Antenna Analyzer Operation

 

Running a Scan of your antenna

Mount the VAA shield on your Arduino R3 and insert the AD9850 into the socket on the shield. Attach your Arduino to a USB port on the PC. The red light will illuminate.

Attach an antenna to the antenna connector on the Shield.

It is important to remember that the VAA program will accurately report the forward and reverse signals generated by the signal injected into the antenna and feedlines to determine SWR values. The most accurate readings are generated when feedlines are at a minimum. While the system is tolerant of long feedlines, antenna switching systems, and other devices in the feed path, they may introduce reflections and pseudo resonances within the system that are not caused by the actual antenna being measured. The best approach is to choose a measurement point that eliminates these areas of problem. The graphic below was generated by testing my tri-band beam with approximately 125ft of feedline and an Ameritron RCS-4 antenna switch.

Start the VAA.exe PC program

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Select the Scan port from the combo box. If the Arduino does not show on a port, Press Port Scan to rescan.

 When the correct port is selected, Press Connect.

A message “Updating Uno Program” will appear. This message means that your Arduino Uno R3 is automatically loading the correct software. This takes less than 10 seconds.

When upload to the UNO is complete, a connection message will display “Port # connected.”

 With the antenna connected to the Shield, start the scan. To start the test Press Scan

 The graph will display the results of this test.

In the view above the resident nodes are those with the lowest SWR. In this case, the tri-band beam shows nodes correctly at 14, 21, and 28 MHz reflections from the feed line and line switches at 6 SWR and above are ignored.

 

Commands

Scanner Port: The PC port on the computer where the scanner is located. This value is read at startup and should be the accurate pc port. If you have multiple ports and connected devices, press Port Scan to re-scan.
Connect: After selecting the port press here to initiate a connection.
Start MHz; The frequency to start the visual scan. Also, used for setting the single frequency output used for testing.
Stop MHz: The frequency to stop the scan.
Smoothing: This function instructs the Uno to perform repeated SWR data reads of each frequency. Reading each frequency multiple times provides an integrated value of the average of all reads for the frequency being tested, which is then used in the SWR calculation. This provides a smoothing of the graphic curves and a more accurate view of the correct value. The default is 8 reads.
Divisions: The program evaluates the frequencies between the Start MHz and Stop MHz and divides this area by the number of Divisions selected. For example, if the start MHz is 10 and the Stop MHz is 20 and the Divisions is 10 then readings are made at 10,11,12,13 MHz etc.  Measurements are made at each of these points. A label located above the Divisions label shows incremental value.
Resolution : Selecting 1 provides a normal view across the entire SWR range. This is the default. A selection of 2 drills down to nodes under about SWR of 4 and provides a more finite reading of low SWR nodes.
Scan: Starts the scanning process.
Continuous: Provides a repeating scan. The selection box to the left of Continuous shows the time delay between each scan. This is helpful when adjusting active chokes in certain antenna systems, or other times when you want repeated scans.
Output Singl e: Provides a single frequency output which is controlled by the Start MHz value. This is a toggle, after setting the Start MHz value, press the button. A second press will discontinue the operation. Used for generating a continuous output frequency as a source for testing.
Show Length Analysis: Displays the length analysis panel for calculating antenna lengths for any node. Set the desired node frequency in the box provided. Now using your mouse cursor move over the current lowest node frequency. The frequency will be shown in the panel and the number of adjustments required to the length of the antenna will be shown in inches. Be sure to select the correct wavelength configuration for your antenna as this affects the calculated result.
Print Graph: Sends a copy of the graphic display to your printer.
SWR slide r: Moving the slider adjusts the graphic display, with an SWR of 2 showing the lowest drill-down value. After a scan is completed, you may use this to Zoom in to the resonate points discovered and displayed.
Left Clicking: on the graphic display. You can choose to drill down on an area of the display by using your mouse to left-click and drag a box around an area on the graphic.  After surrounding the area with the box right-clicking will provide you with the zoom commands and other information.  

Other applications

The VAA.exe interface will function as a dip resonance meter and some interesting and productive uses may be developed using a loop of wire or capacitor probe connected to the antenna connector.

I was able to measure the choke point of choke and capacitor systems on my Cushcraft R7 vertical antenna that isolates multiple resonance points in the multi-band unit, by placing the loop around the unattached choke and scanning the appropriate frequency range. I was able to visually discover the resonating point and the high and low cutoff points.

 

Antenna Length Analyzer

Interpretation of Scans

When Show Length Analysis is pressed the length calculator box is shown. The box will display the calculated antenna length for any target frequency entered.  The calculated length for a target frequency is based on the speed of light in a vacuum and is the ideal length for the target frequency.

Select the appropriate antenna type and move your cursor over the red line display of the current frequency on the graph. Based on frequency the measured length is displayed. A length modification to match the target frequency is then presented. A positive value means that the measured antenna should be lengthened. If the value is negative the antenna should be shortened.

Make sure to choose the correct antenna type.

Signal generator function.

Using the diagnostic single-frequency output, the VAA will function as a signal generator.

The PC card mounts an AD9850 signal generator module, which is a very accurate, digitally controlled signal generator of low power. It generates a spectrally pure, frequency programmable, analog output sine wave.

It is a handy source of RF signals for testing. Connecting a loop to the output can be used as a quick signal generator. While it does not replace a commercial lab signal generator it is a handy source of signal, and programable from our VAA.exe software for any frequency between 1MHz and 55MHz. So, you can easily maximize your investment as it is a multipurpose instrument. 

 

 

Correctly Tuned Antennas

 

When reading the results of an analysis of your antenna these are some of the visual displays that might be seen. The first is a well-tuned antenna system.

 This is a Force 12 tri-band beam for 20, 15, and 10 meters. The blue line indicates the lowest SWR point for this scan, however, due to several factors this line might be shown at another node point.

 

 

 

 

 

 

 

Open Feed Lines

The next scan shows a 58-ohm feed line without a connection to your antenna. 

The multiple dips are due to the reflected nature of the cable bouncing back from the end. It of course does not resonate, and only generates regular reflected signals as the signal bounces.

 

If you see this happening when attached to an antenna it may be due to a faulty connector, a bad or not closed relay in a remote system, or humidity within the cable system.

 

 

 

My fix here is to run a little Rf from my transmitter through the system which usually clears humidity problems if your antenna has not been recently used. Make sure to check your connections as these may also be the cause.

Not Connected to Feed Line

 A blank scan indicates that the VAA is not connected to a feed line. Check your connection.

 

Trap Vertical Antennas

 

This is a partially tuned trap vertical. It is adjusted by lengthening or shortening the distance between each trap.

In this example, the 10-meter adjustment is within tolerance as well as the 15-meter adjustment. The next trap is for 17 meters and the scan indicates that the distance between the 15-meter segment and the 17-meter segment should be increased.

 Once that has been done, and a scan shows that the resonance point has moved downward, we will adjust the next segment until the entire antenna spacing is correct and each segment is at a point of resonance.

Wire Antennas

Multiband wire antenna's resonate points depend on multiples of the lowest frequency for which the antenna is cut. Most require an antenna tuner for final adjustment. Generally, I have found that if the antenna’s base frequency for its length is adjusted correctly the other multiples become close to naturally resonating with little external tuning required.

 This is a scan of a balun end-fed multiband antenna from Maple Leaf Studio Antenna Company. With a tuner, it will resonate nicely, but the scan shows that it is a little short in its raw form as the 80-meter base resonate point is at about 4 MHz.

Extending this wire would help all multiple bands get closer to their ideal resonate point.

After some adjustments to the antenna length most of the bands are very close to the desired frequency resonate points. I still use a tuner to fine tune this wire since I am feeding the system with a SB220 linear which does not have a built-in tuner. There are some other resonating points outside of the permitted ham frequencies, but they can be disregarded