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THE FOLDED UNIPOLE

Frequently Asked Questions About the Folded Unipole Antenna

Dependability

The Nott Ltd Folded Unipole is completely rustproof! This means many years to trouble-free service!

Broad Bandwidth
For better sound-mono for stereo and effective range increase.

Grounded Antenna
Helps eliminate lightning and static electricity problems.

Eliminates Isocouplers
VHF and UHF Antennas on a unipole are fed without discontinuity in their transmission lines.

Elminates Lighting Chokes
Transformers, base insulators, static drain chokes and spark gaps.

Fully Acceptable by FCC

Can Be Used in Directional Antenna Systems.

The folded unipole has been in use in AM broadcasting for more than four decades, providing adequate time to view it in retrospect. Perhaps the earliest instance that it appeared in print was in "Radio Antenna Engineering" by E.A. Laport in 1952. The fundamentals have not changed much since then, but experience has provided several improvements and taught us some weaknesses as well as good points. There is more to it than just suspending some wires alongside a tower.

If the skirt wires are placed too close to the tower, the shunt capacity between them can defeat the advantages. The wires must be placed at least 18" out and preferably 30" except in special cases. Early unipoles were built using guy wire in the skirts, but it has been found that the steel wire can cause perhaps 20% or more loss in effeciency. For good efficiency, skirt wire must be copper or aluminum clad. The wire should also be stranded to prevent wind vortexing, which causes vibration.

Installing a folded unipole does not guarantee broad bandwidth. In general, retrofitting a thin series fed tower will result in an improvement, but if the antenna is significantly shorter than a quarter wave, bandwidth may still be limited unless special care is taken. If top loading can be incorporated on a short tower, the unipole can be designed to provide support brackets for the top loading as well as the skirt wires. An advantage of the folded unipole on a short tower is that it normally transforms a low drive point resistance upward, which can improve efficiency and provide easier impedance matching.

Fallacies went through the broadcast industry some years ago, which stated that the unipole was a "gain" antenna and could also provide circular polarization. Neither of these is true. In some instances, the folded unipole may provide a very small gain, perhaps a fraction of one dB because of the decrease in length to diameter ratio. This causes a slight reduction in the velocity of propagation within the antenna, which may make the tower appear to be slightly taller than it actually is, but in any case there is no dramatic improvement. In instances where the bandwidth has been improved, there have been apparent improvements in the effective range of the station due to improved sideband VSWR.

The folded unipole can be modeled using NEC programs, but this is generally very time consuming. The late John Mullaney had a program written specifically for designing this antenna that is very fast and usually accurate. Two interesting phenomena have been discovered while using this program:

1. Bandwidth may be optimized. While other factors are also involved, an antenna height of between about 105 and 115 electrical degrees may be found that has extremely broad bandwidth. This has been confirmed in practice.

2. Impedance of short towers may be transformed upward. When a short tower is being retrofitted with a folded unipole, an inductor may be placed across the base insulator that will increase the drive point resistance. In some cases, this can improve the antenna efficiency. It must be pointed out that for very short antennas with substantial transmitter power, RF current through the inductor may be high.

Another advantage of the folded unipole is that it is installed on a grounded tower that allows installation of UHF and VHF antennas for rental income on the tower. This also eliminates lighting chokes and provides a more direct path to ground for lightning. The unipole performs well in directional antenna service if it is properly incorporated into the design. When installed in an area prone to flooding, it can be designed to continue functioning when series fed antennas fail.

In some instances, the unipole can allow an FM or communications tower to serve as its supporting structure. If the structure is too tall, the upper portion can be detuned, but bear in mind that the guy wires must be segmented which may be the most expensive part.

Again, installation of a folded unipole is more than just hanging some wires on a tower. If properly designed and constructed of quality, long life materials it can solve many problems for an AM broadcaster.

Additional Information

The Useful Folded Unipole - Radio Magazine - Oct. 2001

The Folded Unipole - Radio Magazine - Oct. 2002