Vector Network Analyzer Resonance Analysis By Kurt Zeller and Brian Kraft In order to examine the EM Drive phenomenon several design iterations were made of an adjustable partially loaded cylindrical resonant cavity. Several dielectric materials were introduced in varying thicknesses such as high-density Polyethylene (HDPE), Plexiglas, and Nylon and resonance sweeps were performed using an Anritsu MS4622B Vector Network Analyzer (VNA). These experimental analyses were performed over the course of six months and results varied wildly between design iterations. The primary conclusion gleaned is that resonant cavities operating near microwave oven magnetron frequency (2.45GHz) are extremely sensitive to dimensional tolerances, antenna quality, surface finish, and potentially ambient conditions. Preliminary design The first design attempted as seen in the document labeled "BKZ1 Drawings" details an adjustable resonant cavity with the RF source antenna placed directly into the cylindrical cavity. After performing initial VNA frequency sweeps using our home-made antenna seen in document labeled "Magnetron Dissection", it was readily apparent that it would not resonate. Although analytical solutions were performed for a cylindrical resonant cavity as can be seen in "Cylindrical Cavity Design", the antenna placement resulted in a minimum reflection coefficient of -10dB. For perspective, RF engineers commonly cite that a reflection coefficient of less than -20dB is a 'good match' when designing a radome or similar bandpass filter. Because the antenna placement directly in the end of cylinder did not produce the desired resonance, it was determined that a waveguide launcher would be necessary in order to provide an impedance transition between the antenna and the cavity. Secondary design It was quickly realized that the microwave oven has an impedance matched waveguide built in to the structure and that it could be easily cut out with a jig-saw. After cutting out two waveguides and drilling holes in the corners to mount to the end of the cylinder, preliminary VNA sweeps looked very promising. Note that "non conducting/conducting plate" refers to the internal movable plate being electrically attached to the inside of the cylinder. It was later found that this electrical contact was essential to prevent arcing from occurring during full power magnetron testing. The first rounds of VNA sweeps can be seen at the end of this document for 7/29 and 7/30/15. Also note that the Mica insert seen below came stock from the microwave oven in order to prevent food and vapor from entering the waveguide area. Several tests were done with this insert but it was determined to be useless for our purposes.
Transcript
Vector Network Analyzer Resonance Analysis
By Kurt Zeller and Brian Kraft
In order to examine the EM Drive phenomenon several design iterations were made of
an adjustable partially loaded cylindrical resonant cavity. Several dielectric materials
were introduced in varying thicknesses such as high-density Polyethylene (HDPE),
Plexiglas, and Nylon and resonance sweeps were performed using an Anritsu
MS4622B Vector Network Analyzer (VNA). These experimental analyses were
performed over the course of six months and results varied wildly between design
iterations. The primary conclusion gleaned is that resonant cavities operating near
microwave oven magnetron frequency (2.45GHz) are extremely sensitive to
dimensional tolerances, antenna quality, surface finish, and potentially ambient
conditions.
Preliminary design
The first design attempted as seen in the document labeled "BKZ1 Drawings" details an
adjustable resonant cavity with the RF source antenna placed directly into the cylindrical cavity.
After performing initial VNA frequency sweeps using our home-made antenna seen in document
labeled "Magnetron Dissection", it was readily apparent that it would not resonate. Although
analytical solutions were performed for a cylindrical resonant cavity as can be seen in
"Cylindrical Cavity Design", the antenna placement resulted in a minimum reflection coefficient
of -10dB. For perspective, RF engineers commonly cite that a reflection coefficient of less
than -20dB is a 'good match' when designing a radome or similar bandpass filter. Because the
antenna placement directly in the end of cylinder did not produce the desired resonance, it was
determined that a waveguide launcher would be necessary in order to provide an impedance
transition between the antenna and the cavity.
Secondary design
It was quickly realized that the microwave oven has an impedance matched waveguide
built in to the structure and that it could be easily cut out with a jig-saw. After cutting out two
waveguides and drilling holes in the corners to mount to the end of the cylinder, preliminary
VNA sweeps looked very promising. Note that "non conducting/conducting plate" refers to the
internal movable plate being electrically attached to the inside of the cylinder. It was later found
that this electrical contact was essential to prevent arcing from occurring during full power
magnetron testing. The first rounds of VNA sweeps can be seen at the end of this document for
7/29 and 7/30/15. Also note that the Mica insert seen below came stock from the microwave
oven in order to prevent food and vapor from entering the waveguide area. Several tests were
done with this insert but it was determined to be useless for our purposes.
This process was repeated countless times, however VNA results were not recorded as
diligently because efforts were more focused on thrust results, and more particularly,
eliminating arcing within the cavity. The final round of VNA sweeps before the end of Summer
testing is seen below as 9/10/15.
Later Revisions
During Winter quarter more efforts were made to eliminate arcing and new VNA
sweeps were performed using the professionally made antenna as seen below. It was
discovered that the professionally made antenna resulted in dramatically different resonances
than the homemade antenna probably due to the geometric tolerances, quality of electrical
connections, and professional design and tuning. Therefore, all of the VNA sweeps performed
over the summer should be neglected as they have resulted from a poorly created antenna.
Using the professionally made antenna, only a single resonance was found near where
the analytic solutions indicate it should be. Thrust measurements were performed and the
resulting pendulum deflection can be found on Kurt's Youtube page [1]. More detail concerning
this latest test can also be seen on the NASA Space Forum [2].
Conclusions
After months of VNA sweeps it can be safely said that obtaining resonance is not easy.
Using a homemade antenna will most likely produce inconsistent results and may be indicating
other phenomena at play. Every resonance found was extremely sensitive to vibrations,
meaning that tapping on the table could shift the entire log-magnitude plot by 20 dB. Every
thrust test was performed after moving the cylinder between buildings and thus the resonance
was more than likely disturbed. Although EM Pro simulations sometimes correlated with VNA
sweeps, more often this was not the case, however more computational power was needed in
