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1. A typical WERA installation
The figure shows an example of a WERA installation.
If a linear receive antenna array is used, this should be installed
parallel
to the coast to minimize the influence of the signal path over land on
the
receive antenna beam pattern. Receice and transmit antennas should be
set-up on a straight line to ensure
the null in the transmit antenna pattern
to point to the receive antenna.
The required minimal distance between
transmit and receive antennas is 100 m to ensure sufficient
isolation (>70 dB) on the direct path
(WERA uses FMCW, i.e. transmitter and receiver are operated
simultaneously).
The area (angle) covered by the HF radar is
±60° perpendicular to the receive antenna array, if a linear
array is used for receiving. In case of the 4-antenna square array
(surface currents only, no ocean wave measurements possible), the
covered area is given by the transmit antenna pattern.
To maximize the working range, the antennas should
either be installed as
close to the water as possible, or on top of a cliff.
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2. Antenna systems
Depending on the working frequency used by the
radar, antenna height
and spacing varies. For a working frequency of 27.65 MHz, antenna
spacing
is 5.42 m (half the electromagnetic wavelength) or for a working
frequency of 16.00 MHz the antenna spacing is 9.35m. So the total
length of the whole antenna array becomes around 160 m.
The antenna height
of a full length quater-wave groundplane is 2.7 m. Below
25 MHz,
the antenna must be electrically shortened using a coil or a wound wire
to ensure enough mechanical stability to withstand stroms.
2.1. Transmit antenna
The transmit antenna consists of two rows of
2-element linear array antennas
with 0.5 Lambda spacing. The two rows are 0.15 Lambda apart. To form
the
antenna pattern, the cables from the
power splitter to the antennas have to be cut to a specific length. The
cables
A and D (to back row) have to be 0.35 Lambda shorter than the cables B
and C
(to front row). The propagation speed factor for RG213/U is V = 0.66
and has been taken into account for the calculations of the values
given in the table below.
The null produced in the antenna pattern
should point towards the receive
antennas to reduce the energy transmitted on the direct path from the
transmit to the receive antenna. The transmit power amplifier, which
also
houses the power splitter, is located at the transmit antenna. A
200 m long RG 213/U cable is required for
the transmit radio signal, a power cable (115/230 V,
3*2.5 mm²) is required for operation of the power amplifier.
Here are photos of the transmit antenna
installed at Gijon (Spain):
2.2 Receive
antenna linear array
If sea state is to be measured, e.g. significant
waveheight and wave directional spectra, a 16-element linear array is
required. A 12-element linear array can also be used, but gives some
coarser azimuthal
resolution. The total
length of 16-element array is 15 * 5.42 m = 81.3 m at
27.65 MHz. The antenna spacing and variances in height of the
mounting points should be
within 1 % accuracy. We often use wooden sticks of ~5 cm *
~8 cm to fix the
antennas to, so fine adjustments can be done during installation.
Each antenna is connected to a separate receiver channel, so
16 cables
RG 213/U, each 200 m long, adjusted to ±20 cm
difference in length, are required.
Frequency |
Spacing |
16-element total length |
12-element total length |
29.85 MHz |
5.02 m |
75.38 m |
55.28 m |
27.65 MHz |
5.42 m |
81.23 m |
59.57 m |
16.046 MHz |
9.35 m |
140.22 m |
102.83 m |
12.50 MHz |
12.0 m |
180.0 m |
132.0 m |
8.00 MHz |
18.75 m |
281.25 m |
206.25 m |
2.3 Receive antenna square
array
If only surface currents are to be measured, a
small 4-element square array
at 5.42 m diagonal spacing (at 27.65 MHz) together with
a direction finding technique for azimuthal resolution may be used.
Frequency |
Diagonal Spacing |
Square edge lengh |
29.85 MHz |
5.02 m |
3.55 m * 3.55 m |
27.65 MHz |
5.42 m |
3.83 m * 3.83 m |
16.046 MHz |
9.35 m |
6.61 m * 6.61 m |
12.506 MHz |
12.00 m |
8.48 m * 8.48 m |
8.00 MHz |
18.75 m |
13.26 m * 13.26 m |
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3. Environmental conditions
The following requirements have to be met to
ensure proper operation of WERA:
- The 16(12)-element linear receive antenna array
must be installed parallel to the coast, e.g. all distances from the
antenna to the sea must be the same.
This ensures that all antennas get the same receive energy of the
backscattered
signal. Variances in distance from the antenna to the sea may be
compensated
by changing the antenna height above sealevel, allowing to set up the
array at some slight angle to the coast line.
- Install the antennas as near to the water and as
high as possible. This
increases the coupling of the antennas to the sea surface, resulting in
stronger signals backscattered from the sea.
- There must not be any metal fences, power cables
or other electrically conductive material within 5 electromagnetic
wavelengths distance around the antennas. Also, avoid electromagnetic
noise in the HF band generated
i.e. by motors or high-voltage power lines.
- The WERA radar itself should be installed in a
metal container to keep
local computer noise off the receive antennas.
- Look for a free operating frequency; avoid
interference with local
wavebouys, which may be operated within the HF radar bandwidth.
- Wavebouys may receive and retransmit the HF
radar signal if they are
located too near to the transmit antenna. Give at least ~1 km
distance
between transmit antenna and wavebouy.
- The radar and antenna installation should be
closed to public.
- The save distance to the transmit antenna for
people using a pacemaker is 3 m at 30 MHz, less than 3 m
at lower frequencies.
- The antenna systems should be installed high
enough to be protected
against storm surges, waves, spray and foam.
4. Power
supply and communication requirements
- The WERA radar itself needs about
1 kW including all computers.
- The container should be air conditioned to keep
the temperature at 18...23 °C. This may take up to 5 kW
and depends on the local conditions (cooling or heating).
- A data link between the WERA radars or the WERA
radars and an operation center is required to on-line calculate
2-dimensional surface current fields. A ISDN telephone link would be
sufficient, if no raw data need to be transferred. The size of a file
containing radial components of the surface current at 60*60 grid
points is 101,888 bytes. Doppler spectra at 60*60 grid points, which
are required for the ocean wave algorithm, take 7,373,312 bytes. To
transfer this file within 5 minutes, a transfer rate of 25 kbytes/s is
needed, which is faster than ISDN (7.5 or 15 kbytes/s).
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