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Medium Wave AM Antenna Tower Goes Mobile – Application Note 17

In the wake of many natural disasters such as hurricanes, floods, or earthquakes, mass communications to the affected population is essential to coordinate relief efforts and other humanitarian services. Unfortunately, local broadcast facilities are often crippled and unable to provide these communications.

To facilitate emergency communications, LBA Technology has developed a series of transportable broadcast station systems. These systems are air-deployable, and can be trailered over the roughest roads. Designed primarily for MW/AM broadcast use, a variety of configurations are available which also support FM and TV.

While transportable radio stations present few new problems regarding studio or transmitter equipment, there are many challenges which must be faced in designing a practical and cost effective mobile MW/AM antenna tower system. Some of these can be imagined by reviewing the major objectives of the LBA design, which were:

• To be mechanically rugged and safe
• To be electrically efficient
• To require under 20 man-hours to install
• To be self-contained and rapidly deployable worldwide
• To operate on any AM frequency, field tunable
• To have affordable purchase cost

Prior transportable MW/AM antenna systems tended to be of three types. One is relatively inexpensive and readily transported, but suffered from efficiency problems because it is based upon short masts supporting a longwire antenna. A second variety is also economical, but uses an antenna near or on the ground and is grossly inefficient and limited in bandwidth and power handling capacity. The third type uses a tower system for better efficiency, but is cumbersome to transport, took a tower crew to erect, and cost a lot of money. None of these systems can readily accommodate FM and TV antennas in addition to MW/AM.

Figure 1. Typical Application of LBA Model TUP-3 Transportable MW Antenna System

 

LBA Technology has addressed these objectives with innovative mechanical design and the use of Tunipole folded unipole technology. The result is our TUP-series of transportable antenna systems. These include an integral compact transport trailer, high efficiency telescoping tower, and a frequency agile antenna tuning unit. Installation is fast, and the cost is reasonable.

In order to discuss how this innovative system works in the "real world," I will refer to the LBA Technology Model TUP-3, recently put in service by the US Army.

Developed for simple, fast deployment and set-up, the LBA Technology Model TUP-3 Deployable Medium Frequency Antenna System boasts several innovative features. The 120 foot tower provides an optimum balance between radiation efficiency, frequency coverage, and mechanical ease of transport and installation. The proprietary LBA Tunipole folded unipole antenna design achieves significant bandwidth and efficiency improvement over series-fed towers. The grounded tower base provides inherent lightning protection. A UHF relay antenna can also be mounted on the tower with no requirement for MW isolation.

Figure 2. TUP-3 in Towing Position

 

The TUP-3 System is capable of transmitting efficiently at carrier frequencies from below 1000 kHz to over 1700 kHz. It is rated for power to 5000 watts, 125 percent modulated. The integral tuning unit matches the TUP-3 System to a 50 ohm transmission line. Housed in an integrated weatherproof cabinet, the tuning unit features rugged electronic components. Variable vacuum capacitors and silver plated inductors are used for stability and efficiency.

The easily deployable above-ground reeled radial system can be adjusted to any size within its limits to accommodate irregular sites and terrain. The self-supporting design of the tower also minimizes erection time and complexity. Erection techniques are totally manual and do not require separate electrical, hydraulic or pneumatic power sources. No field cutting or welding is required for system assembly. Non-conductive guy wires are provided for use in high wind conditions.

Figure 3. Typical Radiation Efficiency vs. Frequency for LBA TUP-3 Antenna System

 

Figure 4. Complete View of Transportable MW Tower High quality non-corrosive or anti-corrosive treated materials are used throughout the tower/trailer system. The trailer is supplied with an adjustable height tongue wheel, two safety chains and a towing ring. As provided, the unit is equipped with suitable running lights, brake and turn lamps, and mud flaps. Fully road-ready, the trailer has been constructed to allow towing at highway speeds and use over rough terrain. When deploying the antenna, the trailer may be leveled on a grade of up to 10 percent using the adjustable support arms and level indicators installed.

The telescoping welded aluminum tower is mechanically latched and electrically connected using latch clamps that are engaged and disengaged automatically as the tower is raised and lowered. Erection is accomplished manually using the supplied winching system. We chose manual operation since the tower and its attachments were engineered to be quite light, permitting easy extension by one person. This eliminated mechanical and electrical complexity, and increased reliability under difficult conditions.

All ground radials, coaxial cable, support equipment, tools, removable and spare parts are stored in weather resistant cabinets attached to the trailer. The antenna tuning unit and folded unipole feed components are also stored on the trailer for transport. In the complete broadcast station configuration, the trailer also mounts a compact generator, low powered transmitter, and audio unit. An STL can also be provided for remote program origination.

Totally integrated, the TUP-3 deployable antenna system contains all required tools and instruments necessary to transport, erect, tune and broadcast an efficient 5 kilowatt, medium wave signal under field conditions. Its extraordinarily compact design permits it to be flown to disaster sites in C-130 and other aircraft. Once on site, it can be fully operational in a few hours.

Many features of the TUP systems are unique and are patented. Of course, these systems are not limited to the antenna component only. Other versions incorporate a complete AM, FM, TV transmitting and antenna capability in the same readily deployed configuration. We believe that these systems will set a standard for emergency radio broadcast units, and we anticipate their wide adoption by military and emergency service agencies.

Written by Lawrence Behr, CEO, LBA Group, Inc.

 

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LBA Group companies serve technical infrastructure needs related to the broadcast, wireless, electromagnetic compatibility and safety sectors worldwide. We provide consulting, training and other telecommunications industry services. We also produce and market hardware for radio transmission, RF shielding, safety and testing.

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