Mastodawn

okay so let's think this out

I have a UHF antenna at a not well known height pinging a location gmaps says is 1084 meters away (line of sight).

We'll call the altitude of that location 0m to simplify maths. Openmaps has it on the low side of a 20m line. That matters to get actual altitude at the end; at most, it's another 2-3m down probably?

At 850m (straight-line) from my 0m location, a signal clears a +70m hill on its way to either end.

So start at 0m and create a right triangle with the signal path, and 70/850 should be the tan() of the angle from the 0m point. Therefore 70/850 = X/1084, and doing the maths gets you X = 90m.

90+20=110m, ish.

But my best direct estimate from the same topo maps is 93-95m.

Being at +93ishm _does_ clear the +70m hill (90m actual) but not at an angle that should let me see the 0m points - but I do.

So.

Bouncy signals? Bad maths? Weird RF-over-angle behaviours? (Same kind of thing gets you picket fencing)

I dunno.

I don't think I'm fucking up my maths but am I?

Or is this "don't mix google maps and open maps it only leads to pain"?

#gmrs #AmateurRadio

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AN/CRM-114 3d ago

@moira ACHIEVEMENT UNLOCKED: knife-edge diffraction

https://www.raymaps.com/index.php/knife-edge-diffraction-model/

Knife Edge Diffraction Model – RAYmaps

Diffraction is a phenomenon where electromagnetic waves (such as light waves) bend around corners to reach places which are otherwise not reachable i.e. not in the line of sight. In technical jargon such regions are also called shadowed regions (the term again drawn from the physics of light). This phenomenon can be explained by Huygen's principle which states that "as a plane wave propagates in a particular direction each new point along the wavefront is a source of secondary waves".

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Solarbird

3d ago

@flyingsaceur That's (part of) the thing that causes picket fencing, innit? Or am I conflating things that I shouldn't be conflating?

anyway that diagramme is helpful thanks, it's a bigger angle add than i expected!

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AN/CRM-114

@moira as I remember it, picket fencing is the result of multipath, when you receive the same signal travelling different distances and arriving out of phase

Knife edge diffraction creates a signal path akin to the light “shining” around a corner. So you’re correct