INTELLIGENT ANTENNA DESIGN
Conflicting technical requirements and a non-negotiable styling surface combined to present our prestige OEM customer with an almost impossible design brief for a telematics antenna. Our creativity, technical know-how and energy solved the problem.
The antenna was required to for use with the on-board telephone system and the VEMS (Vehicle Emergency Messaging System), to be activated following a crash event.
The 1800/1900 MHz GSM/PCS telephone system had a peak transmission power of 8 Watts, necessitating a package away from the occupant cell to avoid exceeding government guidelines for Specific Absorption Rates, published in the (then recent) ‘Stewart Report’ in to the effects of non-ionising radiation on the human body. In comparison, a modern smartphone has a maximum transmission power of between 1.5 – 2 Watts.
Conversely – the requirement for the antenna to remain operational following a crash event in order to transmit the distress signal necessitated a package within the occupant safety cell, as traditional locations for the packaging of antenna (beneath plastic bumpers) were likely to be destroyed in the event of crash.
The OEM’s only proposal up to this point was to position a ‘stubby’ rod antenna on the roof of the vehicle. However this had numerous drawbacks;
- It was vulnerable in ‘rollover’ crash events.
- It had been rejected by the OEM’s styling department, which held significant influence with the organisation.
- It would have meant derivatising not only the extensive roof panel, but also the rear header rail, necessitating tooling changes, re-arrangement of the body assembly production equipment to accommodate the additional part variants, and additional roll-over crash testing.
- In total the cost impact of this solution was an additional $2m over budget.
The proposal also prompted the styling department to issue the edict that no antenna was to be visible either the interior or exterior A-Surface of the vehicle – further complicating the already conflicting requirements.
We were tasked to come up with a solution to this seemingly impossible challenge.
In order to identify a suitable package, we not only engaged traditional tools of a ‘digital buck’, but also used vehicles which had already been crash tested, immediately ruling out any vulnerable locations.
In parallel, we conducted research to identify a highly directional antenna design, thus allowing it to be packaged within the safety-cell but directing it’s power away from occupants.
We verified our research with tests on a range of antenna designs on the vehicle - initially using a handheld electric field meter, and later confirmed with full EMC chamber tests. We identified the ‘Inverted F-Planar’ type of antenna as the most affective.
Our packaging feasibility study identified a void between the ‘parcel tray’ Body-In-White and the associated trim panel, which provided a packaging envelope and a transmission ‘window’ just large enough for a 'PIFA' antenna to work.
We then set about coming up with a concept for an antenna design which would be inexpensive to manufacture, easy to assemble, and straightforward to fit to the vehicle. ‘Poke-yoke’ (the ‘foolproofing’ of the design to prevent mis-fitment of alternative variants) was an important consideration, as the antenna was to be available in alternative waveband versions for different markets into which the vehicle would be sold.
The design we achieved met all of the technical performance requirements and ancillary requirements for ease of manufacture and piece cost. Prototypes were made and component and system level functional tests performed using EMC chambers at MIRA, Gaydon and a testing range at Ford in Detroit. Vehicle level tests were performed in the markets where the feature was to be available prior to formal engineering sign-off.