The Great Circle Map – GCM
When this webpage was loaded the original IP-derived Latitude and Longitude was used to render the Great Circle Map for the Grid Square location as a default. Normally, the map center accuracy should be more than adequate for HF radio DX communication purposes. However, if wish to generate GCM for any other Grid Square location, you may enter a properly formatted Maidenhead Grid below and regenerate GCM.
The GCM generated above, unlike traditional maps of this type, has been rendered in an elliptical shape instead of a circle. This provides an expanded view horizontally thus making it easier to interpret. The map base is colored with each continent colored differently for easier recognition. The color code of the continents is given below the map for easy reference.
To those of us who are exposed to a Great Circle Map for the first time, it may appear a bit unintuitive. However, with a little effort and comparing continental blocks on the GCM with a regular rectangular map, it will eventually become easy to comprehend. For more information on various map types and the significance of a Great Circle Map in relation to HF amateur radio for DX communication, please refer to the article Geodesy for Terrestrial HF Radio.
The center of the elliptical-shaped map is the location of your QTH. The radial lines from the center to the outer periphery represent angular directions in degrees. The north is 0° while the east is 90°, the south is 180°, the west is 270°, and so on. These angles represent the short-path antenna beam headings to various DX destinations.
The map also shows low-contrast concentric circles (ellipses) around the center point. These represent range-circle distances of 5000, 10000, and 15000 kilometers from the center. The outer periphery represents 20000 Km which happens to be the Great Circle antipode distance on Earth. Other than this, the geographic equator and a few other latitude indicators are also provided. However, unless you are familiar with the GCM structure you may find these lines difficult to interpret. You may safely ignore them for now. Similarly, a few important longitude lines (meridians) connecting the geographic north pole to the south pole are also displayed. All these lines are displayed in low contrast to avoid causing confusion to the less discerning users. The GCM map rendering is based on Lambert Azimuthal EquiArea Projection.
For all HF DX radio amateurs, an important takeaway as seen from the GCM is that for long-distance communications, it is not at all necessary to be paranoid about the accuracy of the antenna beam heading. As seen from the beam angle radials from the QTH center, the lines diverge conically, as the flare-out increases with distance. Hence, a far bigger area is geographically covered within any angle as the distance grows.
Typically, the azimuth angle coverage of a directional HF radio antenna is around ±40° to ±50° for normal 2-3-5 element Yagi, 2-3 element Cubical Quad, etc. Hence, such antennas can often broadly cover an entire continent at DX distances without even having to swing the beam around for various QSOs within the region. The bottom line is that during most of the typical DX sessions, at any given point in time, it might rarely be necessary to swing the antenna around too frequently… Take a look at the Great Circle Map once again and see for yourself how often a common central azimuth setting of an antenna would provide you with very large area coverage into DX locations.