How Radio Waves Really Travel: Multipath, Fading, Absorption, and Polarization

Concept

T3B just gave you the physics that makes a radio wave a radio wave: it travels through space at the speed of light, its wavelength and frequency are locked together by that constant, and the VHF, UHF, and HF labels you'll see throughout this course are just names for slices of that same electromagnetic spectrum. This lesson takes those clean, textbook properties and asks a much messier question: what actually happens to a wave once it leaves your antenna and has to cross real terrain, real buildings, real weather, and — for HF — a genuinely turbulent layer of the upper atmosphere, before it reaches a receiver? None of what follows changes the physics T3B established. It's the same wave, obeying the same rules, colliding with a real world that a Technician needs to understand well enough to predict when a signal will be strong, when it will fade, and which way to point an antenna to get the most out of a given path.

Concept

Start with the single biggest reason a signal's strength can swing wildly even when nothing about the transmitter or the distance has changed: more than one copy of the same signal reaching the receiving antenna at once. A direct line-of-sight path is rarely the only path available at VHF and UHF — that same signal also reflects off buildings, hillsides, water towers, even other vehicles, and each reflected copy travels a slightly different distance to get to the receiver. Because radio waves are waves, copies that arrive out of step with each other don't just pile up harmlessly; depending on exactly how far out of step they are, they can reinforce each other and briefly boost the received signal, or they can partially cancel each other and briefly weaken it. This is multipath propagation, and it explains something that seems strange at first: since a difference of just a few feet in antenna position can shift how those reflected paths line up, signal strength on VHF can genuinely swing from strong to weak over a distance far too short to blame on simple path loss. A station moving through that shifting pattern — a mobile driving past a row of buildings, for instance — hears the effect as a rapid, repeating flutter in received signal strength as it crosses in and out of spots where the reflected copies happen to reinforce or cancel. Hams have a name for that pattern: they call it picket fencing, because the rhythm of a mobile signal repeatedly strengthening and weakening as it passes each reflecting or blocking structure in turn brings to mind sunlight flickering through the evenly-spaced slats of a fence going by outside a moving car window.

Concept

HF works by a completely different mechanism than the line-of-sight paths VHF and UHF depend on, and it starts with a single region of Earth's upper atmosphere: the ionosphere, a layer beginning roughly 50 miles up where solar radiation strips electrons from atmospheric gas and leaves behind a region charged enough to bend — and, under the right conditions, fully reflect — HF radio waves back down toward the ground far beyond the horizon. The much lower stratosphere and troposphere don't carry that charge and don't reflect HF energy this way. But the ionosphere isn't one smooth mirror; it's layered, turbulent, and constantly shifting as the sun's angle and activity change, so an HF signal bouncing off it doesn't necessarily take a single clean path. Slightly different paths through a shifting, uneven ionosphere is the same multipath idea from the last block, just happening far overhead instead of off a nearby building — which is why HF skywave signals can fade in a way that seems irregular and unpredictable rather than following any simple pattern tied to distance or time of day. That same imperfect reflection does something else worth knowing: it scrambles a wave's polarization, twisting a signal that left the transmitting antenna cleanly vertical or horizontal into a mixed, constantly-rotating orientation — what's technically called elliptical polarization — by the time it reaches a receiver far away. That's a genuinely different situation from the line-of-sight polarization rules coming up next: for an ionospheric contact, it stops mattering whether the transmitting and receiving stations' antennas share the same orientation, because the ionosphere has already scrambled that distinction out of the signal before it ever arrives.

Concept

Frequency also decides how much a physical obstacle in the signal's path matters, and the general trend runs opposite to what intuition might suggest: it's the higher frequencies, with their shorter wavelengths, that lose the most energy to ordinary physical clutter along the way. Wet vegetation is a good example — leaves and branches sized close to a UHF or microwave wavelength absorb real energy out of a signal passing through them, weakening what a receiver hears well beyond what distance alone would predict, in a way a lower-frequency HF or VHF signal passing through those same trees mostly shrugs off. Precipitation follows the identical pattern for the identical reason: raindrops are physically small, so they only interact meaningfully with wavelengths in their own size range, which puts heavy rain or snow squarely in position to noticeably cut the range of a microwave link, while leaving the much longer wavelengths used on the 10-meter and 6-meter bands largely unaffected by that same storm. It's the same physical mechanism showing up twice — shorter wavelengths run into more of the physical world as an obstacle — just crossing the threshold where it actually bites at different points on the spectrum.

Concept

Polarization is the orientation of that electric field relative to the ground, set by how an antenna's elements are physically oriented, and for a genuine line-of-sight VHF or UHF contact — no ionosphere involved to scramble it away — it matters a great deal that both ends of the contact agree on it. An antenna responds most strongly to a wave whose field lines up with its own elements, so when a vertically-oriented antenna tries to receive a horizontally-polarized signal, or the reverse, it's fundamentally less sensitive to what's arriving, and the operator on the other end sees a real, measurable drop in received strength as a result — not a total blackout, but a real loss that a mismatch in orientation is fully capable of causing on its own, independent of distance or power. That's why VHF and UHF operating communities settled on conventions rather than leaving polarization to chance station by station: FM and repeater work is built almost entirely around vertical antennas, which keeps a mobile whip and a repeater's antenna automatically compatible without either operator having to think about it, while VHF and UHF operators doing direct point-to-point SSB and CW work — no repeater, no FM — have long favored horizontal orientation instead, partly because a horizontal antenna is less sensitive to the mostly-vertical electrical noise thrown off by things like power lines and vehicle ignition systems. Neither convention is a law of physics; they're agreements a Technician needs to know about so a beam or Yagi ends up pointed, and oriented, the right way for whichever mode is actually being worked.

Concept

Multipath's reflected copies aren't only a source of unwanted fading — the same physical reflection that causes fading off a building can also be put to deliberate use. When a direct line-of-sight path to a repeater or another station is blocked by terrain or a structure in the way, a directional antenna gives an operator a tool a fixed antenna doesn't have: the ability to sweep across the compass, actively hunting for some large object positioned usefully between the two stations that will bounce enough of the signal around the obstruction to complete the contact anyway, the same way aiming a flashlight at a mirror can light up a room the bulb itself can't see into. It's worth being clear about what doesn't fix a blocked path, because neither addresses the actual problem: switching which way an antenna's elements are oriented corrects a polarization mismatch, not a blocked line of sight, so it does nothing here; and pushing SWR higher is never a fix for anything — a rising SWR reading means a growing mismatch between the feed line and the antenna, a separate problem entirely, and one that only gets worse, never better, and never had anything to do with clearing an obstruction in the first place.

Concept

That same multipath idea has one more consequence worth knowing, this time on digital modes rather than voice. A digital signal encodes information in precisely-timed transitions, and when several time-shifted copies of that same signal land on a receiver at once, the receiver has to sort out which transition belongs to which copy — a much harder problem than picking a single winner out of two overlapping voice signals fading in and out. The practical result tends to show up as a real increase in how often the receiver gets a bit wrong, since the decoder is effectively being asked to untangle overlapping, time-smeared versions of the same data rather than reading one clean copy — a consequence of multipath being present at all, not something that scales in any fixed, predictable way with exactly how many separate paths happen to exist.

Concept

Put these threads together and a Technician has a working model of why the same station, at the same power, can sound completely different from one moment or one location to the next. Multiple copies of a signal taking different paths — nearby reflections at VHF and UHF, or an uneven ionosphere at HF — reinforce or cancel each other and produce fading, sometimes gently, sometimes as the rapid picket-fencing flutter a mobile station hears passing through a cluttered area. Frequency decides how much ordinary obstacles like foliage or rain get to interfere along the way, with the shorter wavelengths at UHF and microwave losing noticeably more to both than the longer wavelengths below them do. And polarization — a straightforward matter of matching antenna orientation for a line-of-sight contact, a non-issue once the ionosphere has already scrambled it for a skywave contact — rounds out the picture of what an antenna has to be pointed at, and oriented like, to make the most of a given path. That's also as far as multipath and polarization carry the story on their own; the next lesson turns to the specific mechanisms — sporadic E, meteor scatter, aurora, tropospheric ducting — that can open up an unusually long-distance path in the first place, along with the ordinary line-of-sight limit every VHF and UHF signal is working against the rest of the time.