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February 2016
30
W
hether it is a wire dipole,
a doublet or a beam
antenna you are using,
at its core will be an arrangement
to connect the feeder cable to the
antenna. This month’s Antennas is
a general overview of the dipole
centre in terms of examples of
commercially available types and
some homebrew designs.
Dipole centres revisited
A dipole centre’s main purpose is to provide a
reliable connection between the antenna and
the feeder cable from the transceiver/ATU.
However, depending upon the application,
the dipole centre should:
• be lightweight but of a suitably robust
construction
• physically protect the connection between
the feeder and antenna
• be made of a material able to withstand
many years of use outside in various
weather conditions
Then for beam antennas:
• provide a means to fix the driven element
to beam’s central support (boom).
Wire antennas
The arrangement used to connect the feeder
cable to each leg of a wire antenna is often
suspended within the antenna’s span (usually
centrally) and necessitates a physically
strong but lightweight design. Commercially,
there are various designs available and a
typical dog-bone type dipole centre that was
acquired at a radio rally is illustrated in
Photo
1
. This type of dipole centre can be obtained
from various suppliers and there are several
variations available.
Essentially, the dog-bone dipole centre has
a hole at each end through which each wire
leg of the dipole is firmly attached, usually
by tying the wire. A short length of the wire
is left protruding so that a ring terminal can
be soldered to the wire’s end after removal of
the insulation.
The feeder cable, usually a ladder line,
similarly has ring terminals soldered to each
conductor. These are paired up with the
wire leg ring terminals to make the electrical
connection and held in place using nuts
and bolts passed through the dipole centre’s
casing and the terminals. The feeder cable is
held in place by attaching it to the dog-bone
dipole centre using nylon twine, plastic cable
ties or similar.
This arrangement allows the antenna’s
span to be supported at each end using
insulators with the feeder cable hanging
vertically downwards from where it is
connected to the antenna’s span.
Often it is not possible to physically fit a
dipole antenna into the space available as a
straight span, however the space available
may allow the antenna to be installed as an
inverted V configuration
[1]
. The third hole
on the dog-bone dipole centre allows for this
configuration to be installed by using the
third hole to attach the antenna’s centre to
the top of a mast. If a metal mast is used,
a good practice is to attach a short length
of rigid plastic tube to the top of the mast to
act as an insulator and attach the antenna’s
centre to this using a length of nylon twine.
A dipole centre made from a length of
3mm thick uPVC (unplasticised polyvinyl
chloride) is shown in
Photo 2
. This design
has three holes drilled along the upper
section of the dipole centre either side of the
middle as shown. Each wire leg of the dipole
is then held in place by lacing it through the
holes as shown.
Two rows of three holes are then drilled
into the lower section of the dipole centre to
hold the feeder cable in place using cable
ties passed through the holes and feeder
cable (300 ohm ladder line in this case)
in
a
figure of eight
configuration
and tightened.
Two more larger diameter holes are drilled
symmetrically either side of the feeder cable
to allow the antenna to be installed as an
inverted V if necessary.
A short length of heatshrink sleeving
is passed over each dipole leg and the
leg soldered to one of the feeder cable’s
conductors. After soldering, the joint is
protected by wrapping PTFE plumbers’ tape
around it before the heatshrink is slid over
the joint and shrunk using a heat gun to help
weatherproof the joint.
Purely to make the design look neater, the
dipole centre can be finished off by cutting it
into a trapezium type shape before the cables
and feeder are added.
This design allows the antenna’s span
to be suspended horizontally as a straight
antenna or an inverted V with the feeder
cable hanging downwards.
uPVC was chosen for the dipole centre
because this material is designed for use
outside in the wet, is fairly rugged and is able
to generally withstand sunlight without too
much degradation.
Beam antennas
The dipole centre used for a beam antenna,
eg a Yagi, often supports the weight of the
beam’s driven element and this can be a
significant load depending on the band
in use, eg a 6m beam’s dipole has each
leg about 1.5m long, which represents a
significant torque load on the dipole centre.
Therefore, this form of dipole centre is made
more robustly compared to that for a wire
antenna.
Commercially, there are various designs
available and a typical dipole centre is
illustrated in
Photo 3
. In a similar manner
to the dog-bone dipole centre, this type of
dipole centre can be obtained from various
suppliers and there are several variations
available.
The underside of the dipole centre is often
shaped to enable a sturdy fit to be made with
the beam’s central boom and dipole centre
variants include both rounded and square
sections for this reason. The upper section
of the dipole centre usually has two brass
terminal pillars allowing the feeder cable
to be connected. The terminal pillars are
usually connected to the fixings used to hold
the driven element in place, as illustrated in
Photo 4
.
To weatherproof the feeder cable/antenna
connection, this type of dipole centre has a
snap-on lid that has an access hole for the
feeder cable.
A dipole centre can be made up using an
ABS box and aluminium tubing, as shown
in
Photo 5
. Two short lengths of aluminium
tube are used to support the dipole’s legs,
which are made from flush fitting aluminium
tube of a smaller diameter. Each short length
of tube has a lateral segment removed by first
sawing for a short distance lengthwise and
then sawing across the tube, leaving a half-
round section. The segment removed needs
to be cut to leave a gap of about 10mm in
between the two half-round sections when
placed on the ABS box isolating the dipole’s
legs, as shown in
Photo 6
.
Each short length of tube is temporarily
clamped in position to the ABS box, then
holes are drilled through the box and half-
round sections to allow suitable bolts to be
passed through and held in place using nuts
and washers as shown in Photo 6 (slightly
long bolts are used, see later). Sealant should
Antennas
Regulars