Since the beginning of VHF and UHF broadcasting, FM and television stations in the United States and around the world have taken advantage of the obvious economies inherent in sharing a single tower to support multiple antennas. Recently, FM stations and even some television stations use single broadband antennas, which leads to further savings.
There are cases where MW stations exchange traditional serial-powered antennas, but such devices are somewhat complicated by the fact that the antenna exhibits different radiation resistances and electrical lengths at each user frequency, and the combination and isolation components cannot be neatly separated by the user.
A properly folded, unipolar antenna can enable MW stations to implement the same tower distribution and antenna distribution systems that can be used by FM and TV stations. In addition, the properties of the folded unipole allow you to optimize the apparent electrical length and impedance of the travel point for each user in practical limits.
Folded Unipole Basics
Before considering the multi-position folded unipole, review the folded unipolar bases in order. A conventional MW series magnetic field antenna is simply a vertically polarized half-dipole supported at the feed point, but isolated from the ground and its ground system.
If half of the folded dipole, vertically polarized, is installed on the ground with one part of the dipole connected to the ground, and the other part designated by the feed point is the folded monopole or, more often, the folded unipolar antenna.
Folded unipole can be considered as a special case of an antenna connected by a shunt, and, indeed, the regulators of the United States think so. An antenna with a sloping shunt antenna has a grounded base and is powered by a feeder that “tilts” up from the ground at the connection point with the transmission line to the connection point to the tower, which is often chosen as the point at which the impedance resistance of the antenna is 50 Ohms. Some shunt channels are fed from the center of the tower. Typical shunt channels are asymmetrical and use single wires. They often have flaws in distorting the radiation pattern, reducing the bandwidth and creating associated difficulties due to the highly reactive impedances of the excitation points.
A folded unipole can be considered as an antenna with shunt power with vertical, rather than inclined, power wires that are located symmetrically relative to the tower. Wires, also known as skirt wires, drop cables, or bending wires, are attached to a tower at or near the top of the tower.
Since the bending wires are vertical and symmetrical, neither vertical nor horizontal radiation patterns are determined by the presence of wires.
In fact, the theoretical directional pattern of the folded unipole is virtually identical to the theoretical directional pattern of a monopole with a sequential feed with the same height. This was confirmed by research and research by the US Navy group LBA.
Properties of a folded unipole
Tunipole, an advanced folded unipolar system, which can be installed as hardware in a set, to modify an existing heatsink series. Any tower height can be changed to a folded Tunipole unipole with significant advantages. However, if the existing radiator of the series has a length of less than 1/4 of the wavelength in the electrical length, modifying it to the folded Tunipole unipole will provide even greater improvements.
At first, Tunipole will have an increased apparent electrical length and radiation resistance, which will lead to increased efficiency and simpler transmission line resistance and better stability in inclement weather.
Secondly, by adding sufficient coupling chains from the bent shafts to the tower at the second point below the top connections, the base impedance of the Tunipole can be changed. Thus, this circuit can move up and down the tower to adjust impedance, resistance, and reactivity. By combining this setting with the adjustment of the offset distance from the towers, the dynamic bandwidth of the antenna system can be significantly expanded and controlled compared to an equivalent series system.
Thirdly, adding bent wires or a skirt to a radiator with a series feed effectively adds to its cross-sectional area, further increasing the bandwidth.
Fourthly, the base insulator has a short pass, which leads to the fact that the base of the tower has the potential of the ground. Lightning rods can be eliminated, the antenna tuning unit can be mounted on the tower, and the protection from lighting is enhanced. Additional antennas and transmission lines can be added to the tower without the use of isolated devices such as transformers, appropriate pins or insulators. With high power levels, especially with self-supporting towers, this advantage can lead to significant savings in construction costs.
Last but not least, a series-powered antenna connected to a limited ground system, when modified in Tunipole, will have an increased field strength without amplification. This is due to the fact that the radiating current divides between the tower and the sections of the folded wire Tunipole is inversely proportional to the resistance of each of them, increasing the apparent radiation resistance. Thus, when the grounding resistance is higher due to a split or degraded surface system or poor soil conductivity, a greater voltage drop will be observed in the tower, which increases the efficiency of the entire system.
Applications for deadlock stacked unipoles
Taking into account the properties and theory of a folded unipole, some applications become obvious. For existing stations with antennas with a serial antenna of less than a quarter of a wavelength, Tunipole could be used to improve bandwidth, improve stereo performance, improve efficiency, improve stability in bad weather, improve overall performance of a problematic ground system, or convert an AM antenna to a source rental income from additional tower space users.
Tunipole may also be useful for an existing station that changes operation to a lower frequency. Higher efficiency could be achieved by using Tunipole on the tower, which would be electrically shorter at the new lower frequency, and the replacement of the ground system could have been avoided.
Tunipol can be used to add an MW job to an existing tower that is grounded and / or higher than necessary or useful.
If a new station is to be built, Tunipole can be included in the original design to take advantage of a number of savings measures. Since the base insulator, chokes or tower lighting transformers, and a separate installation for the antenna tuner are not required, their cost can be eliminated. Tower shooter can be authenticated, leading to savings on steel (and, possibly, aviation lighting and marking). A shorter tower may be more easily approved by local authorities. If a small piece of land is available or affordable, a perspective ground system can be used. Of course, you can plan a tower to accommodate tenants from the very beginning, since the base will be grounded.
Introducing Combipole - General Folded Unipolar
If one new station can save money on equipment, steel, copper and earth by building a folded unipolar antenna, would it be inconvenient if two or more stations could share such an antenna? Fortunately, this convenience is possible with Combipole - as many as three antennas in one!
As a rule, for a triangular transverse tower, three, six, or nine folds are arranged symmetrically relative to the tower for the Tunipole construction. The amount depends on the power station, the environment and other factors. You can use each of the bends as a separate feed wire for a separate station, or use any combination of bends. Combipole is designed specifically to implement this scheme.
Typical for such systems is Combipole, which serves XEBI, 790 kHz, 5 kW; XEUVA, 1170 kHz, 5 kW; and XERO, 1,490 kHz, 1 kW.
Another typical system Combipole serves XEMTS, 780 kHz with 1 kW and XETO, 950 kHz with 1 kW. They are part of Grupo AS in Tampico.
The original food column was 96 meters high and was covered by a tropical storm. The city would not allow the construction of a replacement tower of the same height, and the land area was limited to 100 feet by 250 feet, which would make it difficult to restrict and limit the ground system.
The new tower was limited to 75 meters, which meant a reduction in electrical length from 109.5 to 85.5 ° for 950 kHz and from 90 ° to 70.2 ° for 780 kHz. In the ground system was less than half the norm! In addition, a six-core FM antenna was installed on its legs from top to bottom to a level of 60 meters.
In the upper part of the tower, an upper loading system was installed with a radial top hat with pleats suspended to it. Fiberglass insulation was used in the aperture of the FM antenna. The conductive part of the bending wires begins at the level of 60 meters of the tower, where they are connected with bridges to the tower.
The bending tower for the two sets of three-piece wires was configured independently to establish the impedance of the folded unipoles. After installation, the impedances were adjusted and measured. At 950 kHz, the impedance was 50 + j400 ohms and 780 kHz, 45 + j128 ohms. The stability of this Combipole system was demonstrated when impedances were re-measured a year later and were considered identical.
Not only is the Combipole system stable, but listener reports confirm an improvement in signal reception with a shorter folded unipole compared to an antenna with a higher frequency, with the same limited ground system.
It should be noted that the individual communication units / settings for each station contain components for rejecting signals from another user, similar to comparable units for multi-user antennas with series power. However, since each station has a separate supply point for Combipole, the tuning and isolation components should be less interactive between users. No serial power, conventional diplexer or triplexer!
In addition, the independent ability to adjust the impedance of a multi-position folded unipolar allows for a better selection and can create the appearance of a larger electrical length, as previously discussed.
The relatively small interaction of the tuning / decoupling components of each station with other stations allows each station to have, on a practical basis, individual pairing and matching functions in separate cabinets under their control - a situation similar to FM and television multi-station antennas.
With proper electrical and mechanical design, you can create a stable and efficient antenna system that can be used by two or more AM stations with the attendant economics of common facilities and the added benefits of a grounded tower and other unipolar properties. The unique Combipole system facilitates such multistation systems and, as has been proven, improves the coverage and quality of MW stations.
