Sharpen voice in why-signals-bend

content/blog/why-signals-bend.md

@@ -10,7 +10,7 @@ draft = false
 
 A line-of-sight microwave link is easy to reason about. Two antennas, straight line, if nothing touches the line the path works.
 
-Ham radio doesn't cooperate with that story. People working the [ARRL 10 GHz and Up contest](https://www.arrl.org/10-ghz-up) log contacts over hundreds of kilometers where a string between the two stations would go straight through an Oklahoma mesa. The path profile looks broken. The contact happens anyway.
+Ham radio doesn't cooperate with that story. People working the [ARRL 10 GHz and Up contest](https://www.arrl.org/10-ghz-up) log contacts over hundreds of kilometers where a string between the two stations would go straight through an Oklahoma mesa. The path profile looks broken. The contact happens anyway. The universe has, as usual, not consulted the path profile.
 
 Below, one layer at a time, with scenes you can interact with. We start from the straight-line-and-obstacle model and break it four times.
 
@@ -20,7 +20,7 @@ Two antennas, flat ground, a hill in the middle. If the hill sticks above the li
 
 <div id="scene-los"></div>
 
-That model is how nearly every commercial point-to-point microwave link is designed. WISPs, utility SCADA, carrier backhaul, all of it. It hides two lies though. The Earth isn't flat, and the signal isn't a line.
+That model is how nearly every commercial point-to-point microwave link is designed. WISPs, utility SCADA, carrier backhaul, all of it. It hides two small lies that the rest of this post is mostly about. The Earth isn't flat, and the signal isn't a line.
 
 ## Earth gets in the way too
 
@@ -38,7 +38,7 @@ The ellipsoid is fattest at the middle of the path. Its radius there is √(λ·
 
 <div id="scene-fresnel"></div>
 
-This is why microwave links behave so much worse than HF over the same obstructed path. Higher frequency, skinnier ellipsoid, easier to clip with a ridge that doesn't look like much on the path profile.
+This is why microwave links behave so much worse than HF over the same obstructed path. Higher frequency, skinnier ellipsoid, easier to clip with a ridge that doesn't look like much on the path profile. The ridge does not care how it looks on the path profile.
 
 ## Ridges leak signal
 
@@ -54,11 +54,11 @@ Knife edges are a convenient fiction. A rounded summit scatters more energy away
 
 <div id="scene-round"></div>
 
-This is where path profile software starts lying to you. If it says a ridge clears by 5 meters and you'll be +3 dB over the noise floor, you can usually subtract another 5–10 dB once you account for the rounding. Most of the time that subtraction puts the link under water. Except the link works. So something else is going on.
+This is where path profile software starts lying to you, politely and with conviction. If it says a ridge clears by 5 meters and you'll be +3 dB over the noise floor, you can usually subtract another 5–10 dB once you account for the rounding. Most of the time that subtraction puts the link under water. Except the link works. So something else is going on.
 
 ## Air isn't empty
 
-Microwave waves travel a hair slower through air than through vacuum, and the slowness changes with pressure, temperature, and humidity. Radio engineers express it as N-units:
+Air, it turns out, cares quite a bit about radio waves. Microwaves travel a hair slower through it than through vacuum, and the slowness changes with pressure, temperature, and humidity. Radio engineers express it as N-units:
 
 > N = (n − 1) · 10⁶
 
@@ -70,7 +70,7 @@ The dashed line is the international standard atmosphere. Drag the slope and the
 
 ## Rays follow the gradient
 
-A horizontal ray in a medium whose refractive index falls with altitude bends downward. The tighter the gradient, the tighter the curve. Under the standard atmosphere the bend is gentle, about a quarter of Earth's own curvature, and engineers paper over it by pretending the Earth is 4/3 its real size and the rays are straight. That's the familiar k = 4/3 factor.
+A horizontal ray in a medium whose refractive index falls with altitude bends downward. The tighter the gradient, the tighter the curve. Under the standard atmosphere the bend is gentle, about a quarter of Earth's own curvature, and engineers handle this by quietly pretending the Earth is 4/3 its real size and the rays are straight. Nobody points out that this is a slightly strange thing to do. That's the familiar k = 4/3 factor.
 
 <div id="scene-raytrace"></div>
 
@@ -84,7 +84,7 @@ A *duct* is a layer of atmosphere where the refractivity drops steeply enough th
 
 <div id="scene-duct"></div>
 
-Green paths in the scene are the rays (ray = the direction the wavefront is traveling) that stay trapped inside the duct. Orange ones escape. The "critical angle" is the elevation window that stays trapped, and it's tiny, usually a fraction of a degree. That's why ducting is so fickle. Your antenna has to be radiating useful power inside that window, *and* the duct has to exist above your station, *and* the station on the other end has to be inside it or close enough for its energy to enter. When all three line up, your 10 GHz rig works a contact 400 km away. When any one of them is off, the path behaves like the terrain profile says it should.
+Green paths in the scene are the rays (ray = the direction the wavefront is traveling) that stay trapped inside the duct. Orange ones escape. The "critical angle" is the elevation window that stays trapped, and it's tiny, usually a fraction of a degree. That's why ducting is so fickle. Your antenna has to be radiating useful power inside that window, *and* the duct has to exist above your station, *and* the station on the other end has to be inside it or close enough for its energy to enter. When all three line up, your 10 GHz rig works a contact 400 km away. When any one of them is off, the path behaves the way the terrain profile says it should, which is to say, not at all. The atmosphere is happy to support this arrangement, briefly, before rearranging itself into something less interesting.
 
 ## Flavors of duct
 
@@ -96,13 +96,13 @@ Surface ducts form overnight as the ground cools faster than the air above it, a
 
 ## A real path
 
-This is the 65 km path I've been staring at: my QTH to the W5HN/B beacon on top of the TWU dorms in Denton. Terrain heights are the actual SRTM elevations along the great circle between the two antennas. Earth's curvature (using the standard k = 4/3 effective Earth) is added on top, which is why the middle of the profile bulges up about 62 m above the endpoints. The terrain obstructs the straight LOS. On the right mornings the path works anyway, because an elevated duct parks over North Texas and both ends get into it.
+This is the 65 km path I've been staring at: my QTH to the W5HN/B beacon on top of the TWU dorms in Denton. Terrain heights are the actual SRTM elevations along the great circle between the two antennas. Earth's curvature (using the standard k = 4/3 effective Earth) is added on top, which is why the middle of the profile bulges up about 62 m above the endpoints. The terrain is unambiguously in the way. On the right mornings the path works anyway, because an elevated duct parks over North Texas and both ends get into it.
 
 <div id="scene-real"></div>
 
 Figuring out whether that duct is there on any given day is the whole point of <https://prop.w5isp.com>. The app pulls HRRR at native hybrid-sigma levels, corrects against ASOS and radiosonde profiles, and computes the refractivity gradient along every path we track. When a duct is sitting on the path the score lights up. When it isn't, the path should behave the way the terrain profile says.
 
-The part that keeps me working on it is how often it *doesn't* behave that way. Ducting, super-refraction, and enhanced tropo scatter combine in ways that still surprise the old guys who've been operating these bands for forty years. The more verified contacts we log with good atmospheric data behind them, the better the model gets.
+The part that keeps me working on it is how often it *doesn't* behave that way. Ducting, super-refraction, and enhanced tropo scatter combine in ways that still surprise people who've been operating these bands for forty years, which is both a bit humbling and a strong argument against ever finishing this project. The more verified contacts we log with good atmospheric data behind them, the better the model gets.
 
 If you run 50 MHz or up, submit your contacts or beacons at <https://prop.w5isp.com>. A continuous, known-distance path is worth a lot more to the model than a pile of contest contacts.