AOs and HFSS

Welcome back to my blog! As promised, I do have a picture of an otter, in case anybody needed some therapeutic cuteness (the AO in the title is obviously the acronym for Adorable Otters). After meeting with Dr. Melde, my advisor, I have begun to shift gears from learning about antennas and earlier designs for tracking collars to learning as much as I can about California otters and their environment. Surprisingly enough, there is a lot of information out there about California sea otters. I recently read an interesting but sad journal article about the bycatching of sea otters, where otters swim into crab and lobster traps but cannot get out and subsequently drown. The study uses statistics from past drownings and an experiment where otters of varying waist sizes and lengths are placed near traps full of food are observed in order to study the likeliness of certain otters becoming trapped (no otters were harmed in the structured experiment).

Hey Steve! There's a bunch of kelp over here!

Otters are very much a beloved species by humans. If you are like me at all, the thought of otters dying as a result of preventable events such as oil spills or fishing traps evokes a very emotional reaction. This is a large part of why I am looking at improving the existing tracking system for sea otters, because as Dr. Melde says, with modern technology, there must be a better way than surgically implanting tracking devices into the otters' abdomens. The device below is what is currently being used in otters, and this is essentially what my research project is all about, because most of my research is being done with the goal of figuring out if there is a viable option for sea otter tracking collars that is non-invasive, but still provides the luxury of a long lifetime.

Yes, this device goes into the abdomen of an otter.

Besides the fact that otters are adorable, there are very practical reasons for devoting resources towards the tracking of sea otters. As many of my peers will remember from the ecology section of biology, otters are an excellent example of a "keystone species," or a species that is so important to its ecosystem that if it is not present, the ecosystem will almost certainly collapse. Sea otters are big consumers of sea urchins, and sea urchins eat kelp. Sea urchin numbers are kept in check enough by otters so that they do not significantly damage the kelp forests, but if sea urchin populations were allowed to grow unchecked by otters, the sea urchin populations would grow exponentially due to the vast resources of the giant kelp forests. Eventually the urchins would eat all of the kelp, and the bottom of the underwater food chain would be eliminated, effectively destroying the ecosystem.
Additionally, I have noticed that many of my readings on otters mention that significant changes in the water environment become apparent in sea otters before they become apparent in other species, giving ecologists warnings about potentially negative changes in the water composition. Further information on that will have to come in a later blog post.

If any tech guys were bored by the last few paragraphs, then I have good news! I have spent a bit of this last week becoming familiar with this amazing computer program called High Frequency Structure Simulator, HFSS for short. Fortunately, trying to say that acronym as an english word is not the most fun part of the program. It allows users to virtually build structures out of myriad materials, and by changing the environment and conditions around the virtual model, the program uses Maxwell's equations to solve for results in order to give the user a ton of information about how their structure will theoretically function. Since I am very new to the physics of antennas and the HFSS program, I certainly do not have a great understanding of the capabilities and nuances of the vast program, but I did manage to "build" a PIFA in the program with the exact same dimensions as the ones that I physically built in order to see what would happen. One of the many features of the program is that it can analyze an antenna much like a network analyzer would, only it's all theoretical. Therefore, the graph of resonance of different frequencies on the right not only looks much prettier and neat than the measurement taken by the actual network analyzer, but the program predicts that the peak resonance of my PIFA will occur at 1.65 GHz instead of the 1.575 GHz that my physical PIFAs did.

My digital PIFA

The virtual network analyzer ( y-axis is in decibels)

The first question that comes to mind is why the program predicts that my PIFA would have its peak resonance at a frequency other than the one it actually had. The answer is obvious: no simulation can completely capture reality, and no device made by my hands will be as perfect as the one in the simulation. However, there are some specific aspects that I believe significantly affected the results. In the program, the plates of the PIFA are planes without any height, but the planes that I used in the physical antennas were over a millimeter in height. Also, my model did not include a coax stem, and there was dielectric inside the top plate of the physical PIFA that may have introduced enough capacitance to affect the frequency resonances. These are all things that I will be further exploring in the HFSS program, and along with researching even more about otters, I will be spending some time next week trying to more accurately replicate my physical PIFA in HFSS.
Have a great week!

Pictures:
http://seaotters.com/wp-content/uploads/2012/03/628x353-otter-cu-yawn.jpg
https://seagrant.uaf.edu/research/projects/10/otter/images/transmitter.jpg