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 The
radio mast, an almost fundamental component of any amateur radio
station was in my case a compromise of a number of mental design
aspirations and by the opportunity of the available hardware that
could be acquired and fabricated. The amateur radio station's
collection of antennas is for me a dynamic thing that will change and
evolve over time, however if the mast is well designed it should
require little modification.
My
first purchased was a 7.7mtr section of lattice tower advertised on
VKHam. I had transported the tower to my home without having fully
determined how it would fit with my plans. I had however formed a bit
of plan that the mast could be a tilt over structure based on photos
that I had seen on the internet without any detail of how these masts
had been constructed.
I
contacted local ham Jim VK6CA for his thoughts on what was possible,
Jim clearly saw the vision of what was needed to be done based several
successful and less successful designs of this type that had been
developed by him and others.
The
final design was for an over all mast/tower consisted of a 5.6mtr
length of 80mm medium duty galvanized steel tube concreted 1mtr into
the ground with a fabricated cradle assembles fitted to the top. The
lattice tower would have a fabricated steel bracket complete with a
pivot rod attached just above centre height of the lattice tower,
about the centre of gravity of the lattice tower. The rod bracket
would sit in the cradle assembly held in place by the weight of the
tower.
The
top of structure would now have a total height above ground of
8.3metres. Finally there wound be an addition 2.0mtr section of 40mm
diameter galvanized steel tube attached to the top of the mast,
achieving total structure height above ground of 10.3mtr.
The
mast is stabilized against high wind loading with three guy wires
positioned approximately 120deg from each other. It is intended that
at least one of the guy wires will be used as a multi-band radiator
covering bands between 7 and 29 MHz.
Swage
connectors were used as part of the termination of the guy wires, these connectors allow
the wires to be bridged from the mechanical termination and bonded to
the mast for example. The 3mm
swage connector is marine grade attachment for small size wire rope
from Zenith, Cat WSS203; a malleable elongated plated copper sleeve
that produces a neat, quality electrical connection with an acceptably
strong physical connection. The swage connector in conjunction with
galvanised wire rope thimbles, D shackles and adjustable turn buckles
creates an effective and adjustable guy wire attachments.
Although
the use of rope
thimbles, D shackles and adjustable turn buckles intentionally allows
for some mechanical movement the connection between these fittings is
not an ideal electrical connection and run the risk of creating
electrical noise as the fittings move due to wind for example. The
solution is to electrically bond across these mechanical connections
by installing a conducting lead from the guy wire to the metal mast
for example.
See photo#3
Operationally
there is no need to climb the tower to carry out work on the antennas.
By disconnecting one of the guy wires and un-bolting the tower
attachment near the base of the structure the lattice tower which is
slightly top heavy can managed easily with ropes and tilts over. The
process is a relatively simple one person operation as is reinstating
the mast to the up right position.
While
I quite like climbing radio towers it is clearly safer and easier to
carry out antenna work from the ground. The completion this project is
not the end; it is the beginning of the real projects.
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| Photo#1
The
tower bracket and rod sitting in the cradle assembly and held in place by the
weight of the
tower.
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Drg#1
Bracket,
rod and cradle assembly.
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| Photo#2 Tower
base attachment.
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Photo#3 Guy wire rope thimble, D
shackle and adjustable turn buckle assembly.
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| Photo#4 Tower
in the tilted over position for installation of new 5λ/8
ground plane antenna.
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Photo#5 Completed
tower assembly in the guyed upright position. |
References
WIRE
SPLICE TERMINATION
Wire splicing for terminating an antenna
aerial wire or guy wire to a strain insulator or thimble.
ARRL
Antenna Hand Book
Links
For key issues to be considered in tower design and maintenance - See:
The
Ten Most Common Tower Building Mistakes
http://www.championradio.com/common.tower.building.mistakes.html
For more information on tower and station earthing - See:
Grounding Systems for Amateur Radio Stations
http://www.ve3sqb.com/hamaerials/kf6gdj/
For more information on
Galvanic corrosion - See:
http://en.wikipedia.org/wiki/Galvanic_corrosion
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