slop-stuff / Embedded & hardware
Radio amateur
The airwaves and the math — region-tagged.
Bands by ITU region, propagation, antennas, SWR/Γ, feedline, modes, safety, and RF kill-list.
Amateur radio is licensed RF: build, experiment, communicate. National rules win. Band edges below are orientation from ITU RR + IARU plans — not your privileges. Deep SDR → sibling sdr/.
Quick reference
λ(m) = 300 / f(MHz)
Free-space λ/2: 150/f m · 492/f ft
Practical dipole: 143/f m ≈ 468/f ft # ~0.95×; start long, prune
λ/4 vertical: 71.5/f m ≈ 234/f ft # needs radials
|Γ| = (S−1)/(S+1)
RL = −20·log10(|Γ|)
refl% = |Γ|² × 100
dBi ≈ dBd + 2.15
EIRP ≈ ERP + 2.15 dB
| SWR | refl% | RL | Vibe |
|---|---|---|---|
| 1.5 | 4% | ~14 dB | fine |
| 2.0 | ~11% | ~9.5 | good enough |
| 3.0 | 25% | ~6 | fix / tuner |
| 10 | ~67% | ~1.7 | stop |
Bands (orientation, not privileges)
Tag ITU R1 / R2 / R3. Frame: ITU RR Art.5 + IARU regional plans + your national license.
| Band | Core | Region edges (approx) | Typical use |
|---|---|---|---|
| 80 m | 3.5↑ | R1→3.8; R2→4.0; R3→3.9 | night regional; NVIS |
| 40 m | 7.0↑ | R1→7.2; R2→7.3 | night DX; day NVIS |
| 20 m | 14.0–14.35 | widely harmonized | daytime DX |
| 10 m | 28–29.7 | FM near top (national) | solar-max DX + local FM |
| 6 m | 50–54 | R1 often 50–52; R2 50–54 | Es “magic band” |
| 2 m | 144↑ | R1 144–146; R2 144–148 | LOS, repeaters, weak-signal |
| 70 cm | 430-class | R1 430–440; R2 420–450 | repeaters, DV, satellites |
| 60 / 4 / 1.25 m | — | highly national — verify locally | don’t invent global rights |
KEY: US Tech/General/Extra edges and ID rules are not world law. Check your regulator + IARU region plan.
Propagation
| Mode | One-liner |
|---|---|
| Groundwave | hugs earth; dies with frequency / terrain |
| Skywave | ionosphere bounce; HF DX |
| MUF | above it → into space |
| LUF | below it → D-layer absorption |
| NVIS | high-angle 40/80/60; low antenna; regional |
| LOS | VHF/UHF optical horizon (+ radio horizon) |
| Knife-edge | diffraction over ridges |
| Troposcatter | weak beyond-LOS VHF/UHF |
| Sporadic-E | short-skip bursts; 6 m / 10 m |
| Cross-pol | ~20 dB hit if mismatched |
VHF is not always LOS-only — tropo, Es, EME exist. Still plan LOS for FM repeaters.
Antennas
λ(m) = 300 / f(MHz)
# practical wire dipole (~0.95 velocity): start long, prune for SWR/resonance
L_dipole_m ≈ 143 / f
L_dipole_ft ≈ 468 / f
L_λ/4_m ≈ 71.5 / f
L_λ/4_ft ≈ 234 / f
| Idea | Rule |
|---|---|
| Dipole free-space | ~73 Ω; ≈ 2.15 dBi = 0 dBd |
| Gain | don’t mix dBi and dBd |
| Power | EIRP vs ERP differ by ~2.15 dB |
| Vertical | needs radials / counterpoise |
| Pattern | height and ground change everything |
SWR / Γ / RL
Low SWR ≠ great antenna (a dummy load is 1:1). Lossy coax hides a bad antenna (looks matched at the shack).
| Myth | Reality |
|---|---|
| SWR 1.5 is “bad” | Usually fine |
| High SWR always kills finals | Modern solid-state folds back; still fix the match / use a tuner carefully |
| ATU = resonant/efficient antenna | ATU matches the radio; antenna+feedline efficiency is separate |
Feedline / matching
| Line | Order-of-mag loss | Notes |
|---|---|---|
| RG-58 | ~6 dB/100′ @146 · ~11 @440 | short runs only at UHF |
| LMR-400-class | ~1.5 @146 · ~2.7 @440 | better for VHF/UHF |
| Ladder / window | low loss even at high line SWR | needs balun / balanced ATU |
1:1 current balun at a coax-fed dipole is the usual default — not “optional forever.” Tuner in the shack ≠ magic wire at full legal power on every band.
Modes
| Mode | When |
|---|---|
| CW | weak-signal / narrow |
| SSB | HF voice DX |
| FM | VHF/UHF local / repeaters |
| AM | vintage / niche |
| FT8 | DX QRP efficiency — high duty → derate PA |
| JS8 | keyboard chat |
| RTTY | contests |
| SSTV | pictures |
| WSPR | beacon / propagation science |
SDR bridge
IQ baseband · Nyquist · for complex IQ, fs ≈ RF bandwidth · transmitting still needs a license and a clean chain. Full story → sdr/.
Safety
| Topic | One-liner |
|---|---|
| Exposure | scales with power × gain × duty × distance — see OET-65 concepts, not a global rulebook |
| Tune | dummy load when possible |
| DC | fuse at the battery |
| Storms | disconnect antennas |
| RF burns / common-mode | choke / bond / don’t grab the radiator |
AI-slop kill-list
- Mixing
468/fft with150/fm constants - SWR 1.5 as “bad”
- “High SWR always kills finals”
- More power fixes a bad antenna / path
- Tuner = resonant/efficient antenna
- Low SWR = great antenna
- US band edges / license classes / ID interval as world law
- ERP = EIRP · dBi = dBd
- “VHF is always LOS-only”
- Invented global privileges on 60 / 4 / 1.25 m
- 100% duty digital at full rated PA
- Eating the whole
sdr/page here - “ATU = any wire, any band, full power, safe”
- Balun optional forever on coax dipole
Refs
- IARU · R1 band plans · R2 band plans · R3 band-plan PDF via IARU R3
- ITU Radio Regulations Art.5 (allocations)
- FCC OET Bulletin 65 + Suppl. B · ARRL RF exposure calculator
- antenna-theory.com — VSWR