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Extremely shortened Monopoles -
they do work Free translation of an article published in
„Funkamateur“, issue July 2000, page 780, ISSN 0016-2833
The shortened monopole described herein is based on a commercial
antenna developed by OE7OKJ, but constitutes a special version ,
tailored for radio amateurs. Permission is granted for public and
private useage and experiments. Commercial applications are covered
by patents. Principle of Operation This antenna constitutes an open, series resonant, electrical circuit.
An inductor is series-connected with a conductive cylinder, together
with its associated distributed capacity to the environment. The
feedline, a halfwave in electrical length, serves as a counterpoise.
The coaxial feedline’s outer conductor is not connected to the
monopole - only its center conductor is connected with the
antenna’s inductor. On one hand the inductor serves as a base
loading coil, on the other hand, as a magnetic field radiator. There
is no great benefit in trying for the ultimate Q of the inductor, as
this would end in greatly reduced operational bandwidth and
increased sensitivity to influences from the antenna’s environment.
The conductive cylinder (radiator) capacitively loads the inductor,
assures resonance at the operating frequency and provides for
sufficient bandwidth, due to its form-factor. The antenna system
described herein was computer optimized, not for optimum Q but for
optimum reliability, i.e. for greatest independence from
environmental influences. Figures 2 through 5 depict the antenna in its sectional plane, together
with all its components. Hints for amateur construction Cut lengths of cover tube (3), length of coil former (5) according to
table 1 (l3 and l5 respectively). Material for both tubes may be
purchased from any hardware shop selling building material. Watch
out, however, as sometimes material available from such shops may
suffer from wide variations from nominal dimensions.
Material with more accurate dimensions are available from shops selling
material for electrical wiring (cable tracks etc.) Machine the mounting block
(2), contact ring (6) and top cover (8) on a lathe, according to the
dimensions
given in figure 2, 3 and 4. Depending on the outline dimensions of
the coax connector (1) you have selected, determine the exact and
required dimensions of the bore, shown on the lefthand side of figure 3 and labelled Ø15.5 x 15.
The connector (1) shall be firmly seated in this
hole. It should slide in,
without applying undue force.Push the starting end of inductor wire
through hole labelled Ø5 x 45° in figure 3 and push until thewire
reaches the left-hand hole Ø15.5 x 15. Remove insulation from inductor wire over a length of approximately 3
mm and solder the wire to the center terminal of coax connector (1) Apply epoxy
resin glue to the relevant mating surfaces of (1) and (2).
Place
connector (1) into the hole of the mounting block (2) until it sits
flush with the block's
surface. Allow for the glue to harden before proceeding. Apply epoxy resin glue to the stud Ø18 x 15 on the right-hand side of
figure 3 and to the mating inside surface of coil former (5).
Slide
coil former (5) over stud Ø18 x 15 and allow for the glue to harden
before proceeding.
By rotating the subassembly (1+2+5), place the inductor wire onto coil
former (5) with the amount of turns "N" specified in table 1. The
individual turns of the inductor coil shall be wound without
spacing.
Let the last turn spread to the right-hand end (figure 3) of coil
former (5).
Cut
the inductor wire, remove about 3mm of its insulation for soldering.
Cut
an axial slot (1mm wide, 5mm depth) into the contact ring (6) at its
lefthand side of figure 4.
This will provide for soldering of wire end. Solder wire end to contact ring (6). Prevent solder from protruding
from the slot so that it does not increase its effective outer diameter (Ø22mm).
Apply
epoxy resin glue to the inside surface of contact ring (6) and the
mating outside surface of
coil former (5). Slide contact ring (6) over coil former (5) until both
components’ right-hand sides (figure 5) mate. Turn the contact ring relative to
the coil former to obtain a flush seat of the inductor
wire
on the coil former's surface. Allow for the glue to harden before
proceeding. Figure 6 shows the assembled inductor coil and contact ring for 3
antenna versions.
Prepare
radiator tube (7). It may be manufactured from
copper tubing or alternately from copper
sheet material. Wall
thickness of copper tubing must not exceed 0.5mm (nominal 0.1mm).
Copper sheet
thickness shall be 0.1mm (maximum 0.5 mm).
If copper tubing (required
length is l7 as per table 1) is used, cut an axial slot along its
side, approximately 1mm in width. This will allow the copper tubing to become
resilient.
If copper sheet material is used, cut the required length (l7 as per
table 1) and width. Using a rigid cylindrical object OD=22.5mm, roll and curl the
copper sheet to form a cylindrical (slotted) tube.
The inner diameter of
cover tube (3) and outer diameter of radiator tube (7) shall mate,
so that the
radiator tube (7) holds it in place when mounted inside of the cover
tube (3). Do not yet apply glue to affix (7) with (3), as radiator tube (7)
eventually needs pruning for fine tuning at the resonance frequency.
Slide
assembly (3+7) over assembly (1+2+4+5+6) until a) radiator tube (7) overlaps contact ring (6) for about 5mm, and Prepare a coaxial feedline which is exactly a halfwave in length
electrically (or an integer multiple thereof). Consult table 2 to find velocity factor
"V" for conventional cable types. Find mechanical
length
"l" in meters for the operating frequency "f0"
in MHz, by solving the equation: l=V*150/f0. Coarse checks of the antenna’s resonant frequency
may be performed indoor, but for accurate finetuning,
it will be necessary to
place the antenna outdoors. Mount antenna atop a non-conducting support tube or pipe, about 3m in
height, running the feedline through its inside. Keep the antenna clear of nearby
conductive or dielectric objects. Check the antenna’s resonant frequency with an
antenna-scope, SWR Analyzer, or your transceiver
and SWR-meter. If required,
prune the length of the radiator tube (7) until the resonant
frequency is
correctly adjusted. Apply epoxy resin glue to inside surface of cover
tube (3), to outside surface of radiator tube (7) and
to the mounting block's
stud labelled Ø22.6 x 10 in figure 3. Assemble (7) with (3).
Slide
assembly (3+7) over assembly (1+2+4+5+6) until a) radiator tube (7) overlaps contact ring (6) for about 5mm, and Allow for the glue to harden before proceeding.
To
make the antenna resistant to weathering and exposure to sea water
spray, fill the cover tube
entirely with a plastic foam. There are several types of foam available.
Do not use the types used in
building construction –
their hardening requires the presence of moisture, but once moisture
is in the
antenna, it is quite impossible to remove it. Only use the double
component PU foam
(polyurethane) for filling. Practical operation Start your experiments with the antenna model for the 40m Band. It is
compact, yet does not require so much precision in its construction process as do
the models for the higher frequencies.
The feedline shall be a
halfwave in length electrically at the design frequency, otherwise a
matching (tuning)
unit becomes indispensable. The latter, however introduces
additional losses into the
antenna and feedline system which may become excessive (50% of total
system loss). A feedline cut for one band may be used as well on harmonically
related higher frequencies. Despite the fact that successful QSOs were made while
the antenna was atop a table in the basement
indoors, it is always a
good idea to mount the antenna in the clear.
Mounting atop a
non-conductive tubular antenna support is the most likely one. Dry
wooden posts,
fibre glass masts or pipes make a perfect antenna support for
stationary and portable operation. Suspending the antenna from a non-conducting support
line, hung in between trees or buildings is
another possibility. For mobile, portable or experimental operation, the antenna may be
simply suspended topside down by its own feedline.
When using PVC tubing for
the construction of the antenna, apply painting with epoxy paint or
acrylic
paint, as the PVC (poly vinylic chloride) is not UV resistant. Do
not use paint or laquer
filled with metallic particles.
Two identical monopoles
may be arranged end-to-end and operated as a dipole antenna. One
monopole
is connected to the center conductor, the other monopole to the
outside conductor (shield)
of the coax feedline. Monopoles for different bands may be operated from one and the same
feedline. Arrange them in quadrature with respect to each other and use coaxial
T-adaptors for connection. Different
monopoles may be arranged
in parallel, if a spacing of at least 1 meter is preserved.
The
author hopes that radio amateurs and CB operators will be successful
in constructing this type
of antenna. For the activity-challenged ones (read = “lazy”),
orders may be placed to [2] Traxel
company exclusively, as
the stock of antennas manufactured by OE7OKJ himself is completely
sold off.
Figures
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