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
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
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
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
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.