I have been playing with HFTA software from the ARRL Antenna Book to determine the typical take-off angles needed to communicate to DX locales. First step was to develop my elevation profile, which I did based upon the instructions included using a program named MicroDEM. While not the most intuitive thing in the world, I did manage to follow the Antenna Book directions to obtain my DEM GIS file from a government location and to create exportable FAN files needed for HFTA. A FAN file is a terrain height profile in a particular direction from my QTH “tower” location.
The false-color map shows relative height above sea-level (ASL). I am at approximately 550 feet ASL, or 168 meters. I recolored the chart and added lines to areas of interest. I used Gimp (a Photoshop clone) to dedit MicroDEM’s map output, reducing the blue to add clarity to the river valley terrain nearby, and the lines correspond to the HFTA FAN files to obtain terrain profiles in the areas of interest.
A FAN file is a very simple data-set: distance from point and height at distance. Here is mine toward Europe:
meters 0 158.0 30 158.43 60 159.22 90 159.88 120 160.98 150 161.90 180 163.29 210 164.45 240 165.00 270 164.73 300 164.37 330 164.10
(And so on out to 4410 meters from my antenna in the direction to Europe.)
Once the FAN files are saved in a location that HFTA can read them, I ran terrain plots for each of the directions of interest. Here is the one to Europe…
If you follow along the line to EU on my terrain map, the terrain profile is up and down (or hills and hollers as they say in Tennessee!) as it continues to drop height toward the riverbed. The terrain plot in that direction is out to 15,000 feet or so from my antenna location (about 3 miles or 4.5 kilometers.) Of course, the terrain illustration is highly exaggerated due the x‑axis scale. The exaggeration highlights the profile changes as you get more distant from the antenna location.
HFTA produces an Assessment Plot for antennas based upon the terrain profile. In the following example I have illustrated a dipole mounted 30 feet high plotted over flat ground of medium conductivity.
The bar graph corresponds to the statistical average of wave angles that signals arrive at my location from Europe on 20M. Notice the predominant angles are 3 degrees, 10 degrees and 15 degrees. Superimposed is the gain line at wave angles for a simple dipole at a height above flat ground of 30 feet (9 meters). This antenna is pretty weak at 20M signals arriving at 3 degrees – too low – but becomes a bit more useful for signals arriving at higher angles up until about 35 degrees where it is strongest. A quick analysis is this antenna will play moderately well into EU at the higher wave angles.
Note also the dipole Max. Gain is listed as 7.4dBi. This is correct for a dipole 30′ over real ground of medium conductivity. A dipole in free space is 2.1dBi, but put it over conductive ground and the horizontal polarization benefits from ground reflection gain. This is NOT the case for vertically polarized antennas however (although I wish it were).
In the next analysis I have graphed the same antenna/frequency except this time using my actual terrain profile pointed towards Europe.
The real ground slopes down towards the river in the direction of Europe enhances the angles below 7 degrees. Statistically on 20M signals do not arrive much at 4 – 7 degrees, but there is a few dB increase at 2 and 3 degree take-off angles. It is a better illustration to compare the antenna referencing another, so I have made a practice to always include the flat-ground models in my graphs. Here is the 20M dipole 30′ up compared to both flat and real ground.
My surrounding terrain enhances signals by about 3dB below 7 degrees on 20M. Still weak though but hey…gain is gain! Simply due to sloping terrain a dipole will have the same gain as a Double-Extended Zepp at this height for TO angles under 7 degrees. So what happens if I go higher?
A higher dipole produces predictably better gain at all take-off angles. Definite enhancement happens at 40′ (12 meters) and even better at 50′ (15 meters). I could derive from this study that 50′ is a great place for a 20M dipole to be placed, but 40′ is not bad at all. In fact, 40′ might provide slightly more utility for domestic signals arriving around 5 degrees.
Lets look at the same situation to EU but this time on 40M. Here is where HFTA is really showing some interesting things!
Statistically, 40M signals arrive from Europe at around 4 degrees and around 13 degrees. Also some higher angles signals arrive at 24 degrees. There is significant terrain enhancement at around 4 degrees, but little difference between the modelled heights! Basically, a low wire on 40M plays just as well as a wire at 50′ into EU for very low TO angles. The difference between a 30′ and 50′ wire is about 4dB at 14 degrees. From my location, a 40M dipole at 9 meters high should play just fine, but being at 50′ makes signals come in that much louder at higher wave angles.
A model is only just that, but my experience validates this exercise. My low G5RV (9 meters above ground) plays very well into EU on 40M most of the time. I do not know enough about propagation at 40M to know when higher or lower wave angles are supported (but will investigate) but seem to recall that gray-line signals seem to be loudest on 40M. My success on 20M is generally less satisfactory. There I am competing with yagi antennas on towers, and as such my signals are always down in the noise by an S‑unit or so. I consider 20M my worst band.
Some final thoughts…
I get out just fine on all ham bands. my G5RV at 30′ has been a great antenna. In less than 2 years of being active again, I have worked all United States on 80 and 40M. I was awarded WAS Triple Play Award #123. I currently have 131 countries worked, with 95 confirmed via LoTW. My worked country totals on 80M: 73, 40M: 95, 20M: 104, 15M: 57. You can work DXCC with one low antenna in a short amount of time!