Terrain Analysis Brian McGinness N3OC Terrain Analysis Now that we know what angle signals arrive at, and what effect antenna height has on takeoff angle, what about the effects of the terrain? Terrain Analysis We have been showing you a scientific approach to designing your contest station antenna systems. 1. Know what arriving signal angles must be covered. 2. Model the antennas and their height and design your system to cover those angles. 3. Be aware of the effects of terrain and take corrective action, if required. N3OC to Europe N3OC to Europe – 10 miles out, using Delorme TopoQuads. N3OC to Europe Delorme Topo3D shows desired downhill terrain out to one mile. But at 2-3 miles out, there is undesirable uphill terrain. What effect does this have on HF signals? ARRL’s HFTA Software One way to answer the question is to purchase the ARRL Antenna Book, which includes HF Terrain Assessment software. HFTA uses public USGS terrain data to model the effects of terrain on your HF signal. Effects of Terrain Generally speaking, flat terrain will produce an even, bell-curve plot on HFTA, on all bands. This is really the lobes of the antenna pattern that you see when modeling an antenna, shown vertically. Effects of Terrain Here is the same antenna, shown on EZNEC. Note the main lobes at 12 degrees and 32 degrees. Effects of Terrain Terrain that slopes down from the antenna will enhance the low angles. This antenna now needs to be lowered a bit on the tower to compensate for the terrain. Effects of Terrain This really becomes a problem on 10 meters! Beware of “mountain” QTHs with too high of an antenna, especially on 10 meters. Effects of Terrain As you might now expect, uphill terrain enhances the high angles, and impairs the low angles. This slide shows the effects on 20 meters, where the gain at low angles has dropped by 5db! Effects of Terrain Here are the effects on 10 meters. Our antenna that was too high for flat and downhill terrain is starting to look a little better! (But still needs to be lowered or stacked). HFTA Case Studies The first example will be a real-world example of a somewhat compromised antenna system, a tribander stack, with average terrain. Since the antennas have to cover three bands, they cannot all be at the optimal height for each band. In this example, the antennas are of the typical “multi-monobander” type of tribander, and are located at 100’, 63’ and 33’. These are not ideal heights, but they are what work on a 100’ tower due to the constraints of the guy wires. Obtaining Terrain Data for HFTA Data must be downloaded and prepared before you can use HFTA. First, you must download the terrain data, centered on your antenna location. Then the street map data is merged with the terrain data. Use of the street map data is optional. Finally, terrain azimuth files are created from the terrain data that you have assembled. There is one file for each five degrees of the compass, from the base of your tower out to 4400 meters. Obtaining Terrain Data for HFTA Alternatively, you can create your own terrain data files by using topographical maps and a text editor. These files contain the terrain elevation in meters, every thirty meters. Terrain data is available for download in the DEM (digital elevation model) or NED (national elevation dataset) formats. Each format has it’s pros and cons. We will use the NED format in the following examples. Downloading Terrain Data NED data is available at: http://seamless.usgs.gov/ Defining the Data Limits Limits are defined 1/10th of a degree each direction from the base of your tower. Request Summary Page Once the limits and output format (tiff) are defined, the data is downloaded to your computer. Downloading & Saving the Data The data is then saved to your hard drive. NED Data is saved in the C:\mapdata\DEMs directory. Opening the Data in MicroDEM The NED data is unzipped, then opened using MicroDEM. Opening the Data in MicroDEM There is no street data yet, just raw elevation data. Download Street Map Data (optional) Street data can be downloaded at: http://www.census.gov/geo/tiger99/tl_1999.html Find the FIPS Number & Download Data Montgomery is 24 031 and Howard is 24 027 Save the Street Data Tiger street data is saved in the C:\mapdata\tiger subdirectory Return to MicroDEM & Merge Map Data Click on Vector Overlay icon. N3OC QTH needs two counties. Completed Terrain for N3OC with Street Data Entering Weapons & Viewshed Parameters Click the Weapons Fan icon, then double-click anywhere on map Enter Tower Location Enter the coordinates of your tower base. Enter ViewShed Parameters Enter the parameters for the radial files. These settings will produce radial files every 5 degrees out to 4400 meters from your tower base. Specify Radials File Name Give a meaningful name to your radial files. MicroDEM will append the degree bearing to this name for each file. MicroDEM Creates 71 Radial Files These radial files contain elevation data every 30 meters from the tower base out to 4400 meters, every 5 degrees. MicroDEM Creates 71 Radial Files HFTA will use these files to model the effects of this terrain on your antennas. Setting up HFTA Analysis Select a radial elevation file for your location and the direction of interest, and enter your antenna type and height. Setting up HFTA Analysis Also you can select the profile for flat terrain to use as a comparison. Setting up HFTA Analysis Select an elevation file to use as a reference for arriving signal angles. We are using W3LPL’s angle data instead of the data that comes with the program. Resulting Terrain Profile Profile of the terrain as specified in the N3OC-45 terrain radial file. Note the antenna heights are shown too. HFTA Terrain Plot for N3OC to Eu on 20m The blue plot shows gain (in dbi) of N3OC’s terrain, and the red plot shows flat terrain, using a 3/3 stack at 100 & 63 feet, on 20 meters. HFTA Terrain Plot for N3OC to Eu on 20m Purple bars show arriving signal angles that need to be covered to Europe, using W3LPL’s data. HFTA Terrain Plot for N3OC to Eu on 20m Normally the program uses angle data referenced to the frequency that a particular angle produces propagation. Some of these angles appear unreasonable. HFTA Terrain Plot for N3OC to Eu on 20m Conclusion is that my terrain slightly helps the signal to Europe on 20m, compared to flat terrain, on the lower angle paths. Not enough to worry about, and may not be noticeable. HFTA Terrain Plot for N3OC to Eu on 15m Lets start on 15m by having a look at the stack compared to flat terrain, to evaluate the effects of the terrain on this band. HFTA Terrain Plot for N3OC to Eu on 15m The downhill terrain has shifted the angles a little to the left, and chewed up the plot a bit, but probably not enough to worry about. HFTA Used to Evaluate Stacks HTFA can also be used to evaluate the angle coverage of individual antennas, and stacks, referenced to the arriving signal angles that need coverage. HFTA Used to Evaluate Stacks This complicated slide shows the plots for the stack (blue), the upper antenna (red), the middle antenna (green), and the lower antenna (cyan). Lets look at them one at a time for simplicity! HFTA Used to Evaluate Stacks First, the upper antenna at 100’. Note the deep nulls at 14 degrees. This antenna covers the low angle paths nicely, but is no good for the high angles. HFTA Used to Evaluate Stacks Next, the middle antenna at 63’. This antenna covers the middle angles, except at 10 degrees thanks to the terrain. If you had to pick one antenna, this would be the one, mounted a little higher. HFTA Used to Evaluate Stacks Here is the bottom antenna at 33’. This is obviously high-angle antenna, probably best suited for sweepstakes! HFTA Used to Evaluate Stacks Finally, the entire stack compared with flat terrain. The stack produces a few db of gain over the individual antennas. Gain is achieved by redirecting the energy to the desired angles. HFTA Used to Evaluate Stacks Just for reference, here is the stack using just the upper two antennas. HFTA Used to Evaluate Stacks And here it is using the lower two antennas. HFTA Terrain Plot for N3OC to Eu on 10m Things change quite a bit on 10 meters. This example shows the result of an stack that is too high – note the deep null at 12 degrees, which is an angle that needs to be covered! HFTA Terrain Plot for N3OC to Eu on 10m Adding a lower antenna to the stack almost fixes this problem. This example shows a 5/5/5 stack at 100’, 63’ and 33’. The ridges out at 2 miles are effecting the signal. HFTA Terrain Plot for N3OC to Eu on 10m The downhill terrain in close helps at 3-4 degrees, but the ridges at 2 miles out hurt the signal at 5 & 9 degrees. But we still need to check the coverage of the individual antennas. HFTA Used to Evaluate Stacks We have only looked at the 45 degree path to Europe. When evaluating your station with difficult terrain, you need to check the entire path to target contest audiences. HFTA Case Studies – K4VV The next example will be a real-world example of a mountaintop QTH, where we might get into trouble with antennas that are too high for the local terrain. I chose to use K4VV’s QTH, since he has a hilltop QTH with complicated downhill terrain, and he is the process of building a station at this location. Let’s see what works at his location! HFTA Case Studies – K4VV Here is K4VV’s MicroDEM data. Note the ridge line running northeast – southwest. HFTA Case Studies – K4VV The street data is added, but this is not required. There is Jack’s street. HFTA Case Studies – K4VV K4VV terrain to Europe, at 45 degrees. This terrain is complicated and runs along the ridge line. HFTA Case Studies – K4VV K4VV terrain to Japan, at 330 degrees. This will have more of an effect. It is downhill all the way. HFTA Case Studies – K4VV Lets start with a 100’ yagi on 20 meters to Europe, and compare it with flat terrain. This antenna is already showing the effects of being too high because of the terrain. HFTA Case Studies – K4VV This is the same antenna, now looking towards Japan. Note the ugly null at 9 degrees caused by the terrain. We need more antennas to fix this, and may need to lower the antennas a bit. HFTA Case Studies – K4VV First lets try a stack, at the traditional heights for a 20 meter stack. It’s getting better… Let’s try lowering the antennas a bit. HFTA Case Studies – K4VV Here are the antennas at 90’ and 45’. Now let’s look at the coverage of the individual antennas. HFTA Case Studies – K4VV Note that the upper antenna alone (red) is about 3db better than the stack at the null at 6 degrees caused by the terrain. HFTA Case Studies – K4VV There is no magic fix for the effects of the terrain. All you can do is move the effects around by varying the antenna heights and using a stack to help control the angles. HFTA Case Studies – K4VV Here is K4VV’s 15 meter path to Japan, again compared with flat terrain. The terrain is working in our favor on this path at this height. HFTA Case Studies – K4VV Lets look at some other directions. HFTA Case Studies – K4VV The same height looks about right for Jack’s path to Europe. What about South America? We haven’t looked there yet. HFTA Case Studies – K4VV Here is Jack’s terrain to the south, very different from his other directions. It is actually slightly uphill for the first mile. HFTA Case Studies – K4VV The path to South America is the most complex, due to variations in propagation, and needs coverage over a wide range of angles. Note the nulls at 6 and 19 degrees that need fixing. HFTA Case Studies – K4VV The null at 6 degrees is caused by terrain and may not be fixable. The null at 19 degrees can be fixed with a lower antenna and a stack. HFTA Case Studies – K4VV Stacking with a lower antenna removes the null at 19 degrees and produces a little gain. The gain is probably not noticeable, the angle coverage is the real benefit of a stack. Conclusions Downhill terrain enhances the lower angles. Uphill terrain enhances the higher angles. Irregular terrain introduces peaks and valleys in the antenna’s vertical pattern that are hard to control. Minor variations in terrain have little effect on the antenna pattern. You will probably only notice problems with terrain that has wide variations. The End.
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