A Experiment Setup
The experiment is set up according to Fig. 7. Each antenna
is connected to the VNA with coaxial cables. The VNA is
used to provide the power source, and to measure the S11 and
S21 parameters of the system. The experiment is conducted at
low power. The system is expected to function similarly in
high power situations as stated in [2].
Fig. 9 shows a picture of the antenna used in the
experiment with the tuning circuit attached to it. The inductor
is made using a coil and a ferrite core, and multiple ceramic
condensers connected in parallel are used to form the
capacitors C1 and C2.
In this experiment, the air gap is fixed at 9cm with the
antennas coaxial (no displacement along the x-y plane). The
coupling coefficient k here is estimated to be 0.135 based on
electromagnetic analysis and pre-match resonance
frequencies of the system.
B. Experiment Results and Comparisons with Simulations
. Fig. 10. shows the comparisons of the simulation results
and the experiment results. Here, the parameters used in the
matching system for both the simulation and the experiments
are C1=32pF, C2=78pF. With inductor L1 fixed at
approximately 5750nH.
The experiment results show that the resonance frequency
can be tuned to match 13.56MHz with an impedance
matching circuit, and the efficiency at that frequency is
increased from 50% to 70%. Also, the equivalent circuit
model can be used to estimate the frequency characteristics of
the pre and post matched systems.
In the post matched system experiment result, the
efficiency η21 at the resonance frequency is lower than that of
the simulation result. However, it can be said that the loss is
not due to a mismatched system as the power reflection ratio
η11 is extremely low, meaning no power is reflected back to
the power source. In other words, the loss is due to other
factors such as radiation loss, the low Q value of the system
(due to the usage of L type impedance matching networks),
loss from the ferrite core used to make the inductor, and
Ohmic loss of the inserted matching circuit.