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Why do line of sight communication links have a restricted range for a given tower height? Is it only because of the Earth's Curvature?

mike65535
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Muhammad Ahmad
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2 Answers2

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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.

Scott Earle
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    RE: " 'line of sight' communication is just that: if you can see it, you can communicate." — However, due to propagation effects for VHF+ frequencies, Earth's radio horizon is somewhat further away from the transmit antenna than the optical horizon, so useful reception range may be greater than expected. – Richard Fry Jun 10 '19 at 10:23
  • And for microwaves, less so. Since specifically “line of sight” was asked about, that’s what I answered. If it’s HF then lots of propagation modes come into effect ... – Scott Earle Jun 10 '19 at 10:49
  • @RichardFry I'd like to explicitly point out that "propagation reaching a target without reflection" is not the same as "line of sight". These are simply two different terms, and shouldn't be interchanged. I specifically dislike the way "radio horizon" is used: it confuses both concepts. It's badly defined – your radio horizon is further away if your receiver is better. That's a useful measure, but not a property of propagation, but of your overall system :) – Marcus Müller Jun 10 '19 at 11:38
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    The question asked specifically about line-of-sight, and that’s what I answered – Scott Earle Jun 10 '19 at 11:40
  • @RichardFry sadly, the useful system "how far can I communicate successfully" measure is not used normally when people refer to "radio horizon". They just use a magical formula from a hard-to-come-by 1967 publication that claims 15% longer service range than line-of-sight range. Well, that applies to some statistics of the specific system operating at the specific bands under the specific atmospheric conditions the author observed. The receivers used in 1967 are technically not as cool as modern receivers, and not everything uses the same modes as the author presumed. – Marcus Müller Jun 10 '19 at 11:47
  • @ScottEarle I'm fully with you on that one :) – Marcus Müller Jun 10 '19 at 11:47
  • It isn't too hard to derive the true line of sight limit equation based on antenna elevations alone. If you want radio horizon, it adds one simple term that based on frequency. The popular equation for this is an approximation that takes both into account. – user10489 Jun 12 '19 at 12:13
  • Even optical line of sight is not a geometric constant on Earth. Depending on the color of the light and any atmospheric temperature gradients or inversions over the path, light can bend over the horizon, or even upward, shortening the visible horizon. Closer to a geometric constant in the vacuum on the moon (unless you find yourself in a crater). – hotpaw2 Jun 14 '19 at 23:02
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In addition to earth's curvature, obstacles in the various Fresnel Zones may reduce received signal strength, and hence lower effective range

Phil Freedenberg
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