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After four years of contesting from this location (grid square DO00hg), I have noticed that sometimes when my antenna is at 25', tower nested down, I do quite well into Canada and the U.S -- perhaps even better than with the tower up another 20' at 45'. But I've never proven the case. Until now. An extensive high-frequency terrain analysis shows why a lower antenna works better for contacting Canada and U.S. stations from here. The analysis reveals how my antenna at 25' and 45' compares to the same antenna over perfectly flat terrain -- looking at the Indian Ocean, Europe, Africa, Canada, the U.S., South America, Japan, and Asia. My elevation above surrounding terrain I have a very steep slope from my back yard to the valley bottom, directly East of our property. Here's how it looks in the Radio Mobile software's modelling: What does the extra elevation mean? Here is a profile of the terrain looking at Europe (30 degrees) from my tower base. The red line in this image shows the height of land. The antenna at 25' up on the tower is the blue dot. The antenna at 45' is the green dot -- the green line shows where the ground would be if the terrain was perfectly flat in all directions (the "reference" line you will see in every image here). Terrain profile looking at Europe You can see how sharply the ground falls away. Here's what that means for my antenna -- a 3-element yagi on a tower that can be 25' or 45' tall -- looking at Europe on 20M (14 Mhz): Antenna gain, looking at Europe on 20M The vertical lines show the takeoff angles of signals arriving from Europe. The longer the line, the more often signals arrive at that angle. (Note: I used a statistics file for Washington State, which is only 80 miles south of my location in southern British Columbia). Remember, the red line is my antenna at 45 feet, the blue line is my antenna at 25 feet, and the green line is the same antenna over perfectly flat terrain (a reference for comparison). In the Europe profile (above), my antenna gain is much better than the reference antenna over flat terrain, especially for signals arriving at very low angles -- which are more predominant from Europe. Very good so far! The
view to Canada and the U.S. With a mountain ridge peaking 4 kilometers away, you can see where low-angle signals are blocked from my location. However, high-angle signals -- anything above 6 degress (the visual height of the offending ridge) -- get out and arrive just fine. Terrain profile looking at the U.S. In fact, as the following chart shows, my antenna is generally more effective than one over flat terrain, and it actually performs rather well at 25 feet or 45 feet: Antenna gain, looking at the U.S. on 20M Comparison with 3-element on 120-foot tower Just for fun, I decided to see how the antenna at 45’ here would compare to a very high 3-el. tribander in a flat location:
Here's how the antennas match up against the same antennas on a 120-foot tower, looking at Europe -- rather well, considering the red line is the antenna at 45 feet up, and the blue line is at 25 feet, and the green line is 120 feet up: Antenna
gain, looking at Europe on 20M Some observations and interpretations Note that the gain here is dBi – not actual gain, just a reference figure that at least can be compared across the many measurements. The software produces a “figure of merit” which is each antenna’s merit based on gain at takeoff angles where, statistically, the most prevalent signals can be expected. If you have great gain at takeoff angles where most of the time no signals are arriving, the FOM is low. However, if you have great gain at takeoff angles where most signals arrive, the FOM is high. For example, on my Europe heading, the 20M Figure of Merit for the 45’ antenna is 11.3, and the FOM for the flat-land reference antenna is 7.3. This indicates the 45’ antenna has a 4 dB advantage at that beam heading. At some takeoff angles, the flat-land antenna might be better, but considering the full range of angles signals can be expected from, the 45’ antenna is quite a bit better.
Over the pole to the Indian Ocean, crossing vast expanses of nature and little else... As you can see, this is a good path directly over the pole, but there are few population centres in this direction. I don't have good coverage for the lowest angle signals, which are some of the most prevalent along this bearing. In practice, however, I hear very well from Kazakhstan and Reunion Island (about as far from B.C. as you can get) when conditions are right. I have worked India a few times, but only on rare path openings. This heading and just east of it, works like gangbusters for the Scandinavian Express -- Sweden, Finland and Norway.
Sharp drop off to EU – my most advantageous direction
See how the 45' yagi compares to one at 120' pointed at Europe
To properly assess the impact -- positive and negative -- of the unusual terrain around my location, I have used special software from the ARRL Antenna Handbook -- the edition is a bit dated now... time to get the latest book and see how it changes things. The ARRL HFTA software uses files output from another program called Microdem. This needs Digital Elevation Model (.dem) files to create information about the topography around your geographical location. I could not find any DEM files for Canada that would work with the Microdem program. I gave up, and forgot all about the idea for a few years. Then, four years after abandoning my first attempts to do the gain-over-terrain analysis, I had an epiphany: I used the free software called Radio Mobile to produce the Height Above Average Terrain (HAAT) elevation data from its high-resolution dataset, and then built the files for HFTA one by one. Essentially, I used the Height Above Average Terrain (HAAT) tool in Radio Mobile to generate elevation figures along lines radiating from my location -- getting an elevation every 100 meters out to 4.4 kilometers on a line pointing to Europe at 30 degrees North, for example. I created a "profile" file for every 30 degrees, plus files for special beam headings that aren't on a 30-degree interval, such as 110 degrees to the southern U.S. and Caribbean. It's not hard at all, but a tad time-consuming as I had to use Excel to strip unwanted columns of data and convert meters to feet, then copy into Notepad for HFTA to use. Here's what one of the output files looks like (this partial list is from a file showing the elevations for 90 degrees due East, VA7ST_090.pro). The columns of data are distance from tower base, and elevation at that point -- all in feet: VA7ST_090.pro
0.00 1620.73 To see what the world looks like from this location, see my great circle bearing map
Radio
Mobile software Great
Circle Bearing Map Software |
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We
moved to the current QTH (location) in April 2004, specifically
because we needed more room for the kids and pets, and we HAD to
get out from under the shadow of a very high mountain ridge that
didn't allow me to do very well in contests.
