Naval Antenna Placement Case Study: FEKO and Antenna Magus

System engineers are often presented with functional requirements for a communication system that has to be operated in a challenging environment.  The task of selecting an appropriate antenna topology and modelling it with FEKO might fall outside their field of expertise.  Antenna Magus will help them to easily select, design and evaluate a suitable antenna model for system analysis in FEKO.

The following example illustrates a workflow that could result from using Antenna Magus and FEKO for such projects.

Requirement:

The functional requirements that are presented for a communication antenna to be mounted on a naval ship are to create a wide-band VHF antenna for ship-ship communication links.  From experience an engineer can write down the following design specifications:

• VHF wide-band requires a frequency span of 150 MHz to 350 MHz, centre frequency 250 MHz, bandwidth of 80% around the centre frequency.

• Wire antennas are preferable as such antennas present little wind resistance and require little maintenance in the challenging environment of operation at sea.

• Communication to the horizon is important and elevation pattern degradation is not a problem.

• The antenna will be mounted on the port-side lower yardarm of rear castle.

Work steps:

Select antenna topology in Antenna Magus:

• Using the find mode, "VHF" is the first keyword that is entered.  From the list of antennas that remains, a folded dipole and cylindrical dipole is added to the antenna collection as they look like typical horisontal communciation antennas.

• Discone antennas are often used in naval applications so "discone" is also entered as a keyword.  One of the filter keyword suggestions from Antenna Magus is "Wire discone" which is selected and the wire discone antenna added to the collection that will be considered.

• Antenna Magus's information browser provides a quick summary of the expected performance characteristics of the antennas in the collection.  Based on the performance bandwidth ratio, it is easy to discard the cylindrical dipole (12%) and folded dipole (25%), while selecting the wire discone (3:1) as the only viable option.
  

Design antenna, estimate performance, iterate in Antenna Magus.

• Using the design mode, the first design is made for a 250 MHz centre frequency with 9 radial elements and the performance estimated.  Antenna Magus is used to estimate performance to find out that the initial design does not meet the required frequency band specifications (S11 < -10dB for 250 MHz to 640 MHz).

• A second antenna is designed with a new centre frequency of 150 MHz and 9 radial elements.  The estimated performance for this design predicts a satisfactory impedance bandwith (S11 < -10dB for 150 MHz to 380 MHz).

Export model to FEKO from Antenna Magus

• The second design is easily exported for FEKO using the Antenna Magus export mode.

Confirm antenna performance with FEKO 

The Antenna Magus model is easily modified in FEKO to simulate across the frequency range of interest, the model meshed and simulated without any problems.  These steps confirm that the stand-alone wire discone antenna easily satisfy the required performance criteria for both impedance bandwidth and radiation pattern.

Antenna placement study with FEKO

Confirm impedance bandwidth, radiation pattern, coupling from VHF whip on rear deck.

• The FEKO model that was exported by Antenna Magus is easily imported into FEKO (import only the geometry, ports are preserved with this option) and translated to the appropriate position just below the port side yardarm where it will be mounted.
• Mounted discone antenna performance simulations are set up in FEKO to evaluate 300 MHz radiation patterns.  These simulation results are easily compared to the results for the standalone wire discone antenna.
• Antenna isolation between the discone antenna and the existing VHF whip antenna on the ship is also considered.  S-parameters are computed to determine how much energy will couple between the wire discone antenna and the VHF whip antenna.

Concluding remarks:

• Antenna Magus assisted in confirming that knowledge of the general naval use of discone antennas was useful and also to help evaluate other possible solutions.  Using Antenna Magus implicitly provides the certainty that a wide range of antennas were considered.
• In free-space the antenna impedance bandwidth specification is easily realised and confirmed by using the CADFEKO model that was exported by Antenna Magus.
• The far-field radiation gain patterns show a loss in gain in the direction of the main mast that the yardarm is mounted on.  System designers either need to incorporate a second discone antenna on the far side of the mast to fill in this null, or need to move the discone antenna to a different less obstructed position.
• Very little energy is coupled into the wire discone antenna from the VHF whip antenna on the rear deck of the ship.  Communication via the wire discone antenna is thus possible even while the VHF whip antenna is transmitting in its band of operation.