We continue with Charlie, N8RR's, article about the use of the HFTA software. We will publish this in three parts here on the WVDXA Blog.
This is Part 2.
How Accurate is HFTA?
In 2007, the answer to this question was unknown to me. Having operated from hilly terrain for much of my radio career, I had some preconceived notions about some of the terrain effects and the results of the initial HFTA runs looked reasonable to me. For example, the general positive effect of sloping hillside under an antenna was known to me; I had successfully placed antennas on hillsides for years to favor a desired direction. The concept of a hilltop antenna being too high was also known; W8NR’s (SK) hilltop 20M monoband yagi was simply not competitive in a DX contest we once operated from his QTH. Contrast that with observations from W8AH (SK) who had a dominant signal on all bands using antennas that were at modest heights compared to many other big gun stations. Al Hix figured out long ago how to use the terrain to his advantage.
The software author made no extravagant claims regarding the absolute accuracy of HTFA predictions. The software performance in rugged complex terrain was also unknown. In 2007, I thought HFTA would be useful for comparing antenna locations and heights in terms of gross results but that it could not be used to fine tune choices. The author did not claim accuracy better than 3 dB. HFTA was used to judge the gross performance differences between options. If the software said one option was 2 or 3 dB better than another, I paid attention. If results were within a dB or so, the result was treated as equal.
It is now 2017 and many HFTA studies have been done for myself and others. To verify software predictions, a considerable amount of on-site testing was performed. The software limitations are now better understood. Overall, HFTA is a useful tool if one has options for placing antennas. It can also be used to evaluate the gross effect of stacking antennas on existing towers. Depending on the terrain, stacking antennas does not always do what you would expect.Limitations
All computer models are simulations. The modeling results are only as good as the underlying assumptions and the accuracy of the data input.
The earth and terrain, especially in front of the antenna, are an integral part of the antenna system. The main elevation lobe is formed by a combination of the direct ray with the earth reflections from below and in front of the antenna. The earth reflections reinforce or cancel the main ray depending on the phase relationship. Standard models show the development of the main lobe over level ground. If the earth around and in front of the antenna is not flat, the elevation angle and amplitude of the main lobe can be drastically different from the flat earth case.
The terrain data used by HFTA to calculate the main lobe amplitude and elevation is an approximation. The software looks straight ahead to points on a given azimuth line, out to about 14,000 feet from the tower base. The data file for each radial fan consists of a series of 147 spots, each representing distance from the tower and elevation at the spot. It is these point elevations where the real variability can occur, because they are derived from USGS data which is based on fairly wide intervals. For example, most elevation contours are on 10 meter intervals and some still use 30 meter intervals.
The electronic USGS elevation for my tower base was off by 20’ on the low side in the initial runs. The problem was discovered and a new USGS file was obtained with correct data. Comparing the HFTA results between the two runs, the numbers are significantly different but the general conclusions reached about options were unchanged.
HFTA can never predict an exact gain or an exact elevation angle, if all else is perfect with the software. Given the terrain accuracy limitations, HFTA does a good job of predicting performance differences between antenna options in gross terms. Don’t expect to use it for fine tuning antennas that are within a dB or so of each other.
The software uses elevation angle statistics calculated by VOACAP for an entire solar cycle on a given band from your general geographic area to a specific target area. HFTA uses these elevation statistics, which indicate what percent of the time a signal arrives at a given elevation angle, and compares this info to your elevation plot, generating a relative performance rating called Figure of Merit, expressed in dB. The software will tell you which antenna option will work the best, on average, to a target area (such as EU). These are approximate because of the terrain limitation AND the elevation statistics are an estimation. The FOM rating is interesting and useful for gross comparison, but I prefer to look at the actual elevation plot.
What I like to see is a high gain elevation lobe, at as low of an angle as possible. What one sees in a positive terrain enhancement situation is a substantial portion of the main lobe that falls in elevation below the main lobe of a flatland antenna. My point of view is that of a DXer, not a contester, who may have other preferences for elevation angle.
There are other limitations which may introduce variability in the results. The software is looking straight ahead on a line of points. The antenna is illuminating the ground in a broad swatch. Terrain behind and to the side of the antenna is not considered. The software would become overly complex if these other terrain factors were considered.
When evaluating stacked antennas, the software author N6BV cautions the antenna spacing must be at least ½ wavelength. If closer spacing is used, HFTA will give an inaccurate (inflated) result.
The most complex part of HFTA was obtaining the electronic terrain file needed to draw the radial fans. An electronic Digital Elevation Model (DEM) was downloaded for the quadrangle in which the QTH was located. Microdem, a mapping program that came with HFTA, was used with the USGS DEM to plot an approximate 14,000’ long radial (also called fan) every 5 degrees around the compass rose.
Using HFTA in 2017 is easier because the process of generating the radial fans has been automated by K6TU. All that is needed is to enter a set of coordinates on K6TU’s site and within minutes the resulting radial fans will be emailed as a Zip file. Microdem is no longer needed by the end user and it is not necessary to know the name of your USGS quadrangle. The K6TU files generate radials in 1 degree increments. What once was complex is now simple. This service is free but one needs to register. http://k6tu.net/
There was good news and bad news at N8RR. The good news was that some of the prime directions were predicted to have significant terrain enhancement. Additional good news was that optimum height for the good directions was fairly low. I would not need to deploy the stacks as planned. The bad news was that some directions have negative terrain consequences, worse than a flat land antenna. In the bad directions, there was nothing practical that could be done to overcome the terrain, including going higher or stacking.
Based on the initial review, the site would be an excellent location for DXing in some prime directions. In other directions, it would be average, and in a few it would be at a competitive disadvantage to a flat land station.
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20 Meters - 45 Degree Azimuth - Stacked C31XR Antennas
There is NO significant improvement to EU predicted on 20M for the stack over a single lower antenna. It turns out 56’ is about the optimum height on this tower site for 20M. The single 56’ high antenna was expected to be a great performer to EU on 20M, outperforming the same antenna at 75’ on flat ground by 2.2 dB Figure of Merit (FOM). The feature I like to see is the shift of the elevation curve favoring lower angles compared to the flat land antenna.
The next graph shows what the predicted 10M performance of the original stack concept would have been at 100’ and 50’, the performance at 56’ for a single antenna, and the performance of the 10M antenna at 25’:
For purposes of this comparison, a 3 element antenna was used, although the C31XR has 7 elements on 10M. Note the stack was only 1 dB better FOM than the single antenna at 56’ although the low angle performance was significantly better for the stack. The FOM for both of these options was significantly worse than for a 10M antenna at 25’ on this tower. The low angle performance of the low antenna was roughly comparable to the 100’/50’ stack. HFTA was indicating the 10M antenna needed to be lower on this hilltop tower. The 15M comparison was similar to 10M.
TERRAIN FACTORS AT N8RR CAUSING ENHANCEMENT OR DEGRADATION
Here is a terrain profile which results in enhancement compared to a flatland antenna, 45 degrees toward EU from the 56’ Rohn 45G tower:
Note the vertical scale is drastically compressed compared to the horizontal scale, which exaggerates the terrain features. It is not as bad as it looks! The dominant feature which creates positive enhancement to low angle radiation is the steep drop in elevation immediately under the antenna. The situation is not as good as it could be because of the approximate 825’ ASL parallel hill at about 1500’ in front of the antenna and in particular the higher 1,020’ hill about 4,000’ out. The later terrain feature impinges on the lowest angles and is close enough to detract somewhat from the performance. As the antenna turns clockwise from 45 degrees toward the east, this particular ridge feature moves closer to the antenna and progressively blocks the lower angles, until at about 90 degrees azimuth the performance is seriously degraded. On the other hand, as the antenna turns north from 45 degrees, the major blocking ridge moves out of the field and the low angle performance recovers. Here is the 90 degree terrain profile:
Again, this looks worse than it is because of the compressed vertical scale. However, it is bad enough, and all HFTA predictions looking east show significant degradation compared to a flatland antenna.
The above chart shows the blockage of low angle signals to Africa at 90’ with the 56’ high antenna (blue line). The red line shows a 100’ high antenna would be significantly worse than the lower antenna. The light blue line shows a 100’ antenna on flat ground. This chart illustrates what terrain degradation looks like.
If we turn toward the north from 45 degrees, the story changes. Here is a look at 10 degrees:
Higher terrain features are out over a mile from the antenna. This is far enough to avoid degradation compared to a flatlander and preserve the enhancement from the steep ground slope under the antenna.
Here is the chart for the C31XR at 56’ looking toward Asia at 10 degrees azimuth, compared to a flatlander at 100’:
Note the big elevation lobe peaking to the left of the flatland elevation lobe. The low antenna on the hilltop should dominate the 100’ antenna on flat ground. The FOM, for what it is worth, is 4.5 dB higher for the hilltop antenna. This is what a favorable terrain situation looks like.
***** This description of HFTA will be finalized in Part 3 *****
Post written by: Charlie, N8RR