MANUAL DE PROCESOS

The prototype antennas, while demonstrating frequency reconfiguration, have not performed up to expectation (low gain, poor S11, and deformed patterns) in either the Midband or Low band reconfiguration state. It was determined later on after many painstaking processes and iterations that these performance deficiencies were caused by the Electromagnetic Interference (EMI) coming from the DC bias traces and wiring.

A. Manipulating the DC Bias Wires

The initial basis for attributing the cause of the performance deficiencies to the DC bias wiring was founded on observations of antenna S11 response when the DC bias wires were manipulated. The S11 shown in for all previous experimental specimens is specific for that arrangement of the wiring and those testing conditions. What was found is that manipulating the DC bias wiring, even subtle manipulations which alter the position of the DC bias wires by only inches, can cause drastically alte red S11 responses.

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Out of the 3 reconfiguration states, the Highband demonstrated the most resilience in the face of DC bias wire changes. There was never an arrangement of DC bias wiring found which destroyed the Highband completely, although it could be greatly perturbed and demonstrate significant changes to bandwidth. However, both the Midband and the Lowband proved extremely sensitive to changes in the DC bias wire positioning. Resonances could be made to disappear and reappear at frequencies sepa rated by 100 MHz or more. Unfortunately, inside the Satimo chamber or near it at the WRCNC we did not have the capability to measure the S11 of the antenna as it appeared under testing conditions with the device positioned on the measurement pedestal inside the chamber. The lack of this capability allows for some doubt about the status of the S11 while the radiation measurements were performed, as the exact S11 response of the antenna as it appeared on the Satimo measurement pedestal is unknown.

B. Studying the EMI from DC Busbars and Wires

To investigate the hypothesis of the DC bias wiring in the presence of the DC busbars causing the performance deficiencies to the pixel patch antenna, a HFSS model was developed to simulate the pixel patch structure and test the effects of the DC bias wiring in a controlled manner. The model includes all the copper traces from the experimental specimen modeled as 0.1mm thick rectangular cross -section solids assigned as copper with a conductivity of 5.8E7 S/m. The anten na substrates are modeled as FR4 with a

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relative permittivity of 4.4 and loss tangent of 0.02. All DC bias wires for the pixel patch and feedline are included in the model. The wires are modeled as cylindrical solids with a radius of 0.25mm and assigned as copper with the conductivity listed above. To reduce simulation time, the Rohacell foam core was neglected from the model. The space between the bottom of the pixel patch substrate and the groundplane is filled with air. The RF MEMS were modeled by creating a short for the ON state, and an open for the OFF state. The frequency reconfiguration states are modeled by connecting or disconnecting the appropriate pixels with a 0.92mm × 1mm × 0.1mm section of copper.

The S11 plot of the Highband, Midband, a nd Lowband before and after including wires to the simulation model is shown in Fig. 2.26.

The results confirm that the DC bias wiring in the presence of the DC busbars is responsible for the performance losses observed from t he experimental prototype test data. The Highband, although showing a decrease to −10dB S11

(a) (b) (c)

Figure 2.26 Simulated S11 measurements showing the effect of DC bias wires on the antenna: (a) Highband, (b) Midband, (c) Lowband.

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bandwidth of approximately 100MHz, remains intact with a bandwidth of 250MHz. Of greater consequence are the S11 responses of the Midband and Lowband after inclusion of the DC bias wiring to the simulation model. The Midband S11 response in particular is adversely affected, with no trace of the “No Wires” S11 response remaining after the inclusion of DC bias wiring.

Further simulated results show more detail of th e effects of the DC bias wires on antenna performance. Fig. 2.27 shows the simulated electric field distributions for the three reconfiguration states of the pixel patch before the inclusion of the DC bias wires.

The electric field magnitudes are calculated at the approximate center frequency of the respective reconfiguration band. The electric field distributions of this figure do not precisely conform to the textbook electric field distribution of a microstrip patch antenna due to the pixelated antenna geometry and DC busbars on the underside of the substrate. However, the

Figure 2.27 Simulated electric field distributions for the pixel patch and DC busbar traces without the introduction of DC bias wiring: (a) Highband at 1.70 GHz, (b) Midband at 1.49 GHz, (c) Lowband at 1.35 GHz. (a) (b) (c) 20 14 5.9 2.5 1.4 0.6 0.3 0.2 V/cm

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electric field magnitude conforms in general to what is expected. The maximum intensity of the electric field occurs mos tly along the edges of the last rows of pixels intended for that reconfiguration state, and the minimum intensity occurs at the center of the pixel patch for all reconfiguration states.

Fig. 2.28 shows the electric field magnitude distributions on the pixelated patch at the same frequencies as above after the inclusion of the DC bias wires to the simulation model.

Once again, the negative effect of the DC bias wiring is immediately apparent. The Highband E-Field is deformed, but still retains some semblance to the original E-field magnitude distribution. Unfortunately, the Midband and Lowband are both completely deformed and no longer recognizable as an electric field typical of a patch antenna.

Figure 2.28 Simulated electric field distributions for the pixel patch after the introduction of DC bias wiring: (a) Highband at 1.7 GHz, (b) Midband at 1.49 GHz, (c) Lowband at 1.35 GHz. (a) (b) (c) 20 14 5.9 2.5 1.4 0.6 0.3 0.2 V/cm

52 C. Other Hypotheses and Outcomes

Our initial hypothesis was that the length of the DC bias wires in the presence of the length of the DC bias traces resulted in a combined length which was proving resonant within our frequency range of interest. If the length of the DC bias system was resonant within the 1−2GHz frequency range, it is implied that increasing or decreasing the length of the DC bias wires would move the resonance outside our desired scope of frequencies. We tested this theory by developing a simulation model and varying the length s of the DC bias wires in the presence of the DC bias traces, the results of which are found in [65]. In summary, there was no indication from the simulated results that altering the lengths of the DC bias wires in the presence of the DC busbars has any significant effect on the S11 response of the antenna. Any length of DC bias wires connected to the DC busbars results in a significant deterioration in S11 response of the antenna for all bands, with the Midband and Lowband being rendered non-functional.

An attempt to limit the surface current present on the DC bias wires was made experimentally with 40kΩ lumped element resistors located between the DC bias wire and the DC busbars to act as a RF choke. This effort also proved unsuccessful. While there di d appear to be a small improvement after the implementation of the 40kΩ choke resistors, it was clearly an insuff icient solution to the problem.

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