The Nott Ltd Folded Unipole is completely rustproof!

FREQUENTLY ASKED QUESTIONS ABOUT THE FOLDED UNIPOLE ANTENNA

1. WHY DOES THE UNIPOLE HAVE A BROADER BANDWIDTH WHEN COMPARED TO A CONVENTIONAL SERIES FED ANTENNA?

The bandwidth of any antenna is affected by its length to diameter ratio. Towers of small cross section in relationship to their height have been determined to have a relatively narrow bandwidth. On the other hand, towers of large cross section have a greater bandwidth. When a folded unipole kit is installed on a tower of relatively small cross section, it makes the tower effectively increase in diameter. This causes a decreased length to diameter ratio with is corresponding broader bandwidth.

2. DOES THE UNIPOLE HAVE A BETTER FIELD GAIN THAN A CONVENTIONAL SERIES FED ANTENNA?

When the length to diameter ratio is reduced as in the preceding answer, it also affects the velocity of wave propagation within the antenna. As the effective diameter is increased, the propagation velocity decreases. This can make a "fat" antenna appear to be slightly taller than it actually is. This may lead to a small increase in the field gain of the antenna. In practice it has been noted that the skywave is apparently reduced to some degree, while adding to the groundwaves significantly. The broader bandwidth of the unipole allows more sideband power to be transmitted, which my result in an effective increase in the coverage of the station. In practice, improvements of from zero to more than 60 percent have actually occurred. The average improvement is on the order of ten to fifteen percent.

3. WHEN WOULD A STATION CONSIDER A FOLDED UNIPOLE ANTENNA RATHER THAN A SERIES FEED?

This answer has several categores. Existing stations that have any of the following problems may wish to consider the unipole:

a) If the station has narrow bandwidth causing degradation of the sound. This can be caused by a combination of several things including a thin tower, an antenna tuning unit that has not been broadbanded or a height that causes steep slopes in the resistance and/or reactance curves.

b) If the station ground system is poor or questionable. Sometimes property boundaries limit the length of the radials. Corrosion from alkaline or acidic soil can tak a toll, eroding the copper with time. Trenching machines for pipelines or cables may cut the wires and they are seldom spliced afterward. The unipole has the reputation of performing very well in spite of a poor ground system.

c) Unstable base impedance. Sometimes the input resistance will vary between wet and dry weather or seasonally. The unipole is a very broadband, stable antenna whose input impedence remains the same throughout widely varying conditions.

d) Lightning and static electricity problems. The unipole is a grounded antenna which provides a low resistance drain path to ground. While it may not totally eliminate lightning strikes, it has earned a reputation for reducing such problems dramatically.

e) If the base impedence is much greater or lower than 50 ohms. The unipole is a tunable antenna, allowing the input impedance to be set to values that are more easily matched to the transmission line. As an example, a tower less than a quarter wavelength in height normally has a low input resistance. The folded unipole can transform this resistance upward, causing the antenna to be more efficient and easier to match its input impedance. Likewise, it can reduce the high resistance of a tall tower to a value near the impedance of the transmission line.

f) If the antenna is a slant wire shunt feed. This antenna has the advantage of being grounded, but the slant wire has negative effects. Since it is part of the antenna, part of the radiation is polarized to its angle, causing an unwanted directional effect. Also, since it is relatively thin wire, it has a very poor length to diameter ratio as discussed previously.

Another category is for stations that are building a new facility or replacing a tower. The base insulator can be eliminated along with lighting chokes, ballgaps and other devices that are needed on an insulated base tower. Isocouplers will not be necessary if VHF or UHF antennas are to be installed on the tower.

Finally, a unipole allows almost any tower to be used as an AM antenna. FM, TV and communications towers may be used, whether guyed or self-supported. Even tall steel monopoles work well as AM antennas.

4. CAN THE FOLDED UNIPOLE BE USED IN A DIRECTIONAL ARRAY?

Because of the broad bandwidth and stability of a properly designed folded unipole, it is very suitable for use in directional arrays. Unipoles are now in service in many directional stations with complete success.

5. CAN TWO OR MORE STATIONS BE MULTIPLEXED INTO A FOLDED UNIPOLE?

Yes. Provided that is is viewed as a system and properly engineered and designed, the unipole performs well in this service.

6. WHAT IS A DETUNING SKIRT AND WHEN IS IT NEEDED?

If a communications tower or other tall structure has been built in the vicinity of an AM antenna, whether directional or non-directional, it may be found that the radiation pattern has been distorted due to re-radiation of that structure. To solve this problem, the unipole may be used as a detuning skirt in order to make the offending structure effectively disappear. It is also possible to detune the upper portion of a tall tower while simultaneously utilizing the lower portion as an AM broadcast antenna with a folded unipole.