A Study of Ground Reflection Gain Using HFTA Software
by Garth Merritt VE5VG
(Pangman, Sask., Canada)
The following illustrates the effects of ground refection gain at varying antenna heights above ground.
At this location, the elevation above sea level is 2350 ft. The topography towards the North-East is gently sloping and rolling topography out to about 2 miles. The elevation varies little after 2 miles and is relatively flat. The antenna location is near the top of the geological land form known as the Missouri Coteau.
Utilizing HFTA¹ software (HF Terrain Assessment by N6BV , ARRL) a comparison was made on antenna wave angles, of the main antenna lobe, between antennas at different heights:
• dipole at a height of 20 ft over flat ground
• 3 element Yagi at 50 ft
• 3 element Yagi at 50 ft over flat ground
• 3 element Yagi at 90 ft
Fig 21.69 of the ARRL Handbook illustrates the elevation plane of a 3-element Yagi placed 1/2 and 1 wave lengths above the ground. In poor propagation conditions, the wave angle of the incoming signals are received at a low angle. The illustration is over flat ground. The lower the antenna lobe is, the better the receive and transmit performance of the antenna is.
The digital elevation map (DEM) was downloaded from the Government of Canada – Natural Resources website (see source below). QGIS² was used to create the PRO terrain file. Terrain elevation points were taken at 30 meter intervals out to a distance of 4400 meters. The comparison was made over average ground at a frequency of 14.200 MHz.
The elevation statistics on the HFTA Output graph reflect the probable angle signals are received and are reported as a percent on the right side of the graph. The elevation statistics are between VE5 and EU.
The majority of the signals received have an angle of less than 15 degrees. The gain is represented in dBi. The elevation angles are shown on the horizontal axis. The graph represents the shape of the antenna lobe at a bearing of 50 degrees (NE).
The HFTA manual defines the Fig.of Merit (FoM) as a weighted average computed by multiplying the gain at each angle by the elevation statistical percentage. The products are averaged and calibrated in dBi. It can be used as a generalized comparison between antennas at various heights.
As the antenna height increases, the effect of irregular ground elevations on the main lobe near the antenna appears to diminish. The sloping topography lowers the elevation angle of the 50 ft antenna lobe slightly compared to over that of flat ground. A deep null can be seen on the 90 ft antenna at 20 degrees between the first and second lobes.
Please note, this is a simplistic view of analyzing a single vector. An antenna lobe is affected by multiple factors within its surroundings (buildings, trees, etc.). However, the graph does illustrate the effects of ground reflection on an antenna lobe, as antenna height is increased, over irregular ground. The photo of the QTH is looking NE. The orthophoto³ and DEM illustrate the local topography.
In summary, my 3-element triband antenna at 50 feet has good receive performance towards Europe, but VK and ZL to the West is a more difficult path when propagation conditions are poor.
1. HFTA - HF Terrain Assessment for Windows is free. The executable file (.exe), its associated files and the user manual (.PDF) come with the ARRL Antenna Book when you buy it.
2. QGIS is a user friendly Open Source (free and powerful) Geographic Information System (GIS) licensed under the GNU General Public License.
3. What is an orthophoto?
“An orthophoto (also known as a orthophotograph) is an aerial image that has been geometrically corrected (ortho rectified) so that the image is uniform from edge to edge. Orthophotos are corrected to remove terrain effects (what happens when you take a 3-D surface and make it into a 2-D product) and distortions that result from the camera’s lens and the angle the photo was taken from the plane. The goal of ortho rectification is to create an image where distance measurements are the same across the entire image.” (Source: https://mapasyst.extension.org/what-is-an-orthophoto/)
4. DEM Source: https://open.canada.ca/data/en/dataset/957782bf-847c-4644-a757-e383c0057995
(More images illustrating this article can be viewed here)
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