**GH Driver Array - Modeling Experiment - Part 2**
Hi Folks,

Experiment #1

Purpose of Experiment:

To observe what happens to the Total Horizontal Forward Gain vs. Channel Number curves as the feedpoint gap distance is changed from 0.5 inches to 3.0 inches in steps of 0.5 inches for several families of the Gray Hoverman Driver Array Assembly.

Figure 1: The Gray Hoverman Driver Array Assembly

In Figure 1, the length of wire number 17 corresponds to the feedpoint gap distance; wire numbers 2 through 7 and 10 through 15 represent the zigzag elements; and wire numbers 1, 8, 9 and 16 make up the stub elements.

Methods and Materials:

At

http://www.digitalhome.ca/forum/showthread.php?t=81982&page=19, Post 284, I found a drawing by Elvis Gump of the First Generation Gray Hoverman Antenna developed by Autofils and others. This seemed like a great place to start since it is a real working and well optimized antenna. The feedpoint gap is 1.77 inches. The length of each zigzag element is 7.07 inches. The length of each stub is 5.59 inches.

In this experiment, I planned to let the gap distances vary from 0.5 inches to 3.0 inches in steps of 0.5 inches and the stub lengths vary from 2.0 inches to 6.0 inches in steps of 1.0 inch. I decided to fix the zigzag element lengths at 7.07 inches in order to keep the total number of antenna modeling files down to a modest count. The next job was to construct all of these models and to save the resulting NEC input files.

The 4nec2 software provides access to four different NEC input file editors. Each has its uses. The most primitive is “Notepad Edit” (Ctrl+F1). Using Notepad to prepare an NEC input file is equivalent to sitting down in front of an old IBM Hollerith Card Keypunch Machine with a deck of blank cards.

The original Numerical Electromagnetics Code (NEC) program was developed in the 1970s for the US Navy at the Lawrence Livermore Laboratory. The software evolved over the next few years. In January 1981, a three part manual was published detailing the NEC2 program description and theory, program listing, and user’s guide. The187 page User’s Guide speaks mostly of preparing an NEC input card deck. Each card type and its associated fields are thoroughly described.

The second editor, called “NEC Editor” (Ctrl+F2), is a much more user friendly interface than Notepad and the 187 page User’s Guide. It knows about each card type and it fields. The user can insert a card into the deck by selecting an appropriate card type and filling in the well labeled fields.

The third editor is called “Geometry edit” (Ctrl+F3). An in depth knowledge of card types and their fields is not required to model an antenna. Instead, one can focus on laying out an antenna’s elements in a three dimensional coordinate system, XYZ space. The geometric editor has buttons to switch from 3D space to the two dimensional XZ, YZ and XY planes. For flat, two dimensional, designs like in this experiment, one can work exclusively in the YZ plane. The geometric editor features a “drag and drop” drawing style for creating straight wire elements. A graph paper like grid is provided in the two dimensional planes for drawing individual straight wire elements. In the “Select object mode”, the properties of any wire can be displayed and/or edited by left clicking on the object of interest. The ends of wires can be easily connected by taking advantage of the “Snap to grid” and “Snap to wire” functions. Beware of “Keep connected” function. When adjusting a wire, there are times to turn it on and times to turn it off. The “Edit/Undo Move” function found on the standard menubar is critical, especially after accidentally making a mess of things.

“NEC editor (new)” (Ctrl+4) is the forth editor. It is a logical extension of the second “NEC editor”. The user interface has subject tabs for “Symbols”, “Geometry”, “Source/Load”, “Frequency/Ground”, “Other” and “Comment”. Most of the tabs provide a spreadsheet like grid for data entry and editing. In some future experiment, I hope to become familiar with using the elegant “Symbols” method for modeling an antenna”. But for now, I will move ahead using the “Brute Force” technique.

I used 4nec2’s geometric editor to build the NEC model files. The stubs were very easy to draw. I was a little worried about the 7.07 inch zigzag elements. The snap to grid and the snap to wire functions made drawing the zigzag elements much easier than I first thought. The strange 7.07 inch length turns out to be the hypotenuse of a right triangle with two 5.0 inch sides on either side of the 90 degree angle. The two 5.0 inch sides neatly correspond to the graph paper like horizontal and vertical grid lines. In the add wire mode, it became a matter of left clicking the mouse to anchor one end of a new wire then dragging 5.0 inches horizontally and 5.0 inches vertically (or the equivalent diagonally in just one step) and releasing the mouse button.

Adjusting the feed point gaps was difficult in the beginning. Then, I read the help file and discovered a drag and drop technique for moving and rotating groups of wires. It goes something like this. While in the geometric editor’s 2D select object mode, left click a wire that will shortly become the “dragging handle”. Move the cursor to an empty spot on the grid. Next, left click and immediately drag and drop diagonally over the group of wires to become a temporary unit. A rectangle will appear showing you what you are about to include. When you let up on the mouse button all of the wires with both ends inside the rectangle will be selected and will turn red. One of the wires will appear thicker than the others. This is the "dragging handle". You can left click and then drag and drop the unit to its new location. It takes a little time to get used to it, but it is very effective. Try it, you will like it.

Altogether, thirty NEC files were created and separated into the following five family groupings:

H01 - 7.07in ZigZags - 2in Stubs - 0.5in Gap.nec

H02 - 7.07in ZigZags - 2in Stubs - 1.0in Gap.nec

H03 - 7.07in ZigZags - 2in Stubs - 1.5in Gap.nec

H04 - 7.07in ZigZags - 2in Stubs - 2.0in Gap.nec

H05 - 7.07in ZigZags - 2in Stubs - 2.5in Gap.nec

H06 - 7.07in ZigZags - 2in Stubs - 3.0in Gap.nec

H07 - 7.07in ZigZags - 3in Stubs - 0.5in Gap.nec

H08 - 7.07in ZigZags - 3in Stubs - 1.0in Gap.nec

H09 - 7.07in ZigZags - 3in Stubs - 1.5in Gap.nec

H10 - 7.07in ZigZags - 3in Stubs - 2.0in Gap.nec

H11 - 7.07in ZigZags - 3in Stubs - 2.5in Gap.nec

H12 - 7.07in ZigZags - 3in Stubs - 3.0in Gap.nec

H13 - 7.07in ZigZags - 4in Stubs - 0.5in Gap.nec

H14 - 7.07in ZigZags - 4in Stubs - 1.0in Gap.nec

H15 - 7.07in ZigZags - 4in Stubs - 1.5in Gap.nec

H16 - 7.07in ZigZags - 4in Stubs - 2.0in Gap.nec

H17 - 7.07in ZigZags - 4in Stubs - 2.5in Gap.nec

H18 - 7.07in ZigZags - 4in Stubs - 3.0in Gap.nec

H19 - 7.07in ZigZags - 5in Stubs - 0.5in Gap.nec

H20 - 7.07in ZigZags - 5in Stubs - 1.0in Gap.nec

H21 - 7.07in ZigZags - 5in Stubs - 1.5in Gap.nec

H22 - 7.07in ZigZags - 5in Stubs - 2.0in Gap.nec

H23 - 7.07in ZigZags - 5in Stubs - 2.5in Gap.nec

H24 - 7.07in ZigZags - 5in Stubs - 3.0in Gap.nec

H25 - 7.07in ZigZags - 6in Stubs - 0.5in Gap.nec

H26 - 7.07in ZigZags - 6in Stubs - 1.0in Gap.nec

H27 - 7.07in ZigZags - 6in Stubs - 1.5in Gap.nec

H28 - 7.07in ZigZags - 6in Stubs - 2.0in Gap.nec

H29 - 7.07in ZigZags - 6in Stubs - 2.5in Gap.nec

H30 - 7.07in ZigZags - 6in Stubs - 3.0in Gap.nec

These Gray Hoverman Driver Array files can be found in the Yahoo Group named AntennaModelingExperiments under Hobbies and Crafts.

A horizontal frequency sweep was produced for each of the above NEC files using 4nec2’s Generate (F7) function. The following parameters were used: Resolution: 5 degrees; Frequency Start: 455 MHz; Frequency Stop: 715 MHz; Frequency Step: 6 MHz; Forward Theta: 90 degrees; Forward Phi: 0 degrees; Forward delta-Phi: 5 degrees; Backward Theta: 90 degrees; Backward Phi: 180 degrees; and Backward delta-Phi: 5 degrees. A 455 to 715 MHz range with steps of 6 MHz was selected to produce points which fell in the center frequencies of each UHF channel from 14 through 51 with three extra points on either side of this channel range.

As the Total Horizontal Forward Gain data were generated for each NEC file, it was immediately transferred to an Excel spreadsheet for later plotting. See

http://www.digitalhome.ca/forum/showthread.php?t=85987&page=14, Post 208 for 300ohm’s method of transferring output data from 4nec2 to a spreadsheet.

Results and Conclusions:

The Total Horizontal Forward Gain vs. Channel plots are shown below. The first family of gain curves shows the Gray Hoverman Driver Array which is most similar to Elvis Gump’s drawing without a reflector.

For this family, 7.07 inch zigzag elements and 6.0 inch stub elements, there appears to be very little practical difference between the Total Horizontal Forward Gain curves with feedpoint gaps distances ranging from 0.5 inches to 3.0 inches.

As the stub element lengths become shorter and shorter, the effect of the feedpoint gap distance slowly begins to appear.

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Experiment #1 Continued in GH Driver Array - Modeling Experiment - Part 3

Due to an Image Limit of 4 per Post

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