Combipole™ Shared Folded Unipole Design – Application 13

A New Approach to Multiple MW Stations on One Tower

Since the beginnings of VHF and UHF broadcasting, FM and TV stations throughout the U.S. and the world have taken advantage of the obvious economies inherent in sharing a single tower for support of multiple antennas. More recently, FM stations and even some TV stations have been sharing single broadband antennas resulting in further economies.

There are instances of MW stations sharing conventional series-fed antennas, but such arrangements are complicated by the fact that the antenna exhibits different radiation resistances and electrical lengths at each user’s frequency, and the combining and isolating components cannot neatly be segregated by user.

The folded unipole antenna, appropriately designed, can afford MW stations the opportunity to realize the same tower-sharing and antenna-sharing economies that FM and TV stations can. Furthermore, the properties of the folded unipole permit optimization of apparent electrical length and drive-point impedance for each user, within practical limits.

Folded Unipole Fundamentals

Before considering a multiple-station folded unipole, a review of folded unipole antenna fundamentals is in order. The conventional MW series-fed monopole antenna is simply a vertically polarized half dipole supported at the feed point by, but insulated from, the ground and its ground system.

If half a folded dipole, vertically polarized, is set upon the ground with one part of the dipole bonded to the ground system and the other part designated the feed point, a folded monopole or, more commonly, a folded unipole antenna is the result.

The folded unipole may be considered a special case of the shunt-fed antenna, and indeed, is so considered by U.S. regulatory authorities. A slant-wire shunt-fed antenna has a grounded base and is fed by a feeder which “slants” upward from near the ground at the point of connection with the transmission line to the point of connection to the tower, which is often chosen as the point at which the resistance portion of the antenna impedance is 50 ohms. Some shunt feeds are fed from the center of the tower.

Typical shunt feeds are asymmetrical and use single wires. These often have disadvantages including radiation pattern distortion, reduced bandwidth, and matching difficulties due to highly reactive drive-point impedances.

The folded unipole can be considered a shunt-fed antenna with vertical instead of slanted feed wires arranged symmetrically about the tower. The feed wires, also known as skirt wires, drop wires, or fold wires, are attached to the tower at or near the top of the tower.

Because the fold wires are vertical and symmetrical, neither the vertical nor the horizontal radiation patterns are distorted by the presence of the wires.

In fact, the theoretical radiation pattern of the folded unipole is virtually identical to that of a series-fed monopole of the same height. This has been confirmed by U.S. Navy studies and research by the LBA Group.

Properties of the Folded Unipole

LBA Technology, Inc. manufactures the Tunipole™, an advanced folded unipole system which can be installed as hardware or in kit form to modify an existing series radiator. Any height tower can be modified to a Tunipole™ folded unipole, with significant benefits. However, if the existing series radiator is less than 1/4 wavelength in electrical length, modifying it to a Tunipole™ folded unipole will provide even greater improvements.

First, the Tunipole™ provides increased apparent electrical length and radiation resistance, resulting in higher efficiency, easier impedance matching to the transmission line, and improved stability in inclement weather.

Second, by adding suitable connecting circuits from the fold wires to the tower at a second point below the top connections, the base impedance of the Tunipole™ can be changed. This circuit can be moved up and down the tower to adjust impedance (both resistance and reactance). Combining this adjustment with fold-wire spacing control can significantly broaden and stabilize the antenna system bandwidth over an equivalent series-fed system.

Third, the addition of fold wires (skirt) effectively increases the radiator’s cross-sectional area, further increasing bandwidth.

Fourth, the base insulator is shorted across, bringing the tower base to ground potential. Lightning transformers can be eliminated, the antenna tuning unit can be mounted on the tower, and defense against lightning is strengthened.

Additional antennas and transmission lines can be added to the tower without the use of isolation devices such as transformers, matching stubs, or isocouplers. At high power levels, particularly with self-supporting towers, this benefit can result in significant construction cost savings.

Last, but by no means least important, a series-fed antenna associated with a limited ground system, when modified to a Tunipole™, can exhibit increased unattenuated field intensity. Radiating current divides between the tower and the fold-wire portions in inverse proportion to their resistance, effectively increasing apparent radiation resistance. Where ground resistance is higher due to foreshortened/deteriorated ground systems or poor soil conductivity, greater voltage drop occurs in the tower portion, increasing total system efficiency.

Product Bulletin: Tunipole™ Folded Unipole Antennas

For specifications, configurations, and key performance benefits, download the Tunipole™ product sheet:

? Tunipole™ – Folded Unipole Antennas (Product Sheet PDF)

• Retrofit kits and new-construction solutions
• Improved radiation resistance and apparent electrical length
• Broader bandwidth and improved operational stability
• Grounded tower base for easier lightning protection and site integration
• Supports co-location and future tower revenue opportunities

Applications for Tunipole™ Folded Unipoles

With the properties and theory of the folded unipole in mind, several applications become apparent. For existing stations with series-fed antennas shorter than a quarter wavelength, a Tunipole™ may be used to improve bandwidth for enhanced audio performance, improve efficiency, improve stability in bad weather, improve performance with a problem ground system, or transform the AM antenna into a source of rental revenue from additional tower-space users.

The Tunipole™ may also be useful for an existing station changing operation to a lower frequency. More efficiency can be achieved by using the Tunipole™ on a tower that becomes electrically shorter at the new lower frequency—often avoiding ground-system replacement.

A Tunipole™ can be used to add MW operation to an existing tower that is grounded and/or taller than necessary for conventional series-fed operation.

If new station construction is contemplated, a Tunipole™ folded unipole can be incorporated into the original design to enable cost-saving measures. Since a base insulator, tower lighting chokes or transformers, and separate ATU mounting are unnecessary, their costs can be eliminated.

A shorter tower may be erected, saving steel (and potentially aviation lighting/marking), and may be easier to gain local approval. Where land is limited, a foreshortened ground system may be utilized. Plans can also be made for future tower tenants from the outset since the base will be grounded.

Introducing the Combipole™: A Shared Folded Unipole

If one new station can save money on hardware, steel, copper, and land by constructing a folded unipole antenna, wouldn’t it be convenient if two or more stations could share such an antenna? Such a convenience is possible with the LBA Technology Combipole™—as many as three antennas in one.

Normally for a triangular cross-section tower, three, six, or nine fold wires are arranged symmetrically about the tower for a Tunipole™ design. The number depends upon the station’s power, the environment, and other factors. It is possible to use each fold wire (or groups of fold wires) as separate feed systems for separate stations. The Combipole™ is designed specifically to implement this arrangement.

A typical system is a Combipole™ shared folded unipole built for Radio Grupo in Aguascalientes, Mexico. Electrically designed and fabricated by LBA Technology, Inc., in cooperation with Lawrence Behr Associates, Inc., the system serves XEBI, 790 kHz, 5 kW; XEUVA, 1170 kHz, 5 kW; and XERO, 1490 kHz, 1 kW. (The same design team has also implemented a number of three-station detuning systems using the same principles.)

Another Combipole™ system was designed and fabricated for two stations in Tampico, Mexico: XEMTS at 780 kHz (1 kW) and XETO at 950 kHz (1 kW).

The original series-fed tower was 96 meters high and was toppled in a tropical storm. The city would not allow a replacement tower of the same height, and land was limited to 100 feet by 250 feet, making guying difficult and limiting the ground system.

The replacement tower was limited to 75 meters, reducing electrical length from 109.5° to 85.5° for 950 kHz and from 90° to 70.2° for 780 kHz. The ground system was limited to less than half of normal, and a six-bay FM antenna was leg-mounted from the top down to the 60-meter level.

A radial top-hat top-loading system was installed at the top of the tower with fold wires suspended from it. Fiberglass isolation was used in the aperture of the FM antenna. The conducting portion of the fold wires begins at the 60-meter level where they are connected with jumpers to the tower.

Tower-to-fold-wire circuits for the two sets of three fold wires were adjusted independently to set the impedances. Following installation, measured impedances were 50 + j400 ohms at 950 kHz and 45 + j128 ohms at 780 kHz. One year later, the impedances were measured again and were found to be identical.

Listener reports confirmed improved reception with the shorter folded unipole compared to a taller series-fed antenna with the same limited ground system. Similar reports were received after the Aguascalientes system was placed in service.

Each station’s coupling/tuning unit includes components to reject the other users’ signals, similar to comparable systems for series-fed multi-user antennas. However, because each station has a separate feed point on the Combipole™, tuning and decoupling components are generally less interactive. No conventional series-fed diplexer or triplexer is used.

The independent impedance adjustment capability of the multi-station folded unipole allows better matching and can create the appearance of greater electrical length, as discussed earlier.

Reduced interaction between user networks also allows each station to maintain its own coupling and matching functions in separate cabinets under its control—analogous to FM and TV multi-station antennas.

With proper electrical and mechanical design, a stable, efficient antenna system can be shared by two or more AM stations with attendant economies of shared facilities and the added benefits of a grounded tower base and other folded unipole properties. The Combipole™ system by LBA Technology facilitates such multi-station systems and is field-proven to improve MW coverage and quality.

A full line of Combipole™ antenna systems for all power levels and tower heights are available from LBA Technology, Inc.

Download the Tunipole™ Product Sheet:
? Tunipole™ – Folded Unipole Antennas (Product Sheet PDF)