A more accurate measuring technique for omni-directional antennas has been developed by the National Physical Laboratory (NPL).
Researchers at the NPL have used the technique for verifying a variety of omni-directional antennas for applications such as wireless crop monitoring and implanted medical devices.
Omni-directional antennas, such as those used in mobile phones, radiate radio frequency (RF) signals at 360°. Martin Alexander, principal research scientist at NPL, said engineers have been challenged to accurately measure these signals.
The reason, he added, lies in creating a suitable testing environment that does not reflect radiation.
All antenna measurements require a transmitting antenna and a receiving antenna.
The receiving antenna is normally connected to the measuring receiver by a coaxial cable. The receiving antenna is rotated and the cable relays information on the amplitude and phase of the RF signals over a sphere with the antenna at the centre.
Testing occurs inside an anechoic chamber and researchers can view the antenna’s 360° radiation pattern in an adjacent room on a computer screen.
Alexander said coaxial cables emit radiation that can interfere with the antenna.
He added that radiation from the cable and the radiation from the antenna can cancel each other out if they are in anti-phase, so the antenna will pick up virtually nothing.
If they are both in phase then the antenna will pick up a bit more, but this potential advantage is outweighed by the disadvantage of signal dropouts in some directions.
‘Either way you want to measure what the antenna is doing. You don’t want to measure what the antenna and cable are doing.’
The NPL has solved this problem by replacing the coaxial cable with an optical fibre. The fibre, which is made of dielectric, is very thin so it does not interfere with radiated RF signals.
The unique feature is the miniaturised RF-optical link. A lithium-niobate crystal is contained in a screened metal box measuring 20mm x 10mm x 10mm. The box is attached to an SMA connector, which in turn is connected to the antenna.
‘Our technique cleans up the pattern and gives antenna designers a much quicker and more efficient turnaround of model to production,’ said Alexander.
Siobhan Wagner
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