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  How to set up a WERA Site

WERA is a HF radar using electromagnetic waves between 6 and 30 MHz (50 m to 10 m wave length) to measure surface current velocities, ocean wave height (spectra) and wind. The transmitted vertical polarized electromagnetic wave travels along the sea surface beyond the horizon and is backscattered by ocean waves of half the electromagnetic wavelength (Bragg Scattering). Typical working ranges are 15...30 km for ocean waves and 40...50 km for surface currents at 27.65 MHz working frequency, 35 PSU salinity and medium sea state. Here is a table on working ranges and further remarks on other radar frequencies.

As one WERA radar measures radial components, two or more radars are required to measure the full 2-dimensional values. The distance of the two WERA radars should be around 15 km at 27.65 MHz, if sea state measurements are required. This is to ensure a sufficient signal-to-noise ratio for the wave algorithm. If only surface currents are to be measured, increasing the distance to around 35 km reduces the measurement errors due to geometry.

To decide on the measurement area and the radar sites, a nautical map is required to know about ship traffic, light houses, and water depth / bottom topography structrue. A topographical map is needed to plan site logistics like access by roads as well as the height of land and islands.

1.  A typical WERA installation

WERA Site

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.

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). 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.

Transmit Antenna

Frequency 0.5 Lambda 0.15 Lambda Length difference A,D to B,C Cable length A,D Cable length B,C
29.85 MHz 5.03 m 1.51 m 2.32 m 6.63 m 8.958 m
27.65 MHz 5.42 m 1.63 m 2.51 m 7.16 m 9.678 m
16.045 MHz 9.35 m 2.80 m 4.32 m 12.34 m 16.66 m
12.50 MHz 12.00 m 3.60 m 5.54 m 15.84 m 21.38 m
8.00 MHz 18.75 m 5.63 m 8.66 m 24.75 m 33.41 m

Note, that the length of cables A and D can be selected to some practical value (to mechanically install the power amplifier at a convenient pace) and cables B and C have to be 0.35 Lambda * V longer.

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.

2.1.  Alternate transmit antenna

If the transmit antenna can not be installed to direct the null in the antenna pattern towards the receive antenna, a second null can be generated 42 degrees to the back side of the transmit antenna by keeping the mechanical distances of 0.15 * 0.5 Lambda, but increasing the electrical cable length difference from 0.35 Lambda to 0.40 Lambda. Here is the resulting antenna pattern.

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 * 4.88 m = 73.24 m at 27.65 MHz (0.45 Lambda spacing). The antenna spacing should be configured for the highest frequency used by the system. 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.

Receive antennas at Gijon

Frequency Spacing 16-element total length 12-element total length
29.85 MHz 4.52 m 67.84 m 49.75 m
27.65 MHz 4.88 m 73.24 m 53.71 m
16.046 MHz 8.41 m 126.20 m 92.55 m
12.50 MHz 10.80 m 162.0 m 118.8 m
8.00 MHz 16.88 m 253.13 m 185.63 m

The direction of the array is required to correctly map the measurements to geographical coordinates.
This is done by passing "True North" to the processing software.

If the linear array is operated in beam forming mode, these are the expected antenna patterns (beams) for 16 antennas when steering the beam to 0 degrees (perpendicular to the array) or 45 degrees. Note, that due to the reduced active aperture length, the beam gets wider when steered to the side.

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.

4-Element Receceive Array

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

The direction of the array is required to correctly map the measurements to geographical coordinates.
This is done by passing "True North" to the processing software.

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).

5.  Radio Transmit License required

For operation of the WERA radar, a HF transmit license is required. This must be granted by the local FCC administration. The characteristics of the transmitted signal are:

Parameter Value
Center frequency 29.85 MHz, 27.65 MHz, 16.045 MHz, 12.50 MHz
Bandwidth @ 3.0 km resolution 50 kHz (±25.0 kHz), see remark below
Bandwidth @ 1.2 km resolution 125 kHz (±62.5 kHz), see remark below
Bandwidth @ 0.3 km resolution 500 kHz (±250 kHz), see remark below
Transmit power 30 W continuously
Transmit antenna 4 Elements, directed to the sea (antenna pattern)
Modulation FMCW, linear frequency chirp
Duration of the chirp 0.26 s

The required bandwidth B [kHz] as a function of the selected range resolution Rresol [km] is:    B = 150 / Rresol

 
 
 
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