Underground and Spreading Antennas
Underground and “spreading” antennas are normally not used by the radio amateurs. Usually there is enough area in which it is possible to put up an antenna of choice. However, with cities getting larger, and architectural demands to keep the exterior of houses and communities attractive, placing the antenna underground may be one of the only solutions. Earlier, here in the USSR, underground and as well as other concealed types of antennas were used on military classified signal operation centers. Now with development of satellite communication systems, these unusual antenna systems are used less often. The items of information about underground and concealed antennas were not written about in unclassified documents not only in the USSR, but also other countries. Therefore, how these types of antennas were installed and used was considered highly confidential in earlier times. Now all these classifications have been removed from this data and the items of information about underground and concealed antennas has become accessible to the radio amateur.
Some Background
The “spreading” antenna is an antenna lying on the top of the ground which follows and blends with the irregularities of the earth’s surface and vegetation coverage. For the radio amateur, the concern is limited to concealment of the antenna from the neighborhood. But, during the First World War, an antenna farm of field radio set masts having a significant altitude 15-30 meters (49-98 feet) represented an easy target for artillery and thus, concealment was an absolute necessity. It was found that spreading the antennas on the surface of the ground enabled communications to be maintained, with the signals propagated in the same direction of the antenna wire leading from the transmitter. Thus, it was essential for the antenna wire to be pointed in the direction of the receiver’s location.
Also, at the same time of building submarine fleets, a need to communicate with the submarines was apparent. Radio communication on submarines used an antenna consisting of wires installed at a very low height of about 1-2 meters (3.2-6.4 feet) above the body of the submarine. This antenna type allowed communications to the bases on shore only when the sub was on the surface.
The development of the spreading ground surface antennas found its beginning in 1923 with the origin of the Beverage, followed later by the rhombic antenna. But, the evolution of the low-profile antenna used on the submarine to the more efficient DDRR type of antenna has taken almost 50 years. The theory for both underground vertical, and underground horizontal antennas was rather detailed. Spreading antennas were used for espionage over a 50-year period. To help conceal the spy antenna, they were commonly placed behind a city so as not to be easily seen and to defy directional-finding equipment.
Later, underground and disguised antennas were developed and widely used for classified underground military bases. For a while, information was readily available in the public libraries about the use of the stationary underground antenna for espionage, however, the publications have since been removed. Design of an antenna used for the purposes of spying and reconnaissance units has been classified for the past half century. Usually the antenna was nothing more than an insulated wire lying on the ground behind a city. This is where they were maintained for operation with the purposes of concealment and to hamper the ability to determine their bearing with directional finding equipment. Submarines submerged could send messages by radio communication using their regular short, low-height antennas. Articles are available now on this subject of underground usage, but they are hard to find. But, these types of antennas exist and have a use in the environment of the radio amateur. Let’s examine the operation of these underground antennas.
Operation of a Horizontal Antenna
Both vertical and horizontal underground antennas exist and have been used. In my opinion it is improbable that the radio amateurs will use vertical underground antennas. Instead, the use of the horizontal underground antenna is more probable.
As is known from the theory of antennas [1] a horizontal antenna, mounted close to the ground, during excitation establishes in the ground, a mirror image. The currents flowing in an antenna and the image are mutually cancelled at low altitudes foiling the antenna’s ability to radiate waves horizontally polarized. Now it can be said about an antenna installed at a low height of one meter or less, an underground antenna will poorly radiate horizontally polarized signals (Figure 1).
It is known that [2] vertically polarized waves are not absorbed in soil, to such a degree as waves horizontally polarized. Therefore, with the underground antenna, there is little radiation of vertically polarized waves present. It is necessary to pay attention to the fact that in the more conductive soil under an antenna, the antenna will be less effective and the radiation will be less.
The installation of an antenna above a poorly conducting surface, the currents excited in the mirror image antenna will be smaller. In other words, if the antenna is located in a sandy soil, the apparent height will be greater than if it were in moist soil. Thus, theoretically, the antenna height, “Í” of such an antenna at the ground level is in reality more than the “real” height, depending on the type of soil. In practice it means, that the horizontal antenna above the surface can radiate electromagnetic waves not only with vertical but also horizontally polarized.
At the contact on ground of a vertically polarized wave there is an inclination of the wave-front, i.e., the vector electrical component will be tilted with respect to ground, therefore the reception of vertically polarized waves is made possible. The antenna also receives horizontally polarized waves in a plane perpendicular to the elements of an antenna, but is much weaker, than the vertically polarized wave, sky or ground. The sky waves incident on an antenna from an ionosphere, are already tilted due to reflection from the ionosphere, therefore the antenna also receives sky-waves [2]. Sometimes such antennas are used specifically as low-noise receiving antennas, owing to their ability to reject the usual types of atmospheric and man-made interference.
The so-called “barkhan” (dune) antenna is another version of interest for the radio amateur application. The antennas was tried by the military on barkhan of sand deserts in Central Asia. It was found by spreading one of these antennas on barkhan, and appropriate selection of a physical length, frequency, coupled with the dimensions and shape of the barkhan, a type of dielectric lens is achieved causing an overall improvement in the performance of the spreading antenna.
The directive pattern of an antenna is exhibited in Figure 1. The maximum direction of the antenna is in line with the element of the antenna. Depending on construction of an antenna it can be calculated for operation mainly with ground wave signals, and for operation mainly with sky-wave signals. For the radio amateur with restricted operating environments, the construction of an underground antenna is a viable alternative. However, for the radio amateur information on the construction of such antennas has been hard to find. Now, with this article, it is time to present to the radio amateur the most accessible literature I have been able to obtain on this subject.
Input Impedance of the Antenna
Determination of input impedance of an antenna both theoretically, and practically [3] can be difficult, therefore we shall confine our discussion to only items of information necessary for the radio amateur to put into practice applicable to the construction of the underground antenna.
As is known from theory [1], the low-height horizontal antenna has low input impedance. But this applies only to resonant antennas. All low-height and almost all horizontal underground antennas are non-resonant, therefore another approach is necessary and the underground antenna becomes an alternative approach to this solution. So, the wire covered in plastic or aerial isolation by thickness H, is installed into ideally conducting soil (Figure 2).
In this case, the antenna of this type shall be considered as feedlines being buried in ideally conducting soil with the velocity factor depending on the property of the dielectric.The center portion of the line is the antenna while the grounding to the soil takes place on the ends. If the loss resistance of a load is matched with characteristic impedance of a load in a communications link, all power into the antenna is absorbed and radiation does not take place.
In reality the underground antenna lies in or on the soil, in which there are losses of high-frequency power along the wire of which the antenna is constructed. There are losses within an antenna as well as radiation. Thus, it appears a good choice of material for making an underground antenna is coaxial cable with the braid removed, or wire with thick plastic insulation, lying on the surface or immediately below.
Depending on the conductance and the dielectric property of real soil, the characteristic impedance of this type of antenna will be 100-500 Ohms on frequencies from 2-30 MHz. Despite the variation of values of characteristic impedance, usually it is possible to define and to match an antenna if it is aperiodic with load resistance (Figure 3).
Length of the Antenna
The first Beverage antenna (father of underground antenna) had a length of almost 10 miles. So, how does length affect the overall performance of a spreading antenna? As the theoretical analysis is analyzed more thoroughly, the data can be compared to the off-the shelf results discussed in the book, Ground and Underground antennas: The Soviet Wireless [3] publication was previously classified, but is now available. As is known, at propagation in an ideal transmission line there is no radiation loss or fading power. At propagation of a high-frequency current in the spreading antenna there are losses in the antenna – there are radiation losses, and losses in ground – both thermal, and dielectric. It is possible to say, that in radiation from an antenna the basic role is determined by the length of the antenna, during which the amplitude of a current (or voltage) diminishes by a factor of 10 as contrasted to by this voltage in points of power supply of an antenna. It is understandable, that after tenfold reduction of amplitude of a current, that part of an antenna, in which the current flows through, will radiate poorly. In Table 1 (taken from ref. [3] with some variations) the lengths of a wire are reduced after burying in soil with different parameters during which the amplitude of a current diminishes by a factor of 10.
From Table 1 it is seen, using given soil definitions for the underground antenna, there is no benefit from trying to improve overall performance of an underground antenna by increasing the length. For the radio amateur, the antenna installation (on basic types of soils), the length should be not less than 30 meters for effective operation on HF bands. Increasing the length of an antenna more than 60 meters, without raising it above ground, will accomplish little. If the installation is on dry, sandy soil, or on top of deep snow, the length may be greater than 60 meters.
The Type of Antenna
The primary choice for the underground antenna is the aperiodic (broadband) which is a progressive-wave antenna (see Figure 3). This commonly loaded on the end with a resistance equal to the characteristic impedance expected for this type of line forming to the underground antenna. Usage of non-loaded antennas is possible too. These antennas are not loaded, so the distribution of current and voltage is similar to their distribution normally present in resonant antennas.
In contrast to an aperiodic antenna having a large bandwidth, the non-loaded antenna’s parameters are rather unstable. The electrical length and input impedance depend on parameters of soil, on or in which the antenna is installed. As is known, these parameters vary over time and are rather unstable. But, sometimes, with stable parameters of soils (desert sands and other such wastelands), it is possible to tune an antenna to resonance, and have it remain stable. Considering all of the variables in parameters, it is impossible to recommend the proper length for resonance for any given frequency for an antenna in soil. Input impedance and resonance underground is not of prime importance for usage by the radio amateurs. Experimentation with the parameters will be necessary to find the optimum configuration for a specific frequency. Again, information for deriving appropriate theoretical information on constructing these antennas can be found in [3].
It is most expedient to use in radio amateur conditions an aperiodic antenna loaded on the end with a load resistor equal to the characteristic impedance expected for this type of line connecting to the underground antenna. At a length of an antenna of 30-60 meters installed immediately on a surface of average soil, the impedance will be in the range of 300-500 Ohms. That allows the construction of a matched load and the feeding of the antenna through known matching circuits commonly used among the radio amateurs. As the amplitude of the current and voltage on the end of the underground antenna is reduced greatly, the resistance of a load in the range of 100-150 Ohm is then possible for increasing the antenna current and efficiency.
Antenna Efficiency
As shown in [3] with a sufficient approximation it is possible to consider that the efficiency of an antenna (and, therefore the gain) on HF ranges constitutes no more than 10 % in comparison to an antenna of the same length and installed at a height of 2 meters above the same soil operating on the same frequency. But such antenna efficiency is achievable only with many consistent conditions. In reality, it can lie between values 1-10 % depending on quality of installation of the antenna and the parameters of the ground on which the antenna is installed. Though the efficiency is in the 1-10% range, power of more than 100 watts can be used with such antennas and it is possible to make contacts on radio-amateur bands under normal conditions.
The efficiency of low-height antennas is almost similar to the above-mentioned antennas, installed above ground at the altitude of 2 meters, but the vertical angle of radiation of an antenna is going to be 10-20 % more, than for a normal antenna. It allows us to assume, that using an antenna in a range 80 and 160 meters is possible for local contacts (up to 500 km). On bands higher than 40 meters, both local and long-distance communications are possible.
Again, due to variations of conductance and inductance of soil because of rain and other atmospheric conditions, the parameters of such an antenna will vary.
Practical Construction of Underground Antenna
For a construction of an underground antenna, it is desirable to use thick coaxial cable, not less than 9 mm diameter with the braid removed. The wire core of a cable, used as an antenna is weatherproof and the plastic insulation of a cable ensures acceptable installation of the antenna on irregular terrain surfaces of the ground
Grounding is of great importance for the underground antenna and there are two ways of doing it. The grounding is carried out the conventional way on the transmitter side and the ground load is connected to 5-10 un-insulated conductors 0,1 of the length of the antenna buried at a shallow depth in the ground. In Figure 4A the top view of this particular method of grounding is exhibited. In Figure 4B, the arrangement of conductors in the earth’s surface is reduced. It is necessary to pay attention that the radial wires of grounding should go out from the sides of the antenna. If the conductors cannot be buried because of hard soil (rocky), they can simply be laid out on the ground, but the efficiency of the antenna will suffer.
An alternate ground system is exhibited in a Figure 5. Two ground wires are connected on each side of the antenna spaced 1-3 m from the antenna and lie on ground.
The character of radiation and distribution of a current in this antenna differs from the classic spreading antenna. It is possible to assume, that the antenna efficiency factor will be not less than three times below the ideal spreading antenna. Its pattern lobe will show an increase in angle of radiation also. But, when it is impossible to realize reliable grounding (sand, deep ice or snow), the underground or spreading antenna is a good alternative.
Load of the Antenna
The load of an underground antenna can be made in a similar manner to the load used on a Beverage antenna. This is done by using one or more resistors connected to the ground system that will be equal to the estimated impedance of an antenna and to dissipate power loading to the antenna. The load of an antenna by a length more than 30 meters can be calculated for power equal to about 10 % of the input power of the antenna. It is necessary to provide protection of a load against atmospheric effects (rain, fog) using any traditional method.
As the input impedance of an antenna on different amateur ranges varies more on high-frequency and less on low-frequency, it is desirable to use a compromise value of 200 Ohms. In this case, the antenna will work satisfactorily in ranges of 6-160 meters and its input impedance will be in limits of 200-450 Ohms, depending on the range of operation. It is possible for the real spreading antenna to pick up a load on minimum SWR on any HF band, but having picked up this load for actual conditions with normal soil, the same resistance of a load will not be expected or optimum when that soil is frozen, dry or moist.
The input impedance of an antenna can be measured at the input connections. It is desirable to use an RF impedance bridge meter that is battery powered. It is possible to meter input impedance at the transmitter input. In this case the exterior cable braid becomes part of the ground system that will cause an impedance measurement error.
Some Experiments
I made an antenna on a roof at home. For this purpose I used an old coaxial cable by a length of 25 meters with an injured rotten braiding. The outer shell and braiding from a cable was removed and the cable placed immediately on a concrete roof which can be considered as ground. The antenna was located at approximately in the middle of the roof of a house. The end of a load of 300 Ohm was attached to the metal portion of the roof, attached to other metal house components leading to the ground. The antenna was fed through the broadband transformer 1:4, with the help of a coaxial cable of 75 Ohms. The end of the transmitter input to the antenna was also grounded in this manner. For the ranges of 6-160 meters, the load of the antenna had a resistance of 300 Ohms. The diagram of this antenna is exhibited in a Figure 6.
For comparing the efficiency of the spreading antenna, above a metal roof at the height of 1,5 meters, a Beverage antenna was installed having a similar load and power transmitter. The spreading antenna successfully conducted QSOs and compared to the Beverage antenna on HF ranges, the signal was higher from 2 to 8 points. On the low-frequency ranges the spreading antenna was worse than the Beverage and the signal dropped sharply. Some time later I operated a spreading antenna while lying on a loft of a 9-story building. On this occasion, the antenna managed local and long-distance QSOs on 10-160 meters quite well.
Conclusions
The antennas of this type fall into invisible espionage antennas. They can be installed on a roof of a house, on a garden section, being practically imperceptible. At the same time these antennas can provide both short-range, and long-range QSOs. When operating such antennas with a low efficiency factor, it is necessary to use higher transmitted powers and increased receiver sensitivity. It is desirable for this antenna to be made from coaxial cable with braiding removed. At installation of this antenna on or in the ground, it is likely, even at small depth that efficiency will suffer, but at least it will be absolutely imperceptible and allow operation. In my opinion, this type of antenna deserves much more experimentation.
Next month, in Part II, I will discuss more details of the construction of these types of antennas from the descriptions I was able to obtain about such antennas that were previously classified by the military. Until then,
The Literature References
[1] G.B. Belocerkovskyi. A Fundamentals of Radio Engineering and Antennas. Moscow:Radio i Svayz, 1983, 296 pages.
[2] M.P. Doluhanov. A Radio Propagation. Moscow: Svayz, 1965, 399 pages.
[3] G.A.Lavrov, A.S. Knyazev. Ground and Underground Antennas. Moscow: Soviet Radio, 1965, 472 pages.
Underground and Spreading Antennas - Part 2
Editor’s Note: This is an extremely interesting article about the origination of the underground and spreading antenna concept previously used primarily by the militaries in various countries. If you are interested in STEALTH, here it is! However, for the amateur’s application, it is unlikely many if any will want to go to the extent of installing the huge underground centers described for espionage or war-time use. BUT, when the principles described here and in Part 1 are analyzed carefully, the reader will have a good understanding of how it is possible for “backyard” installations where there is a need to operate in restricted or close environments. Igor has become an expert on stealth antennas, or invisible antennas as he calls them and has done actual experiments of his own with these concepts. So, this is far more than just theory! Enjoy! Jack L. Stone
Early Use of Underground Antennas
The experiences on development underground and spreading antennas were widely conducted, as early as the 1930s.
Before the Second World War, early research into the operation of underground antennas was made for the purpose of espionage and invisible antennas. These antennas needed be easily installed and set up in order to have a high useful effect as well as to provide operation of short-wave bands, specifically of 2-5 or 8-12 MHz, which was commonly used by the spies throughout the world for clandestine operation. In that time practically it was revealed, that most effective spreading antennas using a wire of about 1 mm diameter enclosed by a dielectric by with approximately 10mm diameter and inductivity . In those times an effective length of a spreading antenna for operation in the “espionage” ranges of 2-5 or 8-12 MHz required a length of about 30-45 meters.
During the Second World War and during subsequent years, active research was conducted into underground and spreading antennas for the RF ranges of 9 kHz through 50 MHz. The antennas for stationary underground radio centers were then developed. Easily installed espionage antennas for operation in short-wave bands for clandestine operation also were developed for portable use. The results that were obtained by the various countries working on these antennas, were classified by each government at the time. Nevertheless, some parts of the research documents about such activities fell into the public domain and that allowed us to conclude that the development of these types of antennas were actively used in the USSR, and certain other countries.
It was determined that direction-finding equipment had great difficulty in locating underground and spreading antennas operating on the short-wave bands. Spies operating on such antennas from a distance of more than 20 kilometers were practically impossible to locate because they were using the underground antennas. Because of this difficulty in finding the radio’s location, the underground antenna was excellent for field espionage operation from suburbs of large cities, and for stationary operation from places located at remote distances from large cities and direction finding centers. As the field strength from underground and spreading antennas is low even when near them, and the signal radiated even in the immediate proximity sounds like it is coming from a distant station, which is a result of ionospheric reflection, locating these espionage stations operating underground antennas was almost impossible.
In some books and magazines about the spies of the Third Reich operating in England, there are references about these underground antennas. In all cases it is possible with confidence to say in Germany, during the Second World War, underground and spreading antennas were developed extensively and widely used. Undoubtedly, the information about these types of antennas fell into the hands of the USA and USSR allowing these countries, over considerable time to advance in this area through more engineering efforts. The historical records will never list the names of those persons, both scientific and military, that first developed and used these underground and spreading antennas.
After WWII, usage and the development of underground and spreading antennas went in two separate directions. First, was that of creative antennas for stationary underground radio centers operating in a broad frequency range – from 9 kHz up to 50 MHz and secondly, to establish spreading antennas for army movable links on HF. Because times have changed, work on such espionage underground antennas has been curtailed for lack of need. However, now let’s consider the practical construction of the spreading antennas as used by the USSR army.
Practical constructions of spreading antennas in the USSR
It is possible to say, that the antennas, similar in construction, remain in existence of not only Soviet Soyuz, but also other countries.
I managed to get some detailed descriptions on antennas of this type, which has now been declassified. Let’s consider the ground antenna such as SA – 60/15. This antenna represents a copper cable with a steel core inside. The cable passes inside plastic triangular insulators, which help keep it is isolated from grounding (Figure 1). The cable is located in thick plastic insulation. The insulators will be separated from each other by a distance of 1,5 m. The two-wire line in plastic insulation is connected to an antenna. The length of this line is 5 m. The load resistance is a nominal 300 ohms and is protected from mechanical damage. The plastic triangles used for supporting an antenna are driven into the ground and therefore, they were made from strong plastic. The counterpoises were fifteen meters long and made of the same copper wire. They electrically were connected to steel pins that are 70 cm long, which were hammered into the ground at the counterpoise installation site.
Antenna “B” is made from a cable with a width of 4 mm. The counterpoise “A” is made from a cable, similar to the antenna. Counterpoise “C” is made from a cord having a pure dc resistance of 100 ohms. This counterpoise is part of the load of the antenna that allows the use of an antenna without grounding pins on any type ground – sand, permanent ice, etc. A two-wire line (similar to ladder line) with a length of 5 meters is connected to the antenna.
According to technical parameters the antenna works in a frequency band 1-50 MHz and can stand an input of 5 kW. In the files, the antenna is defined as a “Spreading antenna upgraded with counterpoises”. This antenna can be buried at a small depth. Spreading antennas were developed in mind for operation in hostile conditions where an exposed taller antenna could be blown away by an explosion. Such durability was necessary because under hostile conditions, it would not be easy to replace an antenna quickly using just natural objects like trees.
Underground Radiocenters and Antennas
The underground radio centers were established for different purposes. Some were constructed as powerful navigational underground radio centers operating in the range of very low frequencies. These underground radio centers with global coverage operated in a range of very low frequencies, super long, long and medium waves. Other underground radio centers operating only on short-wave bands were also used.
Construction of Underground HF Radio Center
As for use by the radio amateur, the greatest interest would be the operation on short waves.
In Figure 3, the construction of an underground radio center is illustrated. The underground radio center consists of a hardware hall (1) located at a depth of 15-30 meters under ground. The transmitters are connected by feed lines (2) to the hall of matching devices (3). It is at a depth of 2-3 meters from the surface of the ground. The aperiodic loaded antennas are connected to matching devices and the counterpoises are buried close the surface. To increase their efficiency, the antennas are placed in the funnel at a maximum depth of 2-3 meters. The funnel is made of a dielectric and is disguised from above by ground covering. The composition of the dielectric is such that this funnel cannot be detected from the air, seismographic, or other devices (I cannot mention other classified methods, which does allow detection). In the hall of matching devices the antennas can be combined in groups for creation of a desirable directivity diagram. The material these antennas were made of at these transmitting centers were bimetallic tubes (copper plated iron) with a diameter of 20 millimeters. For directional control the phasing method was used where some antennas in the group are powered up applying phase shifting rather than moving them.
To create the necessary directional patterns, some antennas will be fed by a phase-shifted signal. Elementary antenna is one single antenna from the group in which high-power from the asymmetrical ATU, but a lower level is fed to the opposite antenna in the array. Some other antennas which are not used for creating the directional pattern are fed low power also. The antennas which have low power act as a ground element antenna and the antennas with high-power are referred to as working antennas.
It is possible to synthesize practically any directivity diagram by an underground antenna system. For example, altering the angle of radiation as well as azimuth directivity. Sometimes similar radio centers were arranged with extendable masts with VHF antennas installed on them used for radio-relay or special link. With the appearance of satellite communications, underground radio centers as links or relays are no longer essential.
In Figure 4 the top view of the antennas of an underground radio center designed for short wave is exhibited. The length of these simple antennas range from 20 to 50 meters in length on various underground short-wave radio centers. The construction of similar underground radio centers was possible on soils practically of any types and therefore, in any location. The motion of soil, variation of parameters of soil generally made little difference on the parameters of the HF aerials, and it was possible to keep the required directivity pattern of the antennas.
Vertical Underground Antennas
Although the documentation about the design of the short-wave vertical version of underground antennas is rather cryptic, I was able to draw ample conclusions about the operation of such underground radio centers using these vertical antennas.
The vertical underground antennas were placed in tunnels drilled into mountain rocks. These mountain rocks were selected to have low losses on short waves. The inductivity e of these rocks should be no more than 4, conductance no more than 20 millisiemens per meter. In this case, inclination of vector of electrical polarization of an electromagnetic wave passing through mountain rocks is not very great. It can be said that the resulting signal consists of vertically polarized waves.
The vertical underground antennas are usually constructed asymmetrical. It is nearly impossible to build a symmetrical vertical underground antenna because of different soil effects on the components of the antenna system. The altitude of the antennas depends on how deep of a hole can be drilled in a mountain rock and how close it is to an aquifer stratum, which usually is used as the “ground” for the underground vertical antenna (Figure 5). The grounding for such vertical antennas consists of several non-resonant short ground rods and wire counterpoises, which are placed about the antenna base. The vertical radiator of the underground antenna is tuned to resonance with the help of special matching devices when transmitting.
When underground antennas are built according to Figure 5 for fixed-placed mounting in such mountain rocks, certain frequencies can be used where the rock formations are transparent and the signals “see” through the rocks. Thus, the rocks have little or no effect on the vertical polarization of the signal’s radiation if confined to these certain frequencies. If applied for use as horizontal underground and spreading antennas on these frequencies, it would be non-resonant and although it makes for broadband operation, it would not have the range of the vertical.
The underground vertical antenna shown in Figure 5 was used at one of the confidential German underground radio centers during WWII located in the Carpathians (now Western Ukraine), which I found in some historical files. Obviously, the creation of underground centers that use vertical phased antenna systems is possible also, but any reliable information about such centers could not be found.
Cave Antennas
A special class of underground antennas is the rhombic vertical loaded antennas. Before WWII, these were first tested by the USSR in the Crimea and Georgian caves.
The layout of this type of antenna is shown in Figure 6. The part of an antenna AB is immediately on the surface of the ground or at a small depth under the surface. The parts AC, BD, CD are placed in a hole either artificial or natural.
The first trials of these antennas were conducted on mountain plateaus (in USSR in the Crimea and Georgian mountain plateaus) where natural caves were located. The rhombic underground vertical antenna, as well as the above ground version is directional toward the termination load. The rhombic terminated antenna is a progressive-wave antenna and there is less influence by the soil on its operation. The soil does not affect it as much as it does to a vertical underground resonant antenna. By switching the load from a point B in a point A, and accordingly switching feed point from point C to point D, the antenna directional characteristic can be changed 180°. In underground transmitting centers such antennas were used with some phasing couplers to allow transmitting in several directions.
In my research of documents, there was mention of the rhombic vertical-type of installation in rocky formations as military hardened radio centers under Sevastopol (this hardened center is now sealed), designed by General Dmitriy Karbyshev (killed in German Mauthausen P.O.W. Camp in 1945). For construction of these antennas, a wire with a diameter of 1 mm in plastic insulation with a diameter of 10 mm was used. It is like a thick coaxial cable with the outer conducting braid removed. It is not known, with any accuracy, when and in what country for the first time this type of antenna made of coax without the outer braid was used. Then being convinced of its adequate efficiency, this material became commonly used in construction of these particular underground antennas.
In the USSR, for construction of underground antennas, a special “coaxial cable” configured as cable only with a copper center conductor and thick polyethylene insulation was used without an exterior metal braid shield. If a cable like this is ever found, you can be sure it was used for construction of underground antennas. From the regular coaxial cable it is distinguished only by stronger plastic outer insulation above the polyethylene insulation that covers the center wire conductor with absence of the metal braid. The exterior of such cable is shown in Figure 7. The outer shell of this cable is very stiff, hard to manage, mechanically strong and serves as the armored covering of the internal plastic insulation.
Underground radio centers that used the vertical rhombic terminated antennas are restricted to areas in which the terrain has high resistance and a small dielectric constant. Therefore, it exhibits a low degree of absorption of a high-frequency signal. It can be in the mountain areas with natural caves of rocky formation, abandoned mines, or any other similar places.
In my view the use by the radio amateurs of rhombic underground antennas in mountains is improbable, though the modern multi-story buildings and apartments represent a likeness of a rock mountain, and if desired, it is quite possible to install a loaded rhombic antenna in a building. During installation of such an antenna, it must be kept from close proximity from RF absorbing objects and the electrical wiring in the building. As the maximum directivity of an antenna of this type is directional toward the termination, the termination can placed in this case, not only in the upper corner of an antenna, but also in one of its lower corners or in the middle of one of its sides.
Underground Radio Centers of Super-Long Wavelengths
The underground long-wave and super-long-wave radio centers are more widespread in contrasted to short-wave centers. Often, a combination is used such as a long-wave radio center at a short distance from the short-wave radio center. The antennas used on long-wave centers are not commonly used for short wave, though in rare occasions there have been double application.
Underground and spreading antennas used for radio communication on super long and long waves, have similar features. On short waves the centers usually use underground and spreading horizontal progressive-wave antennas or vertical underground antennas. For super long and long waves the operation of spreading progressive-wave antennas are not very useful. The length of spreading antennas in these ranges cannot be more than 150-250 meters. This is because they become too lossy (that issue was discussed earlier in Part I of this series). Therefore, for better efficiency, spreading antennas of super long and long waves use only resonant-type of antennas.
The diagram of a spreading antenna for super long and long waves is exhibited in Figure 8. The antenna consists of bent elements 1 and 3, which with the help of the appropriate matching device, 2 and 3 are tuned to resonance. As shown by Figure 8, the elements of the antenna are bent in the shape of a boomerang. Therefore, such antennas in the USSR use the conventional name of “boomerang “. The construction of an antenna in the shape of “boomerang” on super long-wave band has an advantage. First, the area with defined and fixed parameters of soils is necessary for installation and setup of spreading antennas. At installation of an antenna in the shape of a boomerang, the antenna can be installed in a smaller area therefore it is easier to find a place for antenna installation.
At these underground radio centers, sometimes it is necessary to pump out underground water and freeze the ground under the antennas. In this case it is possible to create soil with low conductance. This considerably reduces loss of high-frequency power into the soil. The construction of underground radio centers in this case is more complicated because of the need for special water pumps and water freezing stations which must be located near the centers.
As seen from Figure 8, the spreading antenna of super long waves represents a symmetrical antenna which contrasts to the asymmetrical antenna in that it can be influenced by the soil characteristics. In this case, the antenna directional characteristic in a vertical plane will be along the conductor of the antenna (Figure 9). The directivity diagram in a horizontal plane of the antenna is perpendicular to the element of the antenna (Figure 9). But the horizontally polarized radio waves will suffer from severe absorption in soil, and owing to a short electrical length of the antenna on super long-wave frequencies, the horizontal radiation component of the antenna is insignificant.
The electrical polarization is equal to the direction of the current in the antenna system. The propagation of the power along the antenna conductors on the surface is such that the main radiation from the antenna is going to become the electrical component of the signal.
The input impedance of a spreading antenna for super long waves is in the very low ohmic ranges which causes engineering difficulties when matching these antennas. The efficiency of such antennas is very low being – 40 to -80 decibels compared to the half-wave radiator. It is necessary to note, that in the super long-wave band, even the stationary vertical antennas have efficiencies in the range of -10 to -20 decibels.
For construction of underground radio centers operating in short-wave bands, a series of antennas such as the “boomerang” will allow directional patterns to be generated by using phasing networks. InFigure 10 the top view of a spreading antenna system of an underground radio center is illustrated (in the USA, these radio centers are called “spider”). For the material to build such antennas, you must use a coaxial cable as described above that has no shielding braid. As such antennas cover large areas, and when in operation, the directional pattern can be largely influenced by soil motion due to erosion or other natural conditions. Also weather conditions, such as snow and rain will change the characteristics of soil conductance and, therefore, increase losses in the antenna system. The range of wave and long waves is used for navigating, global radio communication, command and control of the army in emergencies. Thus, these radio centers are in operation worldwide.
In Figure 11, the plans for the underground radio center for the short wave range is shown. The antennas of the radio center are buried at a shallow depth and 0,5-1 meters is common. In cable tunnels, feed lines for the antennas go to the room where phasing and matching devices are located. There the matching devices match all of the antennas and then the phasing networks generate the required directivity pattern. The hall of phasing and matching devices is, depending on the type of soil, at a depth of 10-15 meters. Detection of similar centers by method of radiolocation and audio locating device such as the oil company seismographic “thumpers” that search for hollow spots in the ground all have great difficulty finding the center since the entire installation is buried.
Just below the phasing and matching devices room at a depth of 15-20 meters, the room for the power supply and transmission devices is located. For operation during normal peacetime the normal electric power grid is used. For operation in emergency situations, diesel or nuclear power stations will be used. At a significant depth under ground, the room for the power supply and transmission devices is located so that it cannot be detected by its thermal radiation from satellites and troposphere reconnaissance flight vehicles (SR71). The cooling of this heat-generating equipment represents a serious problem. The water-pump and the refrigerating stations are partly installed in this room, but at some distance from it. They are also at considerable depth to help prevent detection.
The operations control room and life-support systems are installed below the power supply and transmission room at approximately 15-25 meters under ground. As now illustrated from the Figures and descriptions, the radio center of long wave resembles the construction layout of a radio center for short wave.
For the radio amateurs, spreading and underground antenna for long wave are also of interest. For the radio amateur the 136 kHz frequency is preferred and the construction of radio amateur spreading antennas for operation on long-wave band is quite possible. As of now for these purposes, the radio amateur usually uses vertical loaded antennas, which are complicated for installation and setup. The underground and spreading antennas for operation on long waves are good alternative to the vertical.
Originally posted on the AntennaX Online Magazine by Igor Grigorov, RK3ZK
Last Updated : 3rd June 2024