June 16, 2014
If we were to conduct a contest to award the most abstract use of vector network analyzers (VNAs), the application I’m about to explain would certainly be a finalist. In short, an Anritsu VectorStar™ VNA was used by a team of researchers to bring a whole new meaning to the term “making a beeline.”
We are all familiar with the measurements engineers conduct with VNAs, such as making S-parameter measurements in on-wafer applications or conducting signal integrity tests to verify high-speed commercial and defense designs. Harmonic radar measurements are another common use of VNAs, however, the way these tests were conducted by a group of scientists from the Psychology Division at Queen Mary College London and the Harmonic Radar Group at Rothamsted Research has created a buzz in the VNA world.
The UK-based scientists used radar, motion-triggered webcams, and tiny radar transponders to track bumblebees as they flew to artificial flowers. The “flowers” had landing platforms with drops of sucrose in the middle and the researchers tracked the sequence of flower visits during consecutive foraging bouts. It was found that bumblebees chose the closest flowers first and added new flowers during subsequent bouts.
Radar to Track Bees
Vertical-looking radar (VLR) was used to monitor migratory bee movements at high altitude, while harmonic radar recorded the flight paths of low-flying insects. Scientists also used harmonic radar to track the flight of bees that had attended a "waggle dance" and found that they flew straight to the vicinity of the feeding site, as predicted by Nobel Prize winning zoologist Karl von Frisch. The tracks allowed the scientists to determine how accurately bees translate the dance code into successful navigation, and showed that they correct for wind drift even when flying to destinations they have never visited before.
The VLRs developed for these entomological observations projected a narrow, conical beam vertically upwards from a stationary reflector. The radar beam, offset slightly from the vertical, was rotated continuously, together with its plane of polarization. This produced a narrow-angle, conical scan system that yielded a wealth of information about overflying targets. As the bees passed through the beam, analysis of the radar signals reflected from the bees yielded their speed and direction of movement, body alignment, and estimated mass and body shape.
Transponders Help Monitor Bee Flights
When the insects flew within a few meters of the ground, their radar returns were usually hidden by more powerful echoes from ground features and vegetation. To make sure the radar detected the bees, small transponders were placed on the flying insects (figure 1). The transponders picked up the interrogating radar signal, and immediately emitted a signal at a different frequency that the radar receiver had been selectively tuned. Such a frequency change was easily achieved because the non-linear electrical characteristics of the transponders introduced currents at multiples of the frequency of the original radar signal, and those currents re-radiated harmonic frequencies of the signal.
Given the nature of the research, precise transmission of the radar signals was necessary to acquire credible data. The researchers used the VectorStar MS4644A to conduct a series of harmonic radar measurements so the tracking signals met the desired parameters.
How important was the experiment? We’ll let Head of Computational and Systems Biology at Rothamsted research, Professor Chris Rawlings, summarize. “This is an exciting result because it shows that seemingly complex behaviors can be described by relatively simple rules which can be described mathematically. This means we can now use mathematics to inform us when bee behavior might be affected by their environment and to assess, for example, the impact of changes in the landscape.”