In the last few years, numerous articles have been written about flat-panel antennas (FPAs) and how they stack up against conventional stabilised VSAT antennas. On paper, they offer compelling technical arguments such as low weight, no moving parts and improved aesthetics. Having a lower weight makes FPAs easier to install and reduces the cost of installation, whilst no moving parts mean less wear and tear on belts, motors and bearings resulting in increased reliability. Aesthetically speaking, while some owners don’t mind showing off their Satcom antennas many would undeniably rather have a more low-profile antenna solution – preferably one requiring less onboard real estate.
However, with today’s FPAs generally lower performing compared to parabolic antennas, this new technology is not without its flaws. FPAs have understandably been surrounded by a lot of hype, described as the future of superyacht connectivity. Though it may seem like the technology of the future, this does not necessarily mean it is universally suitable at present. Below, we explore the intricacies of FPAs and weigh them up against conventional antennas to evaluate whether it is worth the switch.
How do they work?
In ideal conditions, it should be possible for FPAs to achieve roughly the same performance as a stabilised parabolic antenna for a given antenna surface area. The underlying issue, however, is that maritime antennas do not generally operate under ideal conditions.
An ‘ideal’ situation would find the satellite located directly above the antenna on the vessel, so that the elevation angle for the antenna plane is 90° (as shown in the illustration below).
Often this is referred to as the antenna being pointed broadside towards the satellite, or having a scan angle of 0° (i.e. scan angle = 90° – elevation angle).
This situation only arises when the vessel is located on the equator, directly underneath the satellite, or when vessel movement is such that the antenna panel is pointed broadside towards the satellite. Naturally, such situations do not arise very often.
A more typical scenario is, for example, a vessel located in the Mediterranean Sea to be communicating via a satellite located somewhere along the geostationary arc over the African continent. The elevation angle towards the satellite is more likely to be in the range of 30°-50° depending on the exact location of the satellite and vessel relative to one another.
As the elevation angle decreases, the effective antenna aperture (in the direction of the satellite) is reduced. Therefore when the elevation angle is 30° the effective antenna aperture is reduced to half of its initial size and the antenna performance drops by 3dB. Often, this is referred to as scan loss for FPAs.
This relationship is further illustrated below. The graph shows an antenna with peak G/T performance of 15dB/K when the elevation angle is 90°. As the elevation decreases the antenna performance gradually worsens, as the degradation roughly follows the line of the elevation angle.
In order to maintain reasonable antenna performance, the antenna must be operated with an elevation angle above roughly 30° – corresponding to 3dB of performance loss.
When elevation angles are low there are also additional performance concerns, such as degradation of the antenna pattern and skew effects. So in real life, one would expect performance to drop off more aggressively than above curve suggests. However, the extent to which these issues impact antenna performance depends on a number of things, including antenna geometry and geo-location. Hence, they are somewhat difficult to quantify without a much lengthier description.
Are FPAs worth it?
Let’s return to our example vessel operating in the Mediterranean with elevation angles in the 30°-50° range. Add vessel movement to this and one can easily see that the effective elevation angle may drop down to 20° – and even lower if the satellite is low on the horizon to start with.
This gradually worsens the further north the vessel goes. In northern Europe, the satellite elevation angle could easily be less than 10° even under ideal conditions with no vessel movement. Add in the vessel movement and you could, in fact, have negative elevation towards the satellite. For this reason, mechanically stabilised antennas often support elevation angles as low as -20° to cater for ship motion.
To improve operating conditions for our Mediterranean vessel the obvious solution is to tilt the panel, taking the average elevation angle to around 40°. If the FPA sat at a tilt of 50°, the panel would be pointed broadside towards the satellite meaning a margin of +/- 40° in elevation where scan loss would be tolerable. Sadly, however, this approach does not handle a vessel changing direction. Hence, multiple panels would be required to provide the vessel with 360° view of the satellite.
You could have 4 panels – each tilted at 50° and mounted at right angles to one another to provide a 360° view of the satellite, while at the same time handling elevation angles between 10° and 90°.
This solution still has the advantage of no moving parts, but it is unlikely that any cost savings would materialise due to the significant costs associated with cabling and installing what would effectively be 4 separate installations. One could also argue that 4 panels would have a form factor equal to or larger than a comparable stabilised parabolic reflector, removing any aesthetical advantage of FPAs.
A vessel operating year-round under favourable conditions, meaning a high elevation angle towards the satellite and calm seas, might be a candidate for using Flat Panel Antenna (FPA). A potential buyer needs to factor in the cost of communication (i.e. a service) as well as the cost of the solution itself. Due to the generally lower performance of the FPAs, the cost of communication may easily offset any savings on the solution itself. However, FPAs are likely to find their early adopters in yachts where owners attribute real value to having a radome-free superstructure.
For most other applications, the traditional stabilised parabolic reflector still holds the upper hand. It is superior with respect to handling a wide range of operating conditions and will generally lower the cost of communication.
FPAs have come a long way in recent years, but appear to have some way to go to be a real threat to the versatility of traditional stabilised antennas.
Ultimately, only time will tell if the FPA manufacturers will be able to solve all the issues needed to make these antennas universally applicable for maritime applications.
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