One important consideration in implementing an HF general coverage
dipole antenna with balanced-wire feedline is that of avoiding
awkward input impedances. In particular, impedance magnitudes
exceeding about 5000W may be outside
the operating range of typical matching networks, and if highly
reactive, may give rise to excessive losses in the matching network.
In general, such problems are confined to low-frequency operation
(160m, 80m and 40m).
A dipole antenna cut to be about
l/2 on 160m (and l
on 80m), will have a low input impedance on 160m and an extremely
high input impedance (8 to 16KW) somewhere
in the 80m band. For the NLO installation however, the feedline
length will be of the order of l/4
on 80m, effecting a transformation from high to low impedance.
Hence, an awkward impedance for driving the antenna on 80m is
not to be expected but, depending on the choice of wire lengths,
awkward impedances on 160m and 40m are a distinct possibility.
Modelling with NEC is one way to
investigate the awkward impedance problem, but there are limitations
to this approach for the present application. Firstly, the Observatory
building complex is directly below the central part of the antenna;
and it is impractical to take all of the wiring, pipework, equipment
and building materials into account. Secondly, the isolated transmission
line approach favoured for most simulations will not be valid
for an asymmetrically connected 4-wire line. This means that
the transmission-line system has to be included in the wire model,
with attendant problems of segment boundary alignment, element
proximity, and inaccurate field simulation in the region of sharp
bends. Hence, the best that can be expected from simulation is
an idea of the frequencies at which the nasty impedances will
occur; taming the beast in practice being a matter of pruning
the wires if a problem occurs.
Initial simulations indicate that
making the four horizontal radiators as single 50m long wires
will give an excessively high input impedance in the 160m band.
The optimum length appears to be 40 to 45m. The far end of the
South wire will be too close to the Observatory boundary if lengths
in excess of 50m are used.
Wire model:
The central antenna mast is at the back of the building (mounted
on the North wall using T&K brackets). The bottom section
is 48mm diameter aluminium tubing from a height of 1m to 7m.
The top section is fibreglass, from a height of 7m to 13m. A
straight coupler of about 0.3m length causes the conductive section
to extend to a height of 7.15m.
The radio room is at the front
(South side) of the building, displaced diagonally from the mast
at a horizontal distance of about 10m. Hence the feedline must
cross the roof. The flat roof of the building is at a height
of about 3m.
When feeding a horizontal dipole
antenna, interaction between the antenna and the feedline is
minimised by projecting the feedline at right-angles to the radiator
for as great a distance as is possible. This means that the feedline
should drop vertically to a height where, from the radiator's
point of view, the roof-crossing section merges with the general
conductive clutter of the building itself. This topology has
the additional advantage that it minimises the risk that the
feedline will snap when the central mast sways in the wind.
The wire model is constructed accordingly,
with the feedline dropping from 13m to 4m, then undergoing a
90° bend to travel South-West for a distance of 10m. A further
drop of 2m with splayed ends represents termination at the radio
room. The feedline spacing has been set at 0.2m on the diagonal
(14.1cm x 14.1cm square) this being a reasonable choice. The
overall arrangement assumes that, in practice, the axis of the
feedline will be kept at least 1m from the roof surface, and
1m away from the metal section of the central mast. The central
mast was found to make no significant difference to early simulation
results, and so the structure is omitted from the simulations
given here.
The existing HF beam antenna installation
just outside the radio room has not been included in the model.
The EZNEC+ v5.0 program was used for the simulations.
The input files listed below use more than 500 wire segments
and may need to be modified for use with some versions of EZNEC.
The models used include wire losses and are simulated over a
realistic ground. The four radiator wires are 45m long and slope
from a height of 13m at the central mast to 11m at the far ends. |