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This is a strange sort of question, as in reality the signal will go infinitely far (effectively) however you are really asking at what distance might a receiver be able to pick up the signal. In testing a 2.4GHz signal with a 100mW omni antenna, the furthest distance I could receive a signal with less than 5% retries (802.11b kit) was 2 miles with a 100mW ...

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As pointed to by Marcus' comment, "line of sight" communication is just that: if you can see it, you can communicate. There are formulae that will calculate the distance to the horizon, given the height of the tower at each end, and it is because of the curvature of the Earth, as you suggested.

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5 GHz and 25 km range is hard. Let's do a quick free space loss calculation: \begin{align} a &= \left(\frac{4\pi d}{\lambda}\right)^2\\ &= \left(\frac{4\pi dc}{f}\right)^2\\ &= \left(\frac{4\pi \cdot 2.5\cdot 10^4 \text{ m} \cdot 5\cdot 10^9\text{ Hz}}{3\cdot 10^8\frac{\text m}{\text s}}\right)^2\\ &= \left(\frac{5\pi}{3}10^6\right)^2\\ ... 3 At VHF and above, communication is easiest when the stations have a clear line of sight. Without line of sight, a link is achieved only by reflection and diffraction off/around other objects in the environment, and this must be offset by higher transmit powers or antenna gain if there is insufficient margin in the link budget. Estimating coverage in the ... 3 The footprint calculated by gpredict (and other satellite tracking software) is the area from which the satellite appears above de horizon (or, equivalently, the patch of earth seen from the satellite). This is independent of any radio link the satellite may possess. So the answer is no. You cannot modify footprints on gpredict. This has nothing to do with ... 3 Google Earth Pro (now free. 0) has a Viewshed tool. "A viewshed is the geographical area that is visible from a location. It includes all surrounding points that are in line-of-sight with that location and excludes points that are beyond the horizon or obstructed by terrain and other features (e.g., buildings, trees)." --Wikipedia https://support.google.... 3 I've used this one several times. added: Go to the graphic under: Surface Elevation Tool. Scroll (zoom out to speed things up) OR use the search to get to your geographic area. Search somewhat reliably recognizes town,state names and zip codes. Zoom into and Click on a location to drop a balloon. Zoom out and scroll to other location - zoom in and click ... 3 You should have no problems doing 7 km with a (presumably 5W) HT even with the rubber duck antenna, outside. If you wanted to talk from inside, you will probably want an exterior antenna, but almost any common 2m antennas will do the job. A simple J pole is popular, you could even use a mobile antenna sited out a window. 2 In addition to earth's curvature, obstacles in the various Fresnel Zones may reduce received signal strength, and hence lower effective range 2 HeyWhatsThat is a free web app that can show you the view from any location and elevation. From what I've seen (I haven't used it seriously), it's a bit clunky but it does the job. You can also see a plot of elevation between two points, which is useful if you want to consider Fresnel zones in your propagation analysis. 2 It's unlikely that the transceiver at the fixed location will hear the relatively tiny signal from your phone 2km away well enough to establish a link. You need to have more power on both sides to make it work. Even then, you might need specialized, large antennas or dishes. Of course if there are no restrictions then you should be able to establish such ... 2 One way to estimate line-of-sight before you even step outside is to calculate the radio horizon. For the horizon distance in miles and antenna height in feet, the formula is \text{horizon} = 1.23 \sqrt{\text{height}} $$For kilometers and meters, it is:$$ \text{horizon} = 3.57 \sqrt{\text{height}}  The horizons of each station add. If you need a ...

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25KM is a really long distance. You need to be up fairly high to beat the curve of the earth. If you are at sea level on a beach, and your eyes are 1.7m above the ground, the horizon would be at 4.7km. Source wikipedia:Horizon In actuality both ends would need to be elevated so that the beam path passes high enough over terrain between the 2 points. A ...

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If you want to calculate the height required for a 25 km radio link. Check out this link. http://www.everythingrf.com/rf-calculators/line-of-sight-calculator This link gives not visual line of sight but also the radio horizon which is slightly longer. At 5 Ghz with sufficient gain antenna to direct the signal as close to the horizon as possible and height ...

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To add to the already given answer. Most countries have a 100mW / 20dBm ERP maximum for license free usage of those bands. (You will have to check if this is your case, but chances are) As this is usually expressed in "ERP" which is "Effective Radiated Power" it will take antenna gains into account. Example by using an antenna with 3dBi gain, you will have ...

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Try using the Friis equation for questions such as this. See http://en.wikipedia.org/wiki/Friis_transmission_equation for more information.

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