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Is there a way to use radio to restrict communication to optical line of site at distances greater than 10 miles?

For example, the signal is transmitted directionally from a mountain top fire lookout, only to be received if the transmitter was also able to hit the receiver with a laser beam. Well, there could be some tolerance for obstructions but not if the receiver is hidden by a hill.

If possible, what frequency range and equipment would be used? It doesn't matter if it's voice or data based communication... I'm more wondering if/how radio is used like this.

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    $\begingroup$ Can you explain what specifically you want to restrict? man-in-the-middling? Eavesdropping? Or just make overwhelming the receiver with an unauthorized signal harder? There's a few simple physical answers here (your laser is just a really high frequency radio transmitter, if you will, and laser comms are very common), but I'm not sure they actually solve the problem you're wondering about. Which I'm not quite sure what it is – could you maybe tell us a bit about the context about why you're wondering? This feels like a very interesting question that's just being half-asked :) $\endgroup$
    – Marcus Müller
    Commented Nov 13, 2023 at 11:02
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    $\begingroup$ The topic is half-baked in my head so it makes sense it seems "half-asked". =D This and the other answer has me re-thinking the way I asked. I'm not worried about security. I just want to know if/how radio would be used to determine if there is a clear path to the receiver for distances of 10 to 20 (or more) miles. $\endgroup$
    – W6M
    Commented Nov 13, 2023 at 20:37
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    $\begingroup$ Ah! Tell me if this captures the essence of what you want to know: "How can I use radio to determine if line of sight exists at a distance beyond what the naked eye can see?" $\endgroup$
    – webmarc
    Commented Nov 13, 2023 at 21:08
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    $\begingroup$ Radio doesn't travel along the line of sight, it bends towards the earth as it travels (due to the moisture gradient and air pressure gradient) so it can go 'over' obstacles. Also all EM waves need a clear region to travel in - they effectively fill the ellipsoid with path length similar to the direct path. Light too but the 'Fresnel zone" is much smaller. So Radio Line Of Sight is not the same as optical line of sight. Can you define your terms more clearly? $\endgroup$
    – tomnexus
    Commented Nov 14, 2023 at 2:40
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    $\begingroup$ Many radio propagation modes exist that are not line-of-sight. If you want to know if a target is line-of-sight, use an optical source and detector...using sunlight with a mirror can work as a source (with Mk 1 eyeball as detector), even for long distances. $\endgroup$
    – glen_geek
    Commented Nov 14, 2023 at 15:02

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if you meant to ask:

How can I use radio to determine if line-of-sight exists at a distance beyond what the naked eye can see?

then I would say:

Using a frequency of something with sufficiently narrow beam-width, yes, you can determine if pracical line-of-sight exists between two points.

A laser may be sufficiently narrow... but it may also be too narrow in that the aiming precision required may give the impression that you don't have line of sight when, in fact, you just aren't able to suitably control targeting of the transmitter and receiver.

Microwave dishes, UHF yagi arrays, or even just a single UHF yagi may be 1) of sufficient beam-width to make aiming feasible and 2) of sufficient narrowness for the purpose.

Given modern ability to measure precise location with GPS and calculate relevant antenna bearings, you might even look into using computer calibrated and controlled antennas to accomplish this.

The actual distance between your 2 points will matter, as will atmospherics (temperature, humidity, precipitation, etc) and other environmental factors.

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    $\begingroup$ I appreciate the thoughtful response and apologize if I misled you, but I'm not concerned about security. Microwaves might be the answer I was seeking. Another way to put is is that I'm wondering if radio could be used to determine if I would have line-of-site to the receiver. For example, the transmission would only succeed if I could see the receiver as if I had super-human vision to see over miles where factors like night or distortion weren't an issue. $\endgroup$
    – W6M
    Commented Nov 13, 2023 at 20:14
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    $\begingroup$ Updated answer to reflect new understanding of the intended question :-) $\endgroup$
    – webmarc
    Commented Nov 13, 2023 at 21:16
  • $\begingroup$ @webmarc how does your scenario rule out a single reflected path, or a receiver antenna, a few kilometer of coax and then another transmit antenna? $\endgroup$
    – Marcus Müller
    Commented Nov 14, 2023 at 2:26
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    $\begingroup$ I'm not sure how beam width matters. An omni-directional source would be fine too. Just as you might use a bright naked light bulb on the mountaintop. $\endgroup$
    – tomnexus
    Commented Nov 14, 2023 at 2:44
  • $\begingroup$ @MarcusMüller it does not rule out either. $\endgroup$
    – webmarc
    Commented Nov 14, 2023 at 4:05
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Both radio waves and light waves are EM waves. The only difference between them is frequency (1MHz to 300GHz vs. 400+THz).

Different frequencies refract in different ways. Lower frequencies refract towards the earth more. (This is somewhat linear.)

Different frequencies pass through obstructions or absorbed or reflect off of things differently. This is very nonlinear with specific ranges of frequencies behaving very differently than other ranges.

So, for instance, light (with a wavelength much smaller than a leaf) is completely absorbed by trees, where VHF and UHF is slightly absorbed by trees, and HF is barely absorbed by trees. In the 2-20GHz range, humidity attenuates the signal very significantly, and in the 10GHz-20GHz range, you can easily bounce (or scatter) signals off of raindrops.

Similarly, the HF range bounces off the ionosphere, where for frequencies above roughly 10MHz (depending on space weather), the ionosphere starts becoming transparent. But then there are random absorption bands for the atmosphere. For instance, some IR, some UV, and most x-rays are absorbed by parts or all of the atmosphere and it is transparent for in between frequencies.

Also, how much you can focus a beam depends on the relationship between the size of the focusing elements (antenna, dish, lens, etc.) and the wavelength in use. In general, to get a tight focus, the dish diameter has to be larger than the wavelength. So it is much easier to get a tighter focus with a higher frequency.

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