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Terrain effect on line-of-sight communications


Templeton
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I live in a pretty flat area, but I was interested to try my future (still in the mail) basic GMRS handhelds in the mountains. If I take them to the mountains at some point, I am curious about the general limitations of line of sight in simplex communications. So, in the not-to-scale drawing below, if I have two GMRS radios, say, about 1/2 to 1 mile apart in a completely treeless area, and one of the radios is on the very top of a ridge 500 ft above the other radio, it seems that 2-way communications should work fine. Or, for this example, lets just assume that it works great in that situation. So, the question involves the radio on the top of the ridge: if I move that radio down the back side of the ridge, how far in elevation does the antenna of that handheld radio have to be below the top of the ridge to make communication difficult or impossible with the other radio? Will placing that radio just 1" below (and behind) the ridge stop communication? Or 1 just foot below/behind the ridge? 10 ft below/behind the ridge? 100 ft below/behind the ridge? More? This is assuming that there is not a significant change in the distance between the radios. I know that there always specifics behind effective simplex range, but I am just curious to know how rigidly the "line of sight" rule applies when there are no other real obstacles other than terrain (i.e., earth).

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Cannot be predicted in advance. Round top of the hill will block better than sharp rocky edge. Also, signal does not have to travel the shortest distance, it can be reflected around your hill by the nearby hills.

Below is an example of fairly reliable communications between two HTs, with a lot of static. Top of the hill is sharp, and there are other taller mountains around, some with granite faces. If there is a mobile radio used from the parking lot at the west location, instead of HT, then static completely goes away.

image.thumb.png.d82a06861951d20317c775101893dfaf.png

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Another real life example. Operator 1 walks on the east-west trail, over the ridge. Operator 2 walks on the level trail that goes around. Op1 and Op2 hear each other only in few spots. But Operator 3 sits in the west location in the parking lot with mobile radio and hears both 1 and 2 all time very clear, and can relay messages. But only on mobile, the HT at Op 3 location does not hear neither Op1 nor Op2, HT transmitting by Op3 is not heard by Op1 and Op2.

 

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I agree with Alex on all points. It's too hard to predict/calculate for all but a chosen few.

And to touch a bit more or the reflection idea... think of radio waves like sunlight. There isn't perfect darkness in the shadows because the sunlight, to varying degrees, scatters off of everything around the item creating the shadow. That includes dust and other things in the air. Radio wave literally do the same thing. 

 

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8 minutes ago, axorlov said:

Operator 3 sits in the west location in the parking lot with mobile radio and hears both 1 and 2 all time very clear, and can relay messages.

And now, Templeton, we've come full-circle back to your original post "Better understanding how GMRS repeaters work". This is a real-world situation. Repeaters are "Operator 3".

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22 minutes ago, WRQC527 said:

Repeaters are "Operator 3"

Sort of, yes. The question was about terrain blocking. In the Pinnacles scenario there is no line of sight between either of the operators, somehow we still have communications. And car with mobile radio did not have any elevation advantage, it is was actually parked lower than the trails. But, yes, this trip made me to get simplex repeater (which appears to be against the rules, so shhh don't tell anyone). It works like a charm and saves the day every time when we use it.

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12 hours ago, marcspaz said:

I agree with Alex on all points. It's too hard to predict/calculate for all but a chosen few.

And to touch a bit more or the reflection idea... think of radio waves like sunlight. There isn't perfect darkness in the shadows because the sunlight, to varying degrees, scatters off of everything around the item creating the shadow. That includes dust and other things in the air. Radio wave literally do the same thing. 

 

Interesting. So, in my theoretical example, let's say that the ridge that I have drawn in my OP, for all practical purposes, is infinitely long and there is no other terrain other than what is pictured (so no terrain reflection). If the radio behind the ridge is, say, 100 or more ft below the ridge peak and simplex communication is possible, then pretty much all of the signal received/transmitted between the two radios is from atmospheric reflection? Is atmospheric reflection so significant that one could have two handheld radios on a low flat plane but completely separated by a high ridge and yet they could still communicate?

Is there any way to roughly predict/estimate how much terrain occlusion is (or would be) tolerated by GMRS 2 way communication? And if there is more dust/smoke in the atmosphere, is atmospheric reflection of GMRS radio waves improved? What about if there is fog? Or rain?

No ironclad answers are expected, just rough estimations based on your knowledge and experience. Thanks!

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35 minutes ago, Templeton said:

If the radio behind the ridge is, say, 100 or more ft below the ridge peak and simplex communication is possible, then pretty much all of the signal received/transmitted between the two radios is from atmospheric reflection?

 

If there was zero other terrain?  That would be correct.  This is exactly why RF LOS terminates about 15% further than visual line of sight.  RF is both scattered and partially refracted over the horizon.  The closer to the horizon line, the more dense the scatter is.

 

35 minutes ago, Templeton said:

Is atmospheric reflection so significant that one could have two handheld radios on a low flat plane but completely separated by a high ridge and yet they could still communicate?

 

It's really hard to answer that question.  For example, are we talking about a building?  A small hill? or a 5,000'+ mountain?  My house is in a recess in the ground by about 100 feet, with the ridge line about 150 yards away.  I can stand in my driveway with a handheld radio and still reach another handheld about a mile away before the shadowing become to great.

 

35 minutes ago, Templeton said:

Is there any way to roughly predict/estimate how much terrain occlusion is (or would be) tolerated by GMRS 2 way communication? And if there is more dust/smoke in the atmosphere, is atmospheric reflection of GMRS radio waves improved? What about if there is fog? Or rain?

 

There is a way, but it is way beyond my skill set and understanding.  The level of effort and all contributing factors vary too rapidly and greatly to attempt to try to calculate it in some practical time-frame, as well.  To make things worse, the results would only be valid for the exact forecasted locations of the two radios.  If you move either radio even an inch in any direction, the values are no loner valid.

 

Items in the sky like rain and fog will attenuate the signal more.  While diffraction, reflection and scattering of radio waves is the principle behind extending LOS, as those conditions increase, the amount of dilution increases, leading to more attenuation, not more extended range.  The PPM of the photons simply drops too low to receive. 

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Remember people....GMRS operates in FM mode (F*@king Magic); therefore you may have comms in areas where propagation software predicts that it is impossible and that the inverse is also true. I think that snow/ice covered terrain may enhance the reflection effect that granite and other minerals create. BLUF...get out there and experiment!

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