Antenna Pattern Plot

Antenna Pattern Plotting Concepts

The Data Graph portion of the main window displays S Meter data as a function of time. The Antenna Pattern Plot dialog box displays the same data, but wrapped around a 360 degree azimuth response plot. The radial axis displays the relative S Meter value.  In order for this part of S Meter Lite to be of any practical value, the test antenna must be mounted on a rotator that can turn the antenna at a constant rate. The software assumes a constant rotation rate. In practice, this means that you can only obtain the antenna pattern plot for directional antennas such as the Yagi and Quad, connected to a rotator, and usually mounted on a tower.

The basic set of steps that lead to antenna pattern plot generation are:

  1. You must provide a stable test signal on a quiet band, from an antenna located a reasonable distance from the test antenna, at a meaningful elevation.
  2. The test antenna rotator is positioned so that it can make at least a full 360 degree sweep.
  3. Make sure that S Meter Lite is running, then start the test antenna turning to move in a complete circle.
  4. Allow the antenna to move for at least a few seconds so that it reaches a constant velocity.
  5. Note a reference point on the rotator compass. Press the Mark button to indicate the start of the data sweep. This is called the Mark 1 point.
  6. When the antenna turns all the way around, and reaches the same reference point, press the Mark button again to indicate the end of the pattern sweep. This is called the Mark 2 point.
  7. Click the Antenna Plot button to display the Antenna Pattern Plot Dialog Box.
  8. Adjust the plot using the controls available on the dialog box.
  9. If desired, save the results, either as the plot itself (copied to the system Clipboard), or, as raw data in a comma separated value (.csv) file.

Please note that there is also a 180 degree data capture mode which can be used in certain circumstances.

The Antenna Pattern Plot dialog box can be resized. Simply grab a corner of the window and drag it to a new size. This allows you to control the size of the plot. You can also change the plot text font.

What are we really measuring?

Before getting into the details of the software, it's probably useful to step back and consider what is really being measured.

The most important observation is that we are measuring the response of a real antenna under some set of circumstances and environment, not making absolute measurements about an antenna design. With the advances in antenna modeling software, access to detailed antenna pattern analysis has become commonplace. Yet, these calculated results represent the performance of perfect models, not real antennas. In most cases, the performance of the model exceeds the performance of the real antenna. When evaluating the antenna pattern plot produced by S Meter Lite, please keep the following factors in mind:

  1. Real antennas are surrounded by objects that can greatly influence the response pattern. This could be another antenna stacked on the same tower. Perhaps it's the influence of a wire antenna some distance away.  Maybe it's power wiring or a metal fence. The influence may be a function of the antenna direction. This could lead to various distortions in the expected pattern.
  2. Real antennas are made of real materials. Elements are subject to the effects of corrosion and imperfections such as variation in length, spacing, and orientation.
  3. Unless you set up a quality antenna test range, which is probably only possible at VHF and UHF frequencies, it will be difficult to know, with certainly, the primary arrival/take-off angle of a signal. The elevation angle of the main lobe of an antenna is an important part of its performance. Ideally, the elevation angle of the antenna (in its environment), matches the angle desired for a given mode of operation. Typical elevation angles for DX communication tend to range from nearly 0 degrees on 28 MHz, to perhaps 20 degrees on 7 MHz. If you wish to evaluate an antenna for DX communication, you should make sure that your signal source is arriving from the desired angle. Transmitting a signal from a mobile station located underneath the test antenna may produce an interesting plot, but it is probably unrelated to the performance of the antenna in a DX situation.
  4. QSB (signal variations) will upset the quality of the result, since the signal levels are changing for the wrong reason. Try to capture data when the particular band is quiet, perhaps even dead. Making measurements in windy conditions, or during rain, may produce different results than under calm and dry conditions. Most rotators will take approximately 30 to 50 seconds to rotate in a complete circle. Signals and conditions must be stable over that period so that you are measuring the antenna pattern, not some strange mix of the pattern and the atmosphere.
  5. You must make sure that the distance between the transmitting and receiving antennas is large enough that the antennas are in each others far field, not near field. The definition and discussion of the near and far field is beyond the scope of this documentation, please consult appropriate books and articles for more information. If the antennas are too close together, however, the measured response will not be indicative of the response at much greater distances. Since we are usually interested in communicating over hundreds or  thousands of miles, the antenna response at 100 feet is not really that important, especially if the response at 100 feet is not the same as it is at 100 miles. The separation between transmitting and receiving antennas should be at least several (5 to 10) wavelengths.
  6. If you want to obtain the highest degree of accuracy from the antenna pattern plot, you should be sure that you have calibrated the dB scale of S Meter Lite. Errors on the order of 1 or 2 dB may be significant, depending upon your goals. Calibration requires a signal generator and step attenuator. The process is described on another page. The antenna pattern plot computations are based upon the dB scale.
  7. Don't forget to factor in the height above ground for both the transmitting and receiving antennas. Height above ground will greatly influence the response of most all antennas. Ground reflection can alter the incoming and outgoing (primary) wave angle of an antenna.
  8. It is usually the case that the most sensitive attribute of an antenna is its front to back (F/B) and front to side (F/S) response ratios. Even if you have a real antenna that has deep nulls, it will be the case that the nulls will only exist over a small frequency range. Unlike gain, which tends to be relatively stable across an amateur band, the nulls which define the antenna response are very frequency sensitive. It may be useful to determine the antenna pattern at a number of points within a given amateur band.
  9. If you wish to consider the signal levels between different plots (gain differences), make sure that your transmitter is putting out a constant power level. In practice, this is very difficult, since, at the least, a quality watt meter is needed, and any variation in the transmitter antenna gain will influence the receiver antenna gain.

One conclusion that we can draw is that we are not so much measuring the response of an ideal antenna in isolation, but instead the overall response of a real antenna in a real environment. In most cases, the environment of an antenna will exert a strong influence over the response of the antenna. Probably the only way to reduce the impact of the environment is to get the antenna relatively high in the air (several wavelengths), and away from other metal that may distort the pattern. In practice, this is a luxury that few amateurs have.

Capturing data for an Antenna Pattern Plot

The first step in generating an antenna pattern plot is capturing data using the Data Graph portion of the main window. This is a busy procedure. You have to start a transmitter, rotate an antenna at least 360 degrees, and operate the S Meter Lite program.  In my case, the procedure is made much easier because I have a Yaesu G-800 rotator that has a preset direction, preset button, and can rotate a total of 450 degrees. I can set the rotator direction for over 360 degrees from its current position, and by pressing the button, it will turn until it reaches that end point. There is no need to continuously hold down a button. My signal source has often been the ICOM 706MKIIG radio, set into low power AM mode. I  press the push to talk microphone button to generate a signal. I place the microphone right next to my computer mouse, and between the microphone in my right hand, and my left hand on the keyboard, I can capture all the data I need. I'm fortunate enough to have several different antennas and radios to choose from, in some cases over 500 feet apart. In most situations, you may have to enlist the help of another amateur who lives close to your location.

The dB scale spans 30 dB. You should adjust your capture environment so that the maximum and minimum signal strength values from the test antenna fall within the range of the dB scale, and are not clipped at either the top or bottom. Adjustments can include the signal generator power, the receiver preamp setting, and the receiver attenuator setting.

A pair of Marks are used to define the set of samples which form a complete antenna pattern. By definition, these two Marks will be the first two marks on the current Data Graph. It is typical, therefore, to begin a data capture with an Erase, which will clear the Data Graph area. This also clears all Marks.

Data Graph data capture
Data Graph Data Capture

This screen capture shows the Data Graph which captured the 360 degree response of my 6 meter Yagi antenna. I pointed my antenna in a direction which was in the overlap zone of my rotator. I erased the Data Graph, then started S Meter Lite running. I then pressed the rotator preset button, which started the antenna turning in a complete circle. In fact, it was more than 360 degrees. At the same time, I keyed my transmitter. When the rotator direction indicator swept over 210 degrees, I pressed the Mark button. This created the first red line shown on the graph (the Mark 1 point). The rotator continued to turn, and I kept the transmitter on. It is possible to get a sense of the pattern of the antenna by looking at the Data Graph. The front lobe should have the maximum gain (S Meter reading), and there may be one or more side or rear nulls. These features can be seen in the previous picture.

After approximately 39 seconds, my rotator made a complete circle. As it crossed over the 210 degree mark, I pressed the Mark button a second time. This created the second red line on the Data Graph (the Mark 2 point). At this point I have captured the S Meter data for an entire 360 degrees of test antenna rotation.

A first-order consistency check is that the S Meter value at both red lines is approximately the same. This should be the case, since it is the same antenna direction. In this example both the Mark 1 point and the Mark 2 point are at -5 dB. So far, so good.

It is necessary to stop the capture of data before the data reaches the right side of the graph, and the graph is automatically erased. Any time the graph is erased, whether automatically or manually, all accumulated data is discarded.

Please note that all of the data on the Data Graph is made available to the antenna pattern plotting dialog. This includes data that is not between the red lines. In practice, it is usually impossible to capture exactly 360 degrees of data. You will press that Mark button either a little too early, or a little too late. The purpose of the Mark 1 Adj. and Mark 2 Adj. controls on the Antenna Pattern Plot dialog is to provide the ability to fine tune the data so that the plotted data represents 360 degrees of rotation.

After the data is captured, click the Antenna Plot to bring up the dialog.

wwv50app.gif (20063 bytes)
Antenna Pattern Plot based upon previous capture

This Antenna Pattern Plot dialog was generated from the previous example Data Graph. The controls on the dialog, as described below, are used to adjust and customize the plot.

It appears as if this antenna has some sort of problem, which is indicated by the hump in the pattern at 45 degrees. The left side of the antenna has a better response. I suspect that the problem is caused by some mechanical asymmetry in the antenna. Now that I can see this problem, I will try to investigate it and see if something can be done to make the pattern more symmetric.

When S Meter Lite analyzes the data between the mark points, it will determine the point with the strongest signal. That point, or the center of the region at that constant signal value, will automatically be placed at 0 degrees azimuth, which is pointing straight up on the screen. That point is assigned the strength value of 0 dB, which is at the other ring of the plot. Since it is the maximum signal level point, all other points will be at negative dB values, relative to it. Each data point is considered in tern, moving from the strongest point to the Mark 2 point, then back to the Mark 1 point, then forward to the starting point. For each of those points, the relative dB difference will be computed. This will be mapped into a radial axis displacement, based upon the coordinate system of the plot. That set of points will be connected together with straight lines. and that set of lines will be drawn.  That is the antenna response.

Anatomy of the Plot

Here is an example pattern plot:

W8WWV @ 28.900 MHz
Example Plot

This plot was captured by clicking on the Copy button which put a copy of the bitmap on the Clipboard. I then pasted the bitmap into the Windows Paint program. From that program, I saved the image as a gif file (by the way, the jitter in the signal at approximately 180 degrees is caused by QSB, which should be avoided).

The components of the plot are:

Plot Component

Description

Title The title is located to the left of the center line, at the top of the plot. You set the title. Click the Set Title button to display a dialog box that lets you set the title text.
Date The time/date that the plot was created is automatically placed on the top line, to the right of the vertical center.
Lower Left Data The text on the lower left line consists of three data items. The first is the outer ring gain. This value is relative to the 30 dB range of S Meter Lite. This is not isotropic-referenced gain, or some other common reference. The second data item is the total number of samples represented on the plot. The third data item is the number of degrees that each sample covers. It is equal to 360.0 divided by the number of samples.
Lower Right Data The text on the lower right line consists of three data items. The first is the sweep type, either a 360 or 180 degree sweep. The second data item is the number of seconds in the sweep. Since samples are taken at a rate of 20 samples per second, the total number of samples divided by 20 indicates the number of seconds in the overall sweep. If you are looking for consistency between separate measurements, you can attempt to use the same number of samples per plot, which is equivalent to the same sweep time (assuming a constant rotator velocity). The third data item is the receiving directivity factor (RDF). It is equal to the maxmimum signal strength minus the average signal strength (on the plot).
Radial Axis The radial axis is marked vertically from the outer ring towards the center. The outer ring is always considered to be at 0 dB, where this is the strongest S Meter reading in the data. All other data will be at a lower strength, and, therefore, the axis becomes more and more negative as it moves toward the center. The axis is always in units of dB.
Outer Ring Axis The outer ring axis shows the azimuth degree. It is marked starting with zero degrees, which is always at the top of the plot. It steps clockwise around the circle, marked in 30 degree increments.
Seam Dot The seam dot is a black dot located on the outer ring. In the example above, it is located at approximately 200 degrees. This marker indicates the point in the captured data where the Mark 2 point folds back on the Mark 1 point. This is the seam in the data, where a linear list is folder back on itself to create a circle of data. This implies that the seam dot marks the point where your rotator both started and stopped (capturing data). Since S Meter Lite will rotate the data to put the strongest signal value at zero degrees, the location of the seam dot can be used to relate the plot back to your actual orientation. It is also the place where discontinuities due to inaccuracy in the Mark 1 and Mark 2 points will be found. If you adjust the Mark 1 or Mark 2 points, you should be watching the part of the plot near the seam dot for a smooth transition.

In the example plot, you will notice that a small glitch or discontinuity exists on the pattern at the angular location of the seam dot, at approximately 200 degrees. This is a visual indication that an adjustment of the Mark 1 point or Mark 2 point may be desirable. The discontinuity is present because the data captured does not represent an exact 360 degree circle.

The S Meter value of a sample controls its distance from the outer ring, inward, towards the center of the plot. Four different plotting scales are supported on the radial axis. They are:

  1. Modified (24 dB) This scale has a logarithmic response. Labels are placed at 0, -3, -6, -9, -12, -18 and -24 dB. Other dB lines which are drawn, but not labeled, are -1, -2, -15, and -21 dB.
  2. dB ^ 0.8 (24 dB) This scale maps S Meter dB values using the function, dB raised to the 0.8 power. Since the exponent is less than one, the scale does become more compressed as the data value increases, but it is not as compressed as the modified scale. Labels and lines are placed at the same dB values as on the modified scale.
  3. Linear (20 dB) This scale maps S Meter dB values onto a linear scale that goes from 0 to -20 dB. Labels are drawn every 2 dB.
  4. Linear (40 dB) This scale maps S Meter dB values onto a linear scale that goes from 0 to -40 dB. Labels are drawn every 10 dB.

Manipulating an Antenna Pattern Plot

Once you have captured the data, click on the Antenna Plot button to display the Antenna Pattern Plot Dialog Box. You can then begin to use the controls on the dialog to change the plot. Each time you change a control, the plot will be updated and redrawn. If you bring up this dialog without capturing data with at least 2 Mark points, some of the controls will be disabled. Here is a list of the controls, and their function.

Coordinate System: Selects the coordinate system for the radial axis.

Sweep (360): Specifies that the captured data represents the full 360 degrees of the pattern.

Sweep (180): Specifies that the captured data represents one-half, or 180 degrees of the pattern. The program will automatically reflect the data onto the other half of the plot to create the full 360 degrees of data. This option can be used when you have a symmetric antenna, and you want to speed the process of capturing data.

Reverse Direction: Reverses the data drawing direction. Normally, the data is drawn in a clockwise direction around the circle. When the control is checked, data is drawn in a counterclockwise direction. This control is helpful when you want to be able to rotate your antenna in both directions to capture data. When you rotated the antenna in the opposite of the normal direction, reversing the direction is software will keep the plot from appearing to switch sides.

Center: This up/down control rotates the plot to the left or right. Normally, S Meter Lite computes the strongest part of the captured data, and places that at zero degrees, or towards the top of the screen. If this orientation is not desired, you can use this control to rotate the plot manually. The amount of adjustment is shown. It is in units of samples.

Mark 1 Adj.: This up/down control moves the Mark 1 point in the overall captured data. The amount of adjustment is shown. The adjustment cannot take the point before the beginning of the captured data, or past the Mark 2 point.

Mark 2 Adj.: his up/down control moves the Mark 2 point in the overall captured data. The amount of adjustment is shown. The adjustment cannot take the point beyond the end of the captured data, or before  the Mark 1 point.

Set Title: Brings up a dialog box that is used to specify the plot title.

Save: Brings up a dialog box that is used to specify the name of the file that will receive the data in the plot. This file format is explained further down on this page.

OK: Dismisses the dialog. You can bring up the dialog again, and so long as you do not erase the data from the Data Graph, you will be presented with the same data.

Copy: Places a copy of the plot on the system Clipboard. Use the Paste command in another application to retrieve the bitmap.

Help: Display the dialog-specific help information.

Changing the Plot Text Font

If you wish to change the font used to draw the text on the plot, right click your mouse while it is over the plot area. This will display a context menu that includes a Set Font command. Click on this command to display a dialog box which allows you to select a new font. The initial context menu also includes commands to display the Set Title Dialog Box, and to copy the plot to the Clipboard.

180 Degree Sweep Mode

Most antennas should have a symmetric response on the right and left sides of their front to back center line. The 180 degree sweep mode is designed to exploit this fact by allowing you to capture only one-half of the antenna pattern. S Meter Lite will automatically reflect the data into the other half of the plot. The value of this option is that a data capture can be done in one-half of the time that it would take to perform a full 360 degree sweep. Of course it is probably a good idea to begin with a 360 degree sweep to verify that the antenna is indeed symmetric.

You should be sure that the one-half of the data that is captured includes the front and back of the antenna. This is because the reflected data contains no new data, just duplicated data. Unlike the 360 degree sweep, which can be done at any arbitrary point on the plot, the 180 degree sweep should begin at either the front or back of the antenna, and continue until the opposite point, 180 degrees away. It can be useful to capture slightly more than 180 degrees of data, and use the Mark 1 Adj. and Mark 2 Adj. controls to locate the exact front and back of the data.

Saving Results: Copy and Save

There are two different ways to save the results of the antenna pattern plot computations.

First, you can copy the plot bitmap to the system Clipboard. Use the Paste command to introduce the bitmap into another application, such as a paint program. The plot can be saved to a file using the File Save capability of this second application. If you are going to use compression to reduce the file size, it is suggested that you use the gif format, as opposed to the jpg format. Click on the Copy button to place a copy of the plot on the Clipboard.

The second way to save data is to click the Save button, and then supply a file name to the presented dialog box. This file will receive that entire data of the graph in the comma separated value (.csv) format. This format can be imported by other applications such as spreadsheets. Here is an example listing of the beginning of a data file.

"50.222 MHz","Sun Nov 18 11:26:14 2001","Outer Ring = 12.91 dB, 649 Pts., 0.55/Pt.","360 Sweep (32.5 Seconds)"
"Degree","dB","maxdB-dB"
"0.00","12.91","0.00"
"0.55","12.91","0.00"
"1.11","12.91","0.00"
"1.66","12.73","-0.18"
"2.22","12.73","-0.18"
"2.77","12.73","-0.18"
"3.33","12.73","-0.18"
"3.88","12.73","-0.18"
"4.44","12.91","0.00"
"4.99","12.91","0.00"
"5.55","12.91","0.00"
"6.10","12.91","0.00"
"6.66","12.73","-0.18"
"7.21","12.73","-0.18"
"7.77","12.73","-0.18"
"8.32","12.73","-0.18"
"8.88","12.91","0.00"
"9.43","12.91","0.00"
"9.98","12.91","0.00"
"10.54","12.73","-0.18"
...

First Portion  of an Example Data File

The first line contains four values. They are the plot title, plot date, lower left data, and lower right data (taken from the plot). The second line contains the column headings for the data, which begins on the third line. The column headings are the azimuth degree of the sample, the actual sample, and the difference between the particular data item and the maximum signal strength found in the data.

Lines three and beyond are the data samples, one line per sample. In this example, the third column of the first line indicates that there will be 649 data samples. The first column is the degree of the data. The degree will always begin at 0, and continue until the last sample before 360 degrees. The angular  increment between samples will be determined by the number of samples in the overall data. For example, if there are 360 samples, then there will be one degree per sample. If there are 720 samples, there will be 0.5 degrees per sample.

The second column contains the actual S Meter reading measured at that angle. The third column contains the difference between the particular data item and the maximum S Meter reading found in the data. This value will be less than or equal to zero, in all cases. It is the value along the radial axis.+ All of the values are real numbers, that is, they can contain decimal points.

Saving Input Data

The main S Meter Lite window supports hidden save and restore commands. These commands were added to make it possible to capture data for antenna pattern plotting and analyze it at a later time. After capturing a useful pattern, and before you bring up the Antenna Pattern Plot window, use the control+s command to save the data to a file. The data saved is the contents of the Data Graph. When you are ready to analyze the data, use the control+o command to restore the data to the Data Graph. At that point, you can bring up the antenna pattern plot window and analyze the saved data.

Hints

Here are a few of the hints I have discovered in using the antenna pattern plot capability.

  1. The test antenna can be either the transmitting antenna, or the receiving antenna. It must, however, be the rotating antenna.
  2. In order to save time when make a set of plots, I rotate the antenna in both directions. I use the Reverse Direction control to reverse one of the two drawing directions so that all of my plots are drawn with the same pattern orientation. You have it correct when the seam dots all end up in the same (and correct) location.
  3. When evaluating a given antenna on an amateur band, be sure to generate plots across the entire band. This will help determine the antenna pattern as a function of frequency. In general, deep side and back nulls are the signature of maximum (relative) gain. Deep side and back nulls will also improve the signal to noise ratio since the antenna is listening to a narrower portion of the entire sky. You may find that a certain pattern is not on the frequency you wish it were. If the antenna is a monobander, there might be some chance that you could adjust it to shift the pattern to the desired frequency. For example, if I found that my 3-element 40 meter Yagi had its maximum front to back ratio at 7.3 MHz, I would probably want to shift it down by at least 150 KHz. If the antenna is a multiband antenna, adjustments tend to effect all of the bands, which creates a very complex situation.
  4. If you are testing a VHF/UHF antenna, you can probably set up your own simple test range all within a reasonable sized back yard. For HF, however, where a wavelength is much longer, it can be helpful to enlist the help of another amateur who lives within a few miles of your location. One station transmits a reference signal, and the other makes measurements from the receiver. The wave angle will be very low, but at least you won't be in the near field of the antenna. You will probably have to use a second communication channel, such as the telephone, to coordinate data capture.
  5. If you want more time to capture data on the Data Graph before it automatically erases, increase the Update Rate. A larger update rate will cause the data to accumulate more slowly on the Data Graph. Since the Antenna Pattern Plot functionality uses the underlying individual S Meter raw data samples, it's operation will not be changed.
  6. As mentioned earlier, the environment should be adjusted so that the maximum and minimum signal strengths from the test antenna fall within the dB scale, and are not clipped on either the maximum or minimum value. If the signal is too strong, you can reduce the transmitter power, or use the receiver attenuator. If the signal is too weak, you can use the receiver preamp. If the difference between maximum and minimum signal strength starts to approach the 30 dB range of the software, this adjustment may become quite sensitive. One trick that I have used to gain a fine control over the signal strength is to slightly tune the receiver off frequency. This will lower the signal strength. Please note that if your antenna actually has a difference of 30 dB (1000X) between maximum and minimum signal strength values, clipping must occur, since the total S Meter Lite dB scale range is only 30 dB.
  7. I have been able to make quality plots from a distance of 35 miles (56 km). In other words, the transmitter and receiver were 35 miles apart. So long as the signals are stable, a larger distance between transmitter and receiver is probably desirable. So far, my experience has been that stable signals require groundwave propagation. Once skywave signals are involved, the level of QSB tends to go up dramatically.

If you generate some interesting plots, and you are willing to provide them as examples, please send them to me, and I will include them in the documentation.


Last update:Sunday, October 27, 2002 06:58:20 PM
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