Question E5D02:
Why is it important to keep lead lengths short for components used in circuits for VHF and above?  

Choices:  
A: To increase the thermal time constant  
B: To minimize inductive reactance  
C: To maintain component lifetime  
D: All these choices are correct    

Correct Answer is  B: To minimize inductive reactance  

Correct answer: B — To minimize inductive reactance

Why B is correct:
- Component leads behave like small inductors at RF. The reactance of this parasitic inductance grows with frequency:
  $$X_L = 2\pi f L$$
- At VHF and above, even tiny lead inductances are significant. Rule of thumb: a straight lead is about $1\ \text{nH/mm}$ (≈ $25\ \text{nH/in}$).
  - Example: $L = 10\ \text{nH}$ at $f = 100\ \text{MHz}$ gives $X_L = 2\pi(100\times10^6)(10\times10^{-9}) \approx 6.3\ \Omega$.
  - A 1-inch lead ($\sim25\ \text{nH}$) at $100\ \text{MHz}$ gives $X_L \approx 16\ \Omega$, and at $500\ \text{MHz}$ about $80\ \Omega$.
- Such unintended inductance detunes filters/matching networks, reduces gain, and can cause instability. Keeping leads short minimizes $L$ and thus $X_L$.

Why the other choices are incorrect:
- A: To increase the thermal time constant  
  Thermal behavior depends mainly on mass and thermal resistance. Lead length is not kept short to increase thermal time constant, and that’s not a design goal for VHF circuits.
- C: To maintain component lifetime  
  Lifetime is driven by temperature and electrical stress, not lead length. Short leads don’t inherently extend lifetime.
- D: All these choices are correct  
  False because A and C are not the reasons for short leads in VHF/UHF design.

Useful rules of thumb:
- Lead inductance: approximately $1\ \text{nH/mm}$ (≈ $25\ \text{nH/in}$).
- Inductive reactance: $$X_L = 2\pi f L$$
- Keep leads short and loop areas small to reduce parasitic inductance (and as a side benefit, stray capacitance and unintended radiation).