Rules of Thumb Cheat Sheet

1. Transmission Lines

• Inductance: ~1 nH per mm (~25 nH per inch) of wire/trace in free space.
• Capacitance: ~1 pF per inch for 50 ohm trace on FR4.
• Delay (Free Space): 1 ns per foot (approx 30 cm).
• Delay (FR4): ~15 cm (6 inches) per nanosecond ($\epsilon_r \approx 4$).
• 50 Ohms: Generic trace width is ~2x dielectric height (Microstrip on FR4).

2. Power & Decibels

• +3 dB = $2\times$ Power.
• +10 dB = $10\times$ Power.
• +30 dBm = 1 Watt.
• 0 dBm = 1 milliwatt.
• -174 dBm/Hz: Thermal Noise Floor at room temperature ($kT$).

3. Passive Components

• Resistors: Have parasitic capacitance (~0.1pF). Behave like caps at high freq.
• Capacitors: Have parasitic inductance (ESL). Behave like inductors after SRF.
• Inductors: Block RF (Chokes), pass DC.
• Capacitors: Block DC, pass RF.

4. Antennas

• Dipole Size: Total length $\approx \lambda / 2$. (e.g. 15cm at 1GHz).
• Patch Size: Width $\approx \lambda / 2$ in dielectric.
• Gain: Higher gain $\rightarrow$ Narrower beam (Balloon squeeze).