by David Fiedler
When German Panzer divisions blitzkrieged into Poland in 1939, their lead reconnaissance and combat-headquarters elements were mounted in modern six-wheeled armored scout and communications vehicles. These vehicles were nomenclatured the Sd.Kfz.232 heavy armored scout vehicle (six-wheel) and Sd.Kfz.263 heavy armored radio/command-post vehicle (six-wheel). The employment concept for this wheeled armor was very similar to concepts now being experimented with 60 years later by the U.S. Army�s initial brigade-combat teams.
Signal soldiers should note that both German vehicles were distinguished by their large-frame (loop) high-frequency radio antennas (about 12 feet by 6 feet) supported about 30 inches above the vehicle�s armored turret. The antenna covered a great portion of the vehicle length (see figure below). Its wide use revealed just how important mobile HF radio communications were to the command-and-control; reconnaissance, surveillance and target acquisition; and cavalry/scout missions in an armored force.
|A German C2 vehicle with HF loop antenna circa 1938. The vehicle was used as a command post for RSTA missions.|
These antennas were connected to FuG-11 100-watt radio sets operating in the lower HF (0.2-7.0 megahertz) frequency band. These HF antennas and radios provided both voice and radioteletype (data) mobile omnidirectional communications over wide areas at distances up to several hundred miles.
By 1939 radio engineers knew well that both vertical and horizontal loop antennas of this type produced high-angle radiation patterns (most of the energy at 45-90 degree takeoff angles) when placed close to the earth or other "ground" structure (in this case the vehicle) (see figures following, first figure). When radio-frequency energy produced on these angles contacted the earth�s ionosphere, much of this energy was reflected back to the earth in a circular pattern if certain conditions were met. This made beyond-line-of-sight radio communications in the HF frequency band possible since the circular pattern�s radius was hundreds of miles (see second figure below).
|Typical high-angle radiation pattern produced by loop, tipped whip, horizontal wire dipole and other antennas located close to the earth.|
|Typical omnidirectional energy pattern after reflection by the ionosphere. Radius is up to 300-plus miles without dead spots or gaps in coverage.|
The ionosphere is a belt of electrically charged gases surrounding the earth at altitudes of hundreds of miles. The ionosphere is created by the sun�s ultraviolet radiation breaking down atmospheric gases into electrically charged molecules (ions). The amount of ion density and thus the ability to reflect radio signals depends on the amount of sunlight and solar activity, and therefore depends on the time of day, time of year and current solar activity.
The angle at which a radio wave strikes the ionosphere and the frequency of the radio signal are the major factors determining if useful signal reflection will occur. RF energy on frequencies too high (usually above 10 mhz in daytime and five mhz at night) or angles too steep for existing ion-density conditions won�t be reflected back to earth and will continue into deep space. Signals on frequencies too low become lost in the ambient noise and can�t be used for communications.
The Germans knew that at all times of the day, year and solar (sunspot) cycle, there was always available a range of radio frequencies between two and 10 mhz that would reflect back to earth and could be used for tactical BLOS communications. That�s why they constructed their rather unique, special antennas. This form of radio propagation was named near-vertical-incidence skywave propagation because of the high-angle (nearly vertical) antenna pattern needed to produce it (see figures above, high-angle pattern graphic).
By the early 1930s, enough was known about radio physics that the solar-dependent, ever-changing (with day, month and year) band of reflecting frequencies could be predicted with good, if not yet perfect, reliability. When propagation is done skillfully, the pattern of energy returning to earth becomes a large circle with a radius of 300-plus miles (figure below). Since energy transmitted is returned to earth on high angles from above, there are no gaps, dead spots or skip zones in the received energy (signal) pattern. Likewise, the German antenna�s horizontal polarization wasn�t a problem since energy-robbing obstructions such as vegetation, buildings or the earth weren�t in the propagation path to cause signal loss.
|Take-off angle vis a vis relative distance. This illustration shows distances at which high-angle signals return to earth. Note there are no gaps in coverage as the signal returns. For simplicity, the ionosphere and earth curvature aren't shown.|
Tactically, this makes NVIS the ideal mode of communications when fighting in cities and on mountains as well as over wide areas. Essentially, NVIS path loss is loss in free space and turnaround loss when energy is absorbed or scattered as the wavefront contacts the charged ionosphere. These losses aren�t large, so BLOS communications reliability � even when using low-power radio sets � can approach that of line-of-sight systems as long as the proper antenna and frequency are selected and the ionosphere isn�t in a highly disturbed condition (seldom encountered).
For more detailed explanations of radio propagation and NVIS techniques, see Field Manual 24-18 or Army Communicator�s Fall 1983 edition.
In many respects, the U.S. Army is now going "back to the future" with HF tactical radios. After almost 30 years of being the only army in the world and the only service in the Defense Department failing to see the continuing military value of HF radio development, the Army has recently done an "about face" on a large scale.
The aviation, Special Operations Forces, medical and other Army branches have long recognized the value of HF radio for long-distance/wide-area communications in both ground operations and when engaged in "nap of the earth" flying. However, of late we in the Signal Regiment have virtually ignored this communications mode despite its success.
Until recently, the Signal branch declared that all BLOS tactical communications would be accomplished via satellite communications or by using ground-based retransmission stations. Signal planners assumed there would always be enough satellites or retransmission assets at the right locations with enough channel capacity, and enough ground equipment and money to handle our entire tactical long-distance/wide-area communication requirements.
This false assumption was adhered to at the highest levels, even when we knew the advent of automated systems would generate massive amounts of additional BLOS radio traffic at almost every location and tactical organization on the battlefield � down to the platform level. Accordingly, HF development ground to a virtual halt in the "big" Army, and the once-extensive HF institutional training and doctrine base of the 1930s, 40s and 50s eroded to an almost-nonexistent capability.
Through the 70s, 80s and 90s, the Signal Center unilaterally declared that remaining tactical HF radio required too much knowledge of frequency and antenna technology for the Signal soldier to employ, so HF radio was declared "user-owned and -operated general-purpose equipment." This allowed other branches like SOF, aviation and the Army Medical Department to provide their own Signal procurement, training and support for their unique HF communications networks, exclusive of any Signal Corps involvement.
This usually meant things were OK as long as everything worked. Unfortunately, plenty of Signal officers caught hell from commanders when things went bad. Unit Signal officers not only couldn�t fix whatever the problem was with these networks and equipment, but even worse, didn�t understand them due to lack of training at the Signal Center. Most commanders didn�t accept the Signal branch�s inability to support these unique communications networks, and I�m personally aware it reflected poorly on many S-6 officer-evaluation reports and on the Signal Corps� professional reputation.
Now, let�s roll the clock forward to the current era of Army transformation and IBCTs. An honest analysis of the brigade�s long-distance communications requirements revealed SATCOM and ground-based radio retransmission couldn�t be the answer for all BLOS communications requirements � for reasons too many to go into here. This has led IBCT designers to recognize the need for mobile-ground and platform-based HF communications. (For locations of equipment and net structures, refer to the applicable IBCT doctrine publications.)
IBCT Signal doctrine still considers HF radio to be user-owned and -operated, but the Signal Regiment has finally agreed we need to have HF-radio-trained staff in the IBCT Signal office (S-6) to sustain non-Signal users of HF systems. It�s now recognized the IBCT Signal staff needs to engineer HF radio nets, integrate HF equipment into brigade operations, know how to use HF communications� advantages for the brigade, and provide training and support to non-Signal users of the HF equipment. The return to tactical HF communications is a major step forward for the tactical Signal community and is now also a new major responsibility for the brigade S-6 in IBCTs.
Since the "big Army" hasn�t fielded any new HF equipment since it adopted the Marine Corps-developed AN/PRC-104/GRC-193 family of HF equipment (known as the improved HF radio) in the 1970s, we had to be judicious in our procurement approach. Due to the requirement to field the IBCTs quickly, time was essential.
The Department of the Army decided a new family of modern HF equipment was required, but it should be procured as a commercial-off-the-shelf, non-developmental item. DA also wisely decided that since the new family of equipment was a significant change in both acquisition approach and technology from previous developments, this effort needed to be project-managed. The result is the Army�s new Transformation High-Frequency Radio System, managed by the project manager for tactical-radio communications systems. (PM-TRCS is a subcommand of the program executive office for command, control and communications systems � both are headquartered at Fort Monmouth, N.J.)
After a long evaluation process, PM-TRCS determined that the HF radio system SOF, AMEDD and others in DoD are procuring would best meet the THFRS requirement with certain modifications and options. This system is the AN/PRC-150(C) family of HF radios, manufactured for the Army by the Harris RF Communications Corporation of Rochester, N.Y.
THFRS hardware is a family of HF radio equipment based on the AN/PRC-150(C) manpack radio (figure below). By adding various power amplifiers, couplers, antennas, software and ancillaries to the manpack radio, various vehicular and base-station configurations can be built. These configurations will eventually have their own separate nomenclatures, but currently all configurations are referred to as the AN/PRC-150(C) family or THFRS family of equipment.
THFRS� heart is the 10-pound, 10 � inches by 3 � inches by 13.2 inches AN/PRC-150(C) manpack radio. This radio and all ancillaries needed for manpack operations are provided to all configurations being built. This was done so THFRS users will have a "jerk and run" HF-radio capability to use in case they must separate from their wheeled, armored or base-station platform.
Two BB-5590, BB-590 or BB-390 standard batteries power the software-controlled manpack radio. This provides an output power level of one, five or 20 watts of amplitude-modulated, single-sideband power across the 1.6-30 mhz spectrum. The radio is also capable of providing one, five or 10 watts of frequency-modulated signal on frequencies between 30 and 60 mhz.
The waveform in this frequency range is either 16-kilobits-per-second wideband frequency-shift keying for data transmission or 16 kbs digital voice, which makes the radio interoperable with the Single-Channel Ground and Airborne Radio System in the non-hopping, encrypted or plain-text digital-voice mode. This mode is a very useful feature when tactical necessity dictates the need for communications with units that have no HF communications. Standard analog-voice FM mode with Vinson communications security is also provided, which will allow interoperability with organizations still equipped with pre-SINCGARS tactical FM radios. This includes many of our worldwide potential allies and our own Reserve Components, which still have large quantities of this equipment type.
Twenty and 150-watt vehicular configurations and a 400-watt base-station configuration complete the current THFRS family.
The AN/PRC-150(C) is a fully-software-defined radio, so new features or new revisions of existing standards can be added to the radio through software � not hardware � upgrades integrated into THFRS� receiver/transmitter, also known as RT-1964D(P)(C). AN-PRC-150(C)�s modes of operation and other features required many separate hardware ancillaries and system interfaces to achieve in the past.
Embedded THFRS operating features include:
|Automatic link establishment � military standard 188-144B ALE is provided in THFRS. ALE is capable of dealing with HF radio propagation variables in real time to establish the best communications link for current conditions. THFRS assigns each member of an HF radio net a unique address. It also assigns each net a list of authorized frequencies. A sounding signal is sent on each frequency authorized to a net at a predetermined time as radios scan the frequencies. If a sounding signal is detected during the scanning process, a link-quality analysis is performed on the received signal. The signal also contains the transmitting station�s address. The radio stores this data and, based on the data, the radio automatically selects the best frequency for communications at that time and the net is opened for traffic.|
The ALE process is repeated periodically. The ALE system accommodates individual calls (point to point), group calls (point to multipoint) and broadcasts. ALE greatly improves the probability of successfully establishing a useable communications system on the first try. This saves net time and reduces channel loading. More importantly, ALE removes the guesswork from the frequency-selection process and eliminates the need for many of the hard-to-train frequency-management and engineering skills past generations of HF equipment required. ALE is a major factor in the resurgence of tactical HF radio because it eliminates the basis of many past circuit-reliability problems that were rooted in poor RF-selection techniques.
For more information on ALE, see Army Communicator Spring 1994.
|Modulators/demodulators (modems) � radios cannot transmit or receive digital-data or digital-voice information directly. Digital signals must be converted into data formats suitable for transmission over the narrowband-voice (three-kilohertz bandwidth) radio channels found in all tactical HF radios, including THFRS. Received signals must reverse this process. In the RT-1964D, this is the function of the built-in modem(s). The RT-1964D provides three basic kinds of modems. They are: 1) slow-speed audio FSK, 2) high-speed parallel tone and 3) high-speed serial (single) tone. The high-speed modems are suitable for both digital-voice and -data communications. Low-speed modems are used for data (message text) communications. A standard low-speed AFSK modem capable of operation at rates of 75, 150, 300 and 600 bits per second is provided in THFRS. This type of signaling is standard in virtually every HF data system built worldwide over the last 40 years.|
At this time, THFRS will use this mode when communicating with older radios still in use by U.S. forces (for example, the AN/GRC-193) or with older HF radioteletype sets used by our allies. FSK signaling, along with analog voice signaling, is the "common denominator" present in almost every HF radio in the world, so it remains useful, particularly in coalition warfare.
A high-speed parallel-tone type of modem is also embedded in THFRS. This modem produces 39 subcarrier audio tones that fit within the three-khz wide radio audio channels. Each of these tones is modulated by a process called quadrature differential-phase shift keying, which simply means the data is represented as a shift in the phasing of an audio tone that can take four (quadrature) possible states. These phase shifts are then distributed, interleaved and synchronized over 39 subcarrier tones contained in the narrowband (voice) channel.
This type of modulation is excellent for recovering signals sent over a radio path that�s subject to multipath and signal-fading effects because all tones won�t fade at the same time. If data is coded and spread over all the subcarrier tones, the signal-processing techniques produce performance that�s many times better than the same radio circuit using older FSK modulation.
The THFRS 39-tone modem provides data rates between 75 and 2,400 bps.
In recent years, the 39-tone modem has been eclipsed by the third type of modem THFRS provides: the serial- or single-tone modem. Serial-tone modems are easier to build and perform better than parallel-tone modems under most conditions. Serial tone will be the most widely used data mode in THFRS. THFRS will normally use the 39-tone waveform only when communicating with other systems that have only 39-tone capability.
The military standard 188-110B serial-tone modem embedded in THFRS uses phase-shift keying on a single-carrier frequency to represent data. Many small shifts in phase can be created to represent various binary states. The addition of amplitude shifts to the phase-shift information can also be used to increase the amount of information contained during any time interval.
The serial-tone modem in THFRS can be used at data rates of 75 to 9,600 bps, depending on path conditions. In addition to U.S. standard serial-tone modem capabilities, North Atlantic Treaty Organization�s standard 4285 (75-2,400 bps) serial-data mode and 4415 (75 bps) robust serial data mode � used when severely degraded circuit conditions exist � are provided to enhance operations with equipment built to NATO standards.
|Electronic counter-counter measures � THFRS provides a robust frequency-hopping waveform to use with serial-tone data (75-2,400 bps) and digital voice (600 or 2,400 bps) to reduce the effects of interfering signals or an enemy�s intentional jamming. This capability also lowers the probability of intercept and detection by hostile forces.|
|Voice communications � both analog and digital-voice modes of communications are provided. In digital mode, voice signals are converted into digital representations and coded to correct errors. The digital-voice representations are then transmitted by the serial-tone modem at rates of 2,400 bps for clear channel operation, or at a slower rate of 600 bps for better performance over degraded channels. Six-hundred bps digital voice can provide effective voice communications in environments where signal strength and noise are actually equal.|
|Cipher modes � THFRS has embedded National Security Agency-certified U.S. Type-1 encryption capability. Standard KY-99, KY-100, KG-84C and KY-57 modes are provided for interoperability with compatible systems. Also embedded in the AN/PRC-150(C) is the internationally used Citadel encryption system � useful for coalition operations.|
|COMSEC key management and fill � THFRS will accept COMSEC keys from a variety of COMSEC load devices such as the KYK-13, KOI-18, KYX-15 and AN/CYZ-10 via the standard fill connector located on the front panel. Unique to THFRS is the COMSEC ignition key, which is contained in the radio�s removable keypad/display unit. By removing the KDU, the COMSEC is rendered inert. The KDU can be easily removed, safely stored and reinstalled. This removes the need to remove and reload COMSEC every time the radio is left unattended.|
|Transparent Internet protocols � in anticipation of the Army�s transformation to an IP-based architecture for the Army Battle-Command System, THFRS has internally provided the necessary connectors and IPs. This will allow seamless integration and direct connections with devices such as commercial and military personal-computer workstations, laptops, routers or servers without needing more external hardware and software "gateways."|
|System management � THFRS includes not only the radio hardware but also the means to manage/select various features and modes of radio operation without having to enter data via the radio�s KDU. Every S-6 section in organizations deploying THFRS will also receive an Army common-hardware-system laptop computer with radio-programming application software. RPA software has a user-friendly Microsoft Windows "look and feel" and will allow the operators to define and configure radio nets, assign ALE addresses, select presets for modems or store communications plans. Information can then be transferred to individual radios via a data-transfer cable to the front-panel radio data connector. This capability greatly reduces operator stress and errors in data entry when compared to using the KDU as the means of setting up the radio. KDU data entry remains available if needed.|
No discussion of THFRS or the Army�s return to HF technology for IBCTs would be complete without a few more words about antennas. As with every radio system, the key to success is always contained in the system antenna and frequency assignment. The IBCT needs low-power (five to 20 watt) manpack and vehicular HF radios for reconnaissance and scout operations; medium-power (20-150 watt) vehicular HF radios for command vehicles and tactical-operations centers; and high-power HF radios (150-400 watt) for reachback operations. The antennas provided need to to achieve communications while on the move (mounted or dismounted), at a brief halt (mounted or dismounted) or when fully deployed (dismounted).
The standard manpack antenna provided is a 3.1-meter vertical monopole antenna (OE-505). This antenna isn�t long enough to be a perfect match for the radio at all frequencies, and it�ll be shortened even more when the radio is manpacked and operated since it�s very difficult to carry even a lightweight radio with a 10-foot antenna. THFRS provides an internal antenna-matching network to produce the most efficient electrical feed possible under constrained conditions to the antenna. Under normal conditions over average terrain, omnidirectional surface-wave communications using this antenna fully extended can be expected at distances beyond 15 miles.
For circuits that require longer-distance omnidirectional communications, the RF-1941 wire antenna kit is provided. Kit components can be configured as a horizontal dipole, sloping dipole, inverted "V" or inverted "L" antenna for omnidirectional NVIS communications out to 350-plus miles. These distances are achieved because the high-angle (skywave) radiation from the antenna is reflected off the earth�s ionosphere and back to earth in a circular pattern without gaps or dead spots (see Figure [xx fied4.tif]), just as the Germans did back in the 1930s. This distance is far greater than the expected area of operations for a deployed IBCT, so it�s expected NVIS will be the most common mode of operation for the organization.
Also, RF-1941 can be configured as a sloping "V" or a vertical rhombic antenna for directional long-distance communications that will go for thousands of miles.
All vehicle-mounted THFRS, no matter what their power level, come equipped with 32-foot AT-1011 vertical monopole antenna. Due to their length and higher efficiency, omnidirectional surface-wave communications beyond 20 miles for 20-watt sets and 50 miles for 150-watt sets can commonly be achieved. Probability of extended surface-wave communications is also enhanced because signals on the lower (HF) frequencies tend to "bend" over the horizon, penetrate obstructions such as buildings and get weaker over normal terrain at a slower rate than higher-frequency signals.
Unique to THFRS, this vertical antenna comes equipped with a RF-1980 tilting antenna base that�s able to tip the antenna into a horizontal position when mounted on the rear of a vehicle or shelter. When units are on the move or at a brief halt, THFRS� vertical antenna can be physically tipped forward or backward. Backward "whip tipping" essentially creates a horizontal dipole with high-angle radiation (NVIS) by using the whip as half the antenna and the vehicle/shelter as the other half (see following figures). This being the case, gap-free omnidirectional communications out to 350-plus miles can also be expected while mobile or at a brief halt simply by using a backward-tipped whip.
|Effects of "tipping" the AT-1011 HF whip antenna on a radiated antenna pattern. Note high-angle radiation (NVIS) pattern when tilted 90 degrees.|
|Whip-tilt adaptor concept.|
Also, just by tipping the whip forward, a similar but lower-gain high-angle antenna pattern is created for on-the-move operation (figure below). This is the exact same effect (antenna pattern) as the elaborate German loop antenna of the 1930s discussed at this article�s beginning. At a more permanent halt site, vehicular radios are expected to deploy the more efficient RF-1941 wire antenna provided with each AN/PRC-150.
|Far-field elevation pattern of a vehicular whip tied to a vehicle's front. On-the-move NVIS is obtained by tying the whip forward.|
RF-382 antenna couplers provided with all 150- and 400-watt THFRS will produce the most efficient electrical match possible for both vehicle whip and wire antennas. Twenty-watt vehicular and manpack configurations will rely on the radio�s internal antenna coupler to get the best electrical performance. THFRS 400-watt base-station configurations are also provided the RF-382 antenna coupler for electrical matching and the RF-1912 dipole-antenna kit. The RF-1912 kit contains masts, guys, cables and other items sufficient to construct a fixed high-efficiency antenna for base-station operations.
By properly using THFRS, IBCTs can expect to have highly reliable BLOS communications over corps-size operational areas without needing SATCOM or ground-based/airborne retransmission stations as long as the S-6 ensures correct antennas and radio frequencies are provided. The direct IP connection will make extension of IBCT data networks such as the tactical Internet easy to accomplish.
THFRS by itself won�t, of course, provide all communications IBCTs require. THFRS will provide a viable means of BLOS voice and data communications for TOC to TOC, reachback and RSTA missions that can�t be performed by other systems under all circumstances. THFRS will certainly both complement and supplement other communications systems when IBCTs fully deploy. Under many conditions, THFRS will indeed be the only mobile tactical-communications link to air, maritime, civil or allied forces, which makes it of critical value to the IBCT.
Over the past three decades, the Signal community has been wrong in neglecting the military value of the communications spectrum�s HF portion. We�re now striving to right that wrong, but we won�t be fully successful until HF technology is again fully addressed in the Signal school curriculum and all the senior Signal leadership again recognizes HF systems� value. It�s a bitter thing to have to relearn lessons our grandfathers learned the hard way, but if that�s what it takes, then we need to get on with it. HF radio has returned to the Army.
Mr. Fiedler � a retired Signal Corps lieutenant colonel � is an engineer and project director at PM-TRCS. Past assignments include service with Army avionics, electronic warfare, combat-surveillance and target-acquisition laboratories, Army Communications Systems Agency, PM for mobile-subscriber equipment, PM-SINCGARS and PM for All-Source Analysis System. He�s also served as assistant PM, field-office chief and director of integration for the Joint Tactical Fusion Program, a field-operating agency of the deputy chief of staff for operations. Fiedler has served in Army, Army Reserve and Army National Guard Signal, infantry and armor units and as a DA civilian engineer since 1971. He holds degrees in both physics and engineering and a master�s degree in industrial management. He is the author of many articles in the fields of combat communications and electronic warfare. His military awards include the Bronze Star and Meritorious Service Medal. He has also received the Commander�s Award for Civilian Service, New Jersey�s Distinguished Service Medal and other state military awards. Fiedler has also been awarded the Chief of Signal plaque by two Chiefs of Signal, and he is also a recipient of the Signal Corps Regimental Association�s Silver Order of Mercury. Fiedler holds the distinction of being awarded the Coast Guard�s Meritorious Service Ribbon for conducting successful search-and-rescue operations while serving as an Army officer aboard the Coast Guard cutter Sturgeon Bay.
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