T3 — Radio Wave Propagation
3 exam questions · 3 groups · 34 questions in pool
How signals get from your antenna to someone else’s: the physical nature of a radio wave, how wavelength relates to frequency, and the modes (line-of-sight, ionospheric, sporadic-E, tropo, and so on) that carry signals near and far. Pure physics — no FCC citations.
T3A — Radio Wave Characteristics; Antenna Orientation
12 questions
What this group tests: the everyday behavior of signals — fading, multipath, polarization, absorption — especially at VHF/UHF.
Foundational concepts
When you move a VHF antenna a few feet and the signal jumps around, that’s multipath: copies of the signal arrive via different paths and either reinforce or cancel each other. Multipath on a moving vehicle produces a rapid flutter called “picket fencing,” and on data it raises the error rate. Ionospheric signals fade irregularly for the same underlying reason — random combining of signals arriving by different paths.
Polarization is the orientation of the wave’s electric field, set by the antenna. For long-distance CW/SSB weak-signal work on VHF/UHF the convention is horizontal polarization. If the two ends of a line-of-sight link use mismatched polarization, the received signal is weaker. (Ionospheric signals are elliptically polarized, so for sky-wave paths either vertical or horizontal antennas work.)
Absorption is the other big theme. Vegetation absorbs UHF and microwave energy; precipitation cuts microwave range; but ordinary fog and rain barely affect 10 m and 6 m, because lower frequencies aren’t absorbed by water droplets the way microwaves are. The ionosphere is the atmospheric region that can refract (bend) HF and VHF waves. And if a building blocks your path to a repeater, a directional antenna aimed at a reflecting surface can bounce the signal around the obstruction.
Key facts to retain
- Multipath = signals via multiple paths reinforce/cancel → fading, picket fencing, higher data error rates.
- Polarization = electric-field orientation; VHF/UHF weak-signal work uses horizontal.
- Mismatched polarization on line-of-sight → reduced signal.
- Absorption: vegetation/precip hurt UHF–microwave; fog/rain barely affect 10 m & 6 m.
- The ionosphere refracts HF/VHF.
External reference anchors
- NCVEC syllabus: T3A — Radio wave characteristics; Antenna orientation
Per-question map
| Q | Asks for | Resolved by |
|---|---|---|
| T3A01 | VHF strength varies with small moves | Multipath cancel/reinforce |
| T3A02 | Vegetation effect on UHF/microwave | Absorption |
| T3A03 | Polarization for VHF/UHF weak-signal | Horizontal |
| T3A04 | Mismatched polarization, line-of-sight | Reduced signal strength |
| T3A05 | Reaching a blocked repeater | Use a reflected path |
| T3A06 | “Picket fencing” | Rapid mobile flutter from multipath |
| T3A07 | Weather cutting microwave range | Precipitation |
| T3A08 | Irregular ionospheric fading | Random multi-path combining |
| T3A09 | Result of elliptical polarization | Either V or H antenna works |
| T3A10 | Multipath effect on data | Error rates increase |
| T3A11 | Region that refracts HF/VHF | The ionosphere |
| T3A12 | Fog/rain on 10 m & 6 m | Little effect |
T3B — Electromagnetic Wave Properties
11 questions
What this group tests: the structure of an EM wave, the speed of light, and the wavelength↔frequency relationship — including the one formula you must be able to use.
Foundational concepts
A radio wave is two fields — electric and magnetic — oscillating at right angles to each other (and to the direction of travel). Its polarization is defined by the electric field’s orientation. In free space the wave travels at the speed of light, about 300,000,000 meters per second (3 × 10⁸ m/s).
From that constant speed comes the inverse relationship: as frequency increases, wavelength gets shorter. The handy approximation to memorize is
wavelength (meters) ≈ 300 ÷ frequency (MHz)
This is why bands are named by approximate wavelength: 300 ÷ 146 MHz ≈ 2 meters, 300 ÷ 28 MHz ≈ 10 meters, and so on — the wavelength name and the frequency are two views of the same thing.
Finally, the spectrum labels by frequency: HF = 3–30 MHz, VHF = 30–300 MHz, UHF = 300–3000 MHz. Notice each band is a factor of ten wide and they tile neatly end to end.
Key facts to retain
- EM wave = electric + magnetic fields, at right angles; polarization = E-field orientation.
- Speed in free space = speed of light ≈ 3 × 10⁸ m/s.
- λ(m) = 300 ÷ f(MHz); higher frequency → shorter wavelength.
- HF 3–30 MHz, VHF 30–300 MHz, UHF 300–3000 MHz.
External reference anchors
- NCVEC syllabus: T3B — Electromagnetic wave properties; spectrum definitions
Per-question map
| Q | Asks for | Resolved by |
|---|---|---|
| T3B01 | E and H field relationship | At right angles |
| T3B02 | What defines polarization | Orientation of the electric field |
| T3B03 | Two components of a wave | Electric and magnetic fields |
| T3B04 | Velocity in free space | Speed of light |
| T3B05 | Wavelength vs frequency | Shorter as frequency rises |
| T3B06 | Frequency→wavelength formula | λ = 300 / f(MHz) |
| T3B07 | Other band identifier | Approximate wavelength in meters |
| T3B08 | VHF range | 30–300 MHz |
| T3B09 | UHF range | 300–3000 MHz |
| T3B10 | HF range | 3–30 MHz |
| T3B11 | Velocity of a radio wave | 300,000,000 m/s |
T3C — Propagation Modes; F-Region Skip; Line of Sight & Radio Horizon
11 questions
What this group tests: the named propagation modes that carry VHF/UHF beyond line of sight, and how HF differs from VHF.
Foundational concepts
The baseline for VHF/UHF is line of sight: UHF signals are rarely heard past the radio horizon because they’re usually not refracted by the ionosphere. But the radio horizon is slightly farther than the visual horizon, because the atmosphere refracts (bends) the waves a little.
Several modes push VHF/UHF beyond the horizon. Sporadic-E produces occasional strong signals on 10, 6, and 2 m from beyond the horizon. Tropospheric ducting, caused by temperature inversions, gives regular ~300-mile VHF/UHF paths. Auroral backscatter off the aurora gives distorted, fluttery signals with widely varying strength. Meteor scatter bounces signals off ionized meteor trails and works best on 6 meters. Knife-edge diffraction lets signals bend over a sharp obstruction like a ridgeline.
On HF, long-distance ionospheric (F-region) propagation is far more common than at VHF. During the peak of the sunspot cycle the F region can carry 6 and 10 meters long distances, with the best 10 m openings running from dawn until shortly after sunset during high solar activity.
Key facts to retain
- UHF rarely beyond the horizon (no ionospheric refraction); radio horizon > visual horizon (atmospheric refraction).
- Sporadic-E → 10/6/2 m surprises; tropo ducting (temperature inversions) → ~300 mi.
- Auroral = distorted/variable; meteor scatter best on 6 m; knife-edge diffraction bends over obstacles.
- HF favors F-region long haul; sunspot peak opens 6 & 10 m, best 10 m = dawn→sunset.
External reference anchors
- NCVEC syllabus: T3C — Propagation modes; F region skip; Line of sight and radio horizon
Per-question map
| Q | Asks for | Resolved by |
|---|---|---|
| T3C01 | UHF rarely past horizon | Not propagated by the ionosphere |
| T3C02 | HF vs VHF characteristic | Ionospheric long-distance far more common |
| T3C03 | Auroral backscatter trait | Distorted, varying strength |
| T3C04 | Strong beyond-horizon on 10/6/2 m | Sporadic-E |
| T3C05 | Signals around obstructions | Knife-edge diffraction |
| T3C06 | Regular ~300 mi VHF/UHF | Tropospheric ducting |
| T3C07 | Best band for meteor scatter | 6 meters |
| T3C08 | Cause of tropo ducting | Temperature inversions |
| T3C09 | Best time for 10 m F-region | Dawn to just after sunset, high sunspots |
| T3C10 | Bands for F-region at sunspot peak | 6 and 10 meters |
| T3C11 | Radio horizon > visual horizon | Atmosphere refracts waves slightly |