RF Fundamentals
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1. Antenna Gain (dBi): Directivity, Not Power
Gain doesn't "create" energy; it focuses it. Think of it as reshaping the signal beam.
Low Gain (3dBi)
- Light Bulb: Radiates in all directions. Best for short range or moving vehicles.
High Gain (12dBi+)
- Flashlight: Narrow, focused beam. Best for long-range, stationary links.
2. Impedance: 50 Ohm vs. 75 Ohm
Incorrect impedance causes signal reflections and potential equipment damage.
| Type | Standard Application | Common Cables |
|---|---|---|
| 50 Ohm | Data & Voice (WiFi, 4G, Ham, GPS) | RG174, RG58, LMR200 |
| 75 Ohm | Video & TV (Cable TV, Satellite) | RG6, RG59, RG11 |
⚠️ Never mix 50 Ohm and 75 Ohm components.
3. Cable Loss: Length Matters
Higher frequencies experience faster signal drop (Attenuation).
Signal Loss (2.4GHz per 10ft)
- 📉 RG174 (Thin): ~3.2 dB loss — Short pigtails only.
- 📊 RG58 (Standard): ~1.5 dB loss — Mobile radio use.
- 🚀 LMR195 (Low Loss): ~1.1 dB loss — High performance.
4. What is VSWR?
Measures efficiency of power transmission to the antenna.
- 1.0 - 1.5: Excellent
- 1.5 - 2.0: Acceptable
- 3.0+: Critical