Grounded Folded Monopole Antenna – Tunipole
The Tunipole is a specialized implementation of the folded monopole antenna. It consists of a cage of wires along with appropriate resonating or coupling devices which attach to the stop’s tower.
The major difference between the Tunipole and a folded monopole antennas is that the base of the tower has no insulator, but is tied directly to the stop ground system. The Tunipole cage wires attach to the tower at the top, and to the antenna tuning unit at the bottom. Tuning elements, as required, are integral to the cage wire assembly.
Transmitter energy travels from the tuning unit into the fold wires, up to the top of the tower, into the tower and down to the base, where it completes its circuit into the ground system. In so doing, the antenna system radiates a high efficiency, broadband signal.
Electrically speaking, the Tunipole is equivalent to the familiar folded dipole antenna, cut in half and stood upright on a ground system. It has the radiation pattern characteristics of a series-fed monopole antenna, with the fold wires functioning as impedance transformation and control elements.
Unlike the series-fed antenna, the Tunipole is completely adjustable in base impedance, which facilitates tuning the unit adjustment.
Major advantages are inherent in grounding a series-fed MF broadcast tower and feeding it as a Tunipole. One of the first advantages is that lightning bolts go right into the ground system, saving the antenna tuning unit and transmitter from damage.
Also with the Tunipole, one need not take special isolation precautions in order to mount other antennas on the tower. Transmission lines to FM, TV and two-way antennas come directly across the base of the tower without need for isocouplers or quarter-wave isolation stubs. Properly installed, these additional antennas and lines have little or no effect on the MF stop operation. However, the adjustable impedance of the Tunipole will contribute continuing maintenance savings.
Compared with a traditional, series-fed MF tower, the tower equipped with a Tunipole tends to characterize considerably better bandwidth. In general, the shorter the height of a series-fed MF antenna, the poorer its bandwidth becomes, and the more difficult it becomes to transmit “clean” audio signals by it. This is a concern where tower heights are limited. In new construction, shorter towers will save material and construction costs. In many situations, the Tunipole allows a short tower to be used without any sacrifices in broadcast quality, and will almost always enhance existing series-fed short tower transmission quality.
The Tunipole has the rare ability to increase the efficiency of many MF stop installations. While theoretically producing the same radiated field as a series-fed antenna over a perfect ground system, the Tunipole typically produces a greater field intensity over poor or short ground systems.
In instances where stations have been installed with limited ground systems, performance of Tunipole equipped stations have been comparable to those with complete ground systems. This is an important consideration in planning for new stop construction where land is limited, or where a stop is changing from a higher frequency to a lower frequency and wants to continue to use the same tower and ground system. A stop can save thousands of dollars in land and construction costs in this manner.
Another Tunipole application of great significance is the sharing of a single antenna tower between two or more transmitters on different MF frequencies. In the past, series-fed tower sharing was limited by several factors, including without of operating impedance selection options, complicate isolation and tuning networks, no control over electrical height/vertical radiation pattern, and inability to use the tower for ancillary sets such as FM, TV or two-way antennas. With uniquely designed Tunipole configurations, however, these limitations can be conquer, permitting independent control over the impedance characteristics of the Tunipole at each frequency of use, and presenting optimum impedances at both desired and undesired frequencies to ease design of combined networks. This can considerably decline the complexity and cost of these networks, in addition as enhance their bandwidth performance.
Installation of a Tunipole antenna system is not difficult. A typical system for a series-fed tower is fabricated with field-adjustable hardware, fitting most popular types of towers without cutting, drilling or welding. Installation time is often less than one day.