Axi_po manual

The Axi_po program is a special tool for an axi-symmetrical single feed dual shaped reflector antenna design. A common requirement for an antenna is the ability to operate at different frequencies. If the frequency span is very large, the antenna system could use the common main reflector and separate feed/sub-reflector assemblies to reduce the manufacturing cost. The Axi_po program performs multi-assembly multi-frequency optimisation to find the optimum common main reflector surface and the optimum sub-reflector surfaces of the antenna system.

The optimisation is performed is a few steps. The starting point is conic axi-symmetrical antenna surfaces (Cassegrian or Gregorian) or shaped surfaces generated by MakeRF program (Cassegrian, Gregorian or Double Gregorian). Geometrical Optics method is used first, then two stage Physical Optics optimisation is applied (coefficents optimisation and analytical gradient optimisation of the whole surface). The multi-stage approach ensures fast convergence and fine control over the optimisation process. For Double Gregorian case use Physical Optics without Geometrical Optics stage.

The program layout is very similar to MakeRF program, so please read MakeRF manual first.

Installation

The program consists of an executable file Axi_po.exe for Windows 95/ 98/NT/2000/XP, a few optimisation engines (*.exe files) used internally by the program, and a few example files. The program does not require any special installation - simply copy all the files from disk1 and disk2 into an empty directory and unzip the contents of disk3 using WinZip (www.winzip.com) into the same directory. You may find it useful to create a shortcut to the program and place it on the Desktop. For SWE calculation you need SPHREX.exe (from disk3) program which has to be in the same directory as Axi_po.exe. If you have a dual processor Windows PC, you can use 2 CPU version of the optimisation engines located in 2cpu.zip. Simply unzip all files from 2cpu.zip, overwriting existing 1 CPU version. 2 CPU version of PO optimisation engine will use both processors reducing to the run time almost by half. If you have a single CPU, leave 2cpu.zip alone.

Quick start

The program has 4 pages: "Antenna assembly", "Target", "Feed" and "Optimise". Having started from the first page simply follow this sequence to generate an antenna set. To view this manual you can click the question mark button in the bottom left corner and select "User manual".

As an example let us generate an axisymmetrical Cassegrian antenna. Run the program (file Axi_po.exe) and select "Antenna assembly" page. To reset all fields to a default state, press "New" button on the left vertical panel, when "Save file" dialog box appears, save file as " example1.apo ". The program stores all data in a single file with default extension ".apo".

The most efficient way is to start optimisation from MakeRF generated surfaces. Click "Load MakeRF file" button marked in the figure 1 below with a red dot. Load "From_MakeRF.rfl" file supplied with the program (the reflector was generated for 3 GHz frequency to produce aperture distribution "From_MakeRF.amp", these files can be viewed in MakeRF). Next click "Add" button marked with green dot in the fig. 1, this adds a copy of the current assembly to the assembly list (marked with blue dot), now we have 2 assemblies.

Lets rename the first assembly to "band A". Double click on the assembly name in the assembly list box (marked with blue dot). The edit box will appear, type "band A" and press Enter to load name. Then double click on the other assembly and rename it to "band B" as shown in fig 1.

To view assembly parameters simply click on the assembly name. Our two assemblies are the same, so nothing changes when we select "band A" assembly or "band B" assembly. In general you can have completely different parameters for each assembly, except three parameters that define the main reflector (common for all assemblies): "Vertical aperture size", "F/D ratio" and "Main reflector" surface, these values are underlined. If you change these fields, these parameters will be updated in all assemblies, the other parameters will be unique for each assembly. When you load data from MakeRF file, the current (the one you view) assembly data will be loaded from file, the other assemblies will update the main reflector parameters only. For the definition of the antenna parameters please refer to the MakeRF manual.

Let us proceed to the next page, "Target" (Fig.2). For each assembly it is possible to define an unlimited number of targets. For each target you need to define:

Select (click with mouse) first target in the target list box (marked with a green dot in the Fig.2 below). Let us set the sidelobe envelope for the first target, the easiest way is to load it from an .env file generated by MakeRF. Click small "Load" marked with a red dot in the figure below, load "27log25.env" file supplied. The sidelobe envelope records appears in the envelope list box and you see the envelope graph in the bottom of the page. For the information how to edit the sidelobe envelope records please refer to the MakeRF manual.

Click "Add set" button marked with blue dot 3 times, this will add 3 more copies of the current target set to the targets list box. You can edit parameters of the selected target using combo and edit boxes under the targets list. Assign the first two target to "band A" assembly, the other 2 - to "band B" assembly. Set the frequency values as shown in the figure below. We have the same sidelobe envelopes for all targets, but you can edit/load/save the sidelobe envelope parameters for each target separately if required.

Now let us set the feed parameters for each target. Select the "Feed" page. The target can be selected from the combo box marked with a blue dot in the figure 3 below or you can use buttons with red arrows to move from the current target to the next or previous one. The feed parameters for each target are completely independent from each other. The feed data can be loaded from MakeRF file, this will load the feed parameters only, the reflector data will not be loaded. Let us set 10dB cosn taper at 20 degrees for the first target (3GHz), 11 dB for 3.5 GHz target, 12 dB for 5 GHz and 13 dB for 5.5 GHz.

Let us proceed to the final "Optimise" page. This page displays the summary of all target parameters. There are additional parameters you can change in each target:

In this example let us increase weight of the first target to 1000. Set "Optimisation stage" to "Stage 1: GO", "Decrease automatically" to "SL envelope" to move automatically the sidelobe envelope down, "Min aperture efficiency" to 0.7, "Max centre field" to 0.0. The last parameter is used to control the field value at the aperture centre. To force the program use "hole in the middle" aperture taper set this parameter to -10...-20 dB.

Make the sidelobe envelope adjustments: "Move envelope by " - 0 dB, "Envelope starts at" 5.3 deg, "Envelope stops at " 30 deg, click "The same for all targets" to set these parameters for all targets. Press "Run" button, this will launch GO engine. The engine output are captured by the Axi_po and displayed every few seconds. You can see the penalty function value for each target, total penalty function value (bottom of the page), iteration number, antenna efficiency and max gain. Also you can choose different plots: far-field, aperture (amplitude and phase), sub-reflector distortion and main reflector distortion.

The final result is shown in fig.4, the envelope has been moved by 4 dB down for all targets. When the program finishes, you could try to press "Run" button again to check that the optimisation cannot proceed any further. The optimisation results were saved as "example.apo" and supplied with the program.

To compete the description of the "Optimise" page: The number of coefficients which control distortion of the reflector surfaces can be set using combo boxes, you can choose from 5 to 10 coefficients, the number of coefficients is common for all targets (but the sub-reflector surfaces are different!). To reset coefficients (if you wish to start optimisation again) click the small button next to the coefficients combo box. The main reflector surface is common, so one click resets main surface for ALL targets. The sub-reflector coefficients are common for each assembly, so one click resets sub-reflector surface for the current assembly/target only.

The results (separately for each target) can be saved to a file in GRASP or AutoCAD .dxf format: use the scroll bar to scroll down the parameters box, select the file format and click "Save target results to file" button.

Antenna assembly page, user-defined reflector surfaces

The initial antenna surfaces (before the optimisation) can be specified as "conic surfaces" or "distorted" (user-defined) surfaces. When you load MakeRF-generated surfaces from MakeRF file, the surface type is automatically set to "distorted". To set the surface manually, choose "Distorted parabola" from "Main reflector" combo box and click the small button next to it. A window as in fig .5 will appear. You can type the numbers in the memo box, paste data from clipboard or load from a file. Do not forget to set the distance between the main and sub-reflectors (as in GRASP format) which controls the absolute position of the main reflector. Click "Update" button. If no mistakes are found, click "OK". If you do not want the data to be loaded into the program, click "Cancel".

Follow the same procedure to load the user-defined sub-reflector surface, set "Sub-reflector" to "Distorted conic surface" and click small button next to it. You might find useful the radio-box which allows the data to be loaded as (x, z) or (x, -z). The latter can be used to load data from GRASP sub-reflector files, when the sub-reflector coordinate system is rotated by 180 degrees, so z-axis become -z.

In this program we use a single coordinate system with origin in the feed, z is antenna boresight and x is the vertical axis.

Feed page, user defined feed pattern and SWE calculation

The cos power n feed pattern can be specified using two parameters: feed taper and the feed semi-vertex angle. Let us study user-defined patterns which allow accurate representation of the feed radiation parameters.

The user feed pattern can be loaded in different formats which are converted to a common form: a set of azimuth (phi) cuts; for each phi-cut data is displayed as Theta, deg/Co-polar value, dB/Cross-polar value, dB/ Phase of co-polar, dB / Phase of cross-polar, dB. The user feed data can be used directly by the program if you select feed type as "User feed far-field pattern in vertical and horizontal plane". The vertical and horizontal feed pattern are used directly to compute the field on the sub-reflector using Ludwig's 3rd definition [2]. A far more accurate approach is to use the feed radiation pattern to compute SWE coefficients [2] using SPHREX program [3]. Once computed, the SWE coefficients can be used to compute field on the sub-reflector surface with high accuracy.

Let us start with description of the user feed pattern input window (fig.6). As an example load "example_SWE.apo" file supplied. The graph on the "Feed" page displays the user feed pattern which has 4 pattern cuts: 0, 90, 180 and 270 deg. The pattern phase is constant (click "Plot: phase" button) and the cross-polar pattern is set to -200 dB (click "amplitude" and "cross-polar").To display the window click "Set user feed taper" button (the button is visible when the feed type is not cos power n).

You can type the data in the memo box, paste from clipboard or load from a file. Select the file format from a combo-box and click "Update one cut" button. Update value of the phi angle for the cut.

To add another cut click "Add" button to make a copy of the current cut, update value of the phi angle for the new cut, then load new data to the memo box and click "Update one cut" button.

Use buttons with arrows to scroll between cuts and "Delete" button to remove cuts. The cuts are always sorted in the ascending order of the phi angle.

Often the measurements are performed at equal angle intervals, so the angle information is not included in the file (i.e instead of 5 columns the file has 4 columns: Co, Cx, Phase co, Phase cx). In this case select "no angle" check box. An edit-box become visible, where you can set the angle increment between points. It is assumed that the first point has zero theta angle.

Another option is to load multiple cuts at once. Before each cut you must supply a line which contains "phi" keyword anywhere in the line and the phi cut value, for example (no angle option must be checked), then click "Update multiple cuts" button.

For SHREX program you need to set the radius of the measurement sphere (for far-field measurements set to -1).

To close the window, click "OK" button. If you do not wish to load the data to the program, click "Cancel".

Let us discuss parameters for SPHREX run. We recommend to read SPHREX manual to become familiar with the SPHREX program. As you close the user pattern window, the program tries to suggest values for the SPHREX parameters, you can change these parameters before the SPHREX run.

Click small "Run" button in the "SPHREX execution control" window. The SPHREX output is captured and displayed in the memo box at the bottom of the window. As SPHREX exits, the SWE coefficients are loaded automatically into Axi_po.

To view the run information window (fig. 8) click "Run info" button. The window displays all SPHREX input/output files and captures SPHREX output to plot input and SWE fitted radiation pattern as shown in fig.8. Windows 95/98 cannot display large files in the memo box, so a warning message will be displayed suggesting to use another editor to view output files (Word for example).

Graphs: copy, save data, customize

Click right mouse button on a graph to display a menu. Select "Copy to clipboard" to copy graph as a Windows metafile, "Run designer" to customize graph parameters and "Save to file " to save graph data into a text file. Some options are disabled on selected graphs when considered impractical.

Keyboard shortcuts

Ctrl-O	Load (open) file
Ctrl-S	Save file
Ctrl-R	Run or Stop the program
Ctrl-1	Select Antenna assembly page
Ctrl-2	Select Target page
Ctrl-3	Select Feed page
Ctrl-4	Select Optimise page
PgDn	Next page
PgUp	Prev page
F1	Help

Note that you can use standard Windows keys (as Ctrl-C or Ctrl-Insert) to copy, cut and paste text.

References

1. Westcott B.S. and Zaporozhets A.A. "Dual-reflector synthesis based on analytical gradient-iteration procedures", IEE Proc., Part H, Vol. 142, No 2 April 1995, pp 129-135.

2. Spherical near-field antenna measurements. Edited by J.E Hansen. IEE electormagnetic wave series, 26. Peter Peregrinius Ltd, 1988.

User manual

Axi_po

PO optimisation of an axi-symmetrical system

Version 1.1

Contents