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 somewhat complicated by the fact that the antenna exhibits different radiation resistances and electrical lengths at each users 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, 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 the United States 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 the disadvantages of distorting the radiation pattern, reducing bandwidth, and creating matching difficulties because of highly reactive drive point impedances.

The folded unipole could be considered as 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 in the form of hardware, 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™ will have increased apparent electrical length and radiation resistance, which results in higher efficiency, and easier impedance match to the transmission line, and better stability in inclement weather.

Second, by adding suitable connecting circuits from the foldwires to the tower at a second point below the top connections, the base impedance of the Tunipole™ can be changed. Thus, this circuit can be moved up and down the tower to adjust the impedance, both resistance and reactance. By combining this adjustment with adjustment of foldwire distance from the towers, the dynamic bandwidth of the antenna system can be significantly broadened and controlled over that of an equivalent series fed system.

Third, the addition of the fold wires or skirt to the series fed radiator effectively adds to its cross-sectional area, further increasing the 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 great 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, will have an increased unattenuated field intensity. This is due to the fact that the radiating current divides between the tower and the fold wire portions of the Tunipole in inverse proportion to the resistance of each, increasing the apparent radiation resistance. Thus when the ground resistance is higher due to a foreshortened or deteriorated ground system or poor soil conductivity, a greater voltage drop will occur in the tower portion, increasing the efficiency of the total system.

Applications for Tunipole™ Folded Unipoles

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

The Tunipole™ could also prove useful for an existing station that was changing operation to a lower frequency. More efficiency could be achieved by using the Tunipole™ on a tower which would be electrically shorter at the new lower frequency, and replacement of the ground system could be avoided.

A Tunipole™ can be used to add MW operation to an existing tower which is grounded and/or of greater height than necessary or useful.

If new station construction is contemplated, a Tunipole™ folded unipole can be incorporated into the original design to take advantage of a number of cost saving measures. Since base insulator, tower lighting chokes or transformers, and a separate mounting for the antenna tuning unit are unnecessary, their cost can be eliminated. A shorter tower can be erected, resulting in savings on steel (and perhaps aviation lighting and marking). The shorter tower may be easier to get approved by local authorities as well. If a small parcel of land is all that is available or affordable, a foreshortened ground system can be utilized. Of course, plans can be made for the tower to accommodate tenants at 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? Fortunately 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 foldwires 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 of the foldwires as a separate feed wire for a separate station, or any combination of the foldwires can be used. The Combipole™ is designed specifically to implement this arrangement.

Typical of such systems 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 Combipole™ system serves XEBI, 790 kHz, 5 kW; XEUVA, 1170 kHz, 5 kW; and XERO, 1490 kHz, 1 kW. (The same design team has also successfully implemented a number of three station detuning systems using the same principles!)

Another typical Combipole™ system was designed and fabricated for two stations in Tampico, Mexico. Station XEMTS operates at 780 kHz with 1 kW and XETO at 950 kHz with 1 kW. They are part of Grupo AS in Tampico.

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 to be constructed, and the land area was limited to a space 100 feet by 250 feet, making guying difficult and limiting the ground system.

The new tower was limited to 75 meters, which represented a reduction in 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! Furthermore, 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 the 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 of the tower 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 impedance of the folded unipoles. Following installation, the impedances were adjusted and measured. At 950 kHz the impedance was 50 + j400 ohms and at 780 kHz, 45 + j128 ohms. The stability of this Combipole™ shared folded unipole system was demonstrated when the impedances were measured one year later and were found to be identical.

Not only is the Combipole™ system stable, but listener reports confirm an improvement in signal reception with the shorter folded unipole as compared to a taller series fed antenna with the same limited ground system. Similar enthusiastic reports were received after the Aguascalientes system was placed in service.

It is noteworthy that the individual coupling/tuning units for each station contain components for rejecting the other user’s signals, similar to comparable units for series fed multiple user antennas. However, because each station has a separate feed point on the Combipole™, the tuning and decoupling components are to be less interactive among users. No series fed, conventional diplexer or triplexer is used!

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

The relatively small interactivity of each station’s tuning/decoupling components with those of the other stations allows each station on a practical basis to have its individual coupling and matching functions in separate cabinets under its own control - a situation analogous to FM and TV multi-station antennas.

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

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