve of the earth. The path losses and power requirements are fairly simple to estimate with the aid of the graphs in the Radio Propagation Handbook. Land is much less conductive than salt water, particularly poorly conductive European land, so the propagation losses are far greater than we calculated in the marine radio articles. Therefore, the usable ranges are much shorter.
We can generally ignore topography for ground wave; it doesn't have a strong influence at the frequencies where ground wave is usable. For that reason, ground wave range offers a conservative minimum level of performance that we can be reasonably confident will be available along any route, regardless of the intervening terrain. If the terrain is favorable, other modes may allow communication with smaller antennas and less power, but if not, ground wave will still be there.
Frequency selection for ground wave is a complicated tradeoff. The lower the frequency, the lower the propagation losses, and the greater the potential range. Unfortunately, the lower the frequency is, the taller the transmitting antenna must be to get reasonable efficiency and the low radiation angle needed to launch its power along the surface. And, the lower the frequency, the higher the atmospheric noise is, so low frequencies require more power to take full advantage of the superior propagation. In the OTL world, very low frequency ground wave signals have traveled to the far side of the world, at the cost of enormous transmitting antennas and colossal power.
With the power levels and antenna heights likely to be feasible by 1636, it would be impossible to exploit low-frequency (under 300 kHz) ground wave to its fullest. As we'll see, though, what can be affordably achieved at practical frequencies is of great value.
Under these constraints, 500 kHz is something of a sweet spot for long-range ground wave. Therefore, we'll calculate poor-earth ground wave ranges at that frequency. We'll also do the calculations at 5 MHz and 15 MHz. Those frequencies are within the capabilities of the first generation of down-time tubes, and they're better suited to the antenna dimensions and power levels of a land mobile station.
Free space propagation mode
Mathematically speaking, pure free space propagation is the simplest to analyze of all modes, and is by far the least lossy. "Path loss" for this mode doesn't involve actual power dissipation along the propagation path at all. It's just a mathematical expression of the continuous decrease in power density as the spherical wavefront expands away from the transmitting antenna and grows in frontal area—the classic "inverse square law" that follows from simple geometry and the capture area of the receiving antenna.
Unfortunately, that ideal can rarely be achieved in practice anywhere near the earth's surface. Even at microwave frequencies, antennas can't be made sufficiently directional to avoid reflections off the earth along point-to-point routes. Consequently, wave interference between direct and reflected paths is unavoidable. About the only place it could be applied in pure form is in high-angle communication with aircraft. That's outside the scope of this article.
However, an approximation to free space propagation can occur over much of a path, if at least one end of the link is many wavelengths above nearby terrain, and the reflections are off lossy surfaces. A common practical case is communication between a hilltop base station and a mobile unit on flat land. While most of that type of path might be unobstructed, the last part of almost any terrestrial path comes within a wavelength or less of the earth as the wave leaves or approaches an antenna near ground level. That terminal portion of the path transitions into high-loss ground wave. The Rural Electrification Administration's publication Power System Communications: Mobile Radio Systems has loss curves for that type of mixed path down to 40 MHz. With an adjustment for the larger capture area of an antenna scaled for 15 MHz, we can extrapolate path loss at the frequencies our 1636-period tubes can handle.
Diffraction is an electromagnetic phenomenon that causes a small portion of a radio wave's power to re-radiate from the edge of an obstruction and propagate into the shadowed space beyond. It's the reason you can hear an FM broadcast station when you're behind a hill. Given the bend angle needed to reach the antenna behind the obstacle, the diffraction loss can be calculated and added to the rest of the path loss terms. With that number, it's possible to calculate the increase in transmitter power needed to overcome the diffraction loss.
Diffraction very conveniently complements free space propagation. In a situation where a fraction of a watt might be enough to reach a receiver up in the clear on a hilltop, several watts to several tens of watts might be needed to be heard in the valley beyond. The synergistic combination of free space propagation and diffraction is a major workhorse of land mobile communication in our own era, and it will be in the 1630s as well—just at lower frequencies for the first decade or so. Interestingly, it will often work better at these lower frequencies, because the longer wavelength results in a larger effective capture area at the edge of the obstruction. Thus, more of the transmitter's power is available to be re-radiated into the shadow.