CPA v4 Tuning.

Date: September 2, 2011
Time: 1300-1700 PDT
Lab: Portland State University Electromagnetics Lab. 1900 Sw 4th Avenue, Portland Oregon.
Investigators: Glenn LeBrasseur, K Wilson, Nathan Bergey

Abstract:

The GPS and ATV Circular Patch Antennas were physically modified in an effort to tune them towards the desired design frequencies of 1.575 GHz and 2.39 GHz respectively. The original frequencies and the effects of incremental and final modifications were measured on an Agilent 5071C ENA Series Network Analyzer.

As a result of these modifications the GPS antenna frequency was changed from 1.521708 GHz to 1.5707520 GHz by changing the vertical dimension of the radiator by (-2.2 mm). The ATV antenna frequency was changed from 2.35 GHz to 2.39 GHz by changing the vertical dimension of the radiator by (-0.8 mm).

Introduction:

After testing at NW EMC (see 2011-08-12) and some experimental work at the PSU Electromagnetics Lab (see 2011-08-19) it was decided to tune the GPS and ATV antennas by modifying the geometry of the antenna radiator sections. No modifications were performed on the feed network.

This report includes the techniques used to modify the CPAs, the measurements obtained at each step of the modifications and the theory used to generate expected results and decide on the magnitude of each incremental modification.

Experiment

The change in antenna frequency is proportional to the change in the vertical dimension (X) of the antenna radiator.

CPA Tuning Diagram

By measuring the current dimension of the radiator, and the current frequency of the antenna an estimate of the necessary adjustment can be made.

Xf = (fc/fg) * Xc

Xd = Xf - X

fg = Xc/Xf * fc

Where:

- X  = original vertical dimension of radiator
- Xc = current value of X (mm)
- Xf = final vertical dimension of radiator
- Xd = delta X, the amount to change X (mm)
- fc = current frequency (Hz)
- fg = goal frequency (Hz)

Dimensional measurement was performed with digital calipers. N.B. Digitial calipers must be calibrated ('zeroed') prior to each measurement.

Each incremental modification to Xd was on the order of (-1 mm). A 'snap knife' or thin sharp blade was used against a metal guide held in place around the cyclinder of the CPA. The metal band and guide are removed during testing of the antenna. The following photo shows the CPA antennas with the metal band. In the background is the Network Analyzer with the calibration setup. From top to bottom the antennas are GPS, ATV and Wifi.

CPA With Metal Guide Band

This photo shows a closeup of the metal band in preparation for a cut.

Early cuts were made with a heavy utility knife and blade. It was found that using a sharp 'snap knife' device, it was easier to control the alignment against the metal guide.

Never Use This.

Cut control was best achieved by alternating cutting and turning the CPA array, while observing the blade from a position that minimized parallax. At least two full rotations were made during cutting. The first orbit cut through the F4 PCB top layer and the second orbit through the thin copper layer. The two cuts will have a different 'feel' to the operator. The F4 layer is fibrous and slower to cut while the copper is smooth and cuts more quickly.

Making A Cut.

Another Cut

Removing Material.

Skive.

Another Skive.

GPS CPA Tuning Results

Initial GPS NA Measurements

img1 img2

GPS Results Table

Xc	Xd	fc	fg	Expected Freq		Measured Freq
62mm	-1mm	1.521708GHz	1.575420GHz	1.54665GHz		1.54422GHz
61mm	-1mm	1.54422GHz	1.575420GHz	1.56954Ghz		1.57052GHz

First Cut NA Measurements

img3 img4

Second Cut NA Measurements

img5 img6

The final frequency achieved was 99.7% of the goal frequency.

It was decided to cease making any adjustments to the GPS antenna at this point.

ATV CPA Tuning Results

Calculating the Xd for this antenna resulted in a requirement for Xd =~ (-0.5mm). This is expected to be the minimum manageable amount of material to skive off the antenna. The actual measured amount removed was closer to Xd =~ (-0.8mm). This is also close to the minimum distance measurement capability of the digital calipers. The actual amount removed is between 0.5mm and 0.8mm. Both values have been entered into the table.

ATV Results Table

Xc	Xd	fc	fg	Expected Freq		Measured Freq
39.6mm	-0.5mm	2.35GHz	2.39GHz	2.380519GHz		2.39000GHz
39.6mm	-0.8mm	2.35GHz	2.39GHz	2.399215GHz		2.39000GHz

Initial ATV NA Measurements

(Not available? See elsewhere?)

Post Cut ATV NA Measurements

img7 img8

Discussion of -5db performance change?