137 MHz NOAA Weather Satellite Band Engineering Guide & Solvers
VHF APT weather satellite reception from NOAA-15, NOAA-18, and NOAA-19 satellites.
Dedicated Calculators Tuned for 137.5 MHz
137.5 MHz Wavelength
Compute resonant wavelength element lengths, matching specs, and RF parameter formulas for 137.5 MHz.
137.5 MHz Half-Wave Dipole
Compute resonant half-wave dipole element lengths, matching specs, and RF parameter formulas for 137.5 MHz.
137.5 MHz Quarter-Wave Vertical
Compute resonant quarter-wave vertical element lengths, matching specs, and RF parameter formulas for 137.5 MHz.
137.5 MHz Yagi-Uda Array
Compute resonant yagi-uda array element lengths, matching specs, and RF parameter formulas for 137.5 MHz.
137.5 MHz Free Space Path Loss
Compute resonant free space path loss element lengths, matching specs, and RF parameter formulas for 137.5 MHz.
137.5 MHz Atmospheric & Terrain Propagation Dynamics
Signals at 137.5 MHz propagate primarily via line-of-sight space waves, with terrain diffraction and atmospheric refraction determining coverage beyond the optical horizon.
- Free-space wavelength of 2.180 meters (218.0 cm) allows compact, high-efficiency antenna elements.
- Geometric path loss is 75.2 dB at 1 kilometer and 95.2 dB at 10 kilometers in unobstructed free space.
- RF skin depth in copper conductors is 5.63 micrometers, requiring high-conductivity plating on high-power tank circuits.
- Tropospheric bending extends the radio horizon approximately 15% beyond optical line-of-sight under standard 4/3 k-factor atmospheric refraction.
Resonant Antenna Cut Dimensions at 137.5 MHz
| Antenna Type | Metric Dimension | Imperial Dimension | Workshop & Tuning Advice |
|---|---|---|---|
| Half-Wave Center-Fed Dipole | 1.036 m (0.518 m per leg) | 3.40 ft (1.70 ft per leg) | Includes 0.95 physical end-effect shortening. Cut 3% longer for tuning with an antenna analyzer. |
| Quarter-Wave Ground Plane Vertical | 0.518 m radiator height | 1.70 ft radiator height | Equip with 4 downward-sloping radials (45-degree droop) to match 50 Ohm coax directly. |
| 5/8-Wave High-Gain Vertical | 1.295 m radiator | 4.25 ft radiator | Provides approximately 3 dB of omnidirectional gain with low take-off angle; requires a base series matching inductor. |
Coaxial Feedline Attenuation at 137.5 MHz (per 100 Feet)
| Coaxial Cable Model | Loss / 100 ft | Power Delivered | Engineering Recommendation |
|---|---|---|---|
| RG-58 (Thin Coax) | 4.5 dB | 35% | Acceptable for short patch cords under 20 feet. |
| LMR-400 (Low-Loss Foam) | 1.0 dB | 79% | Recommended standard for base station antenna feedline runs up to 100 feet. |
| 1/2" Heliax Hardline | 0.4 dB | 91% | Optimal for commercial repeater masts and weak-signal satellite stations. |