Question E9F08:
Which of the following is a significant difference between foam dielectric coaxial cable and solid dielectric coaxial cable, assuming all other parameters are the same?  

Choices:  
A: Foam dielectric coaxial cable has lower safe maximum operating voltage  
B: Foam dielectric coaxial cable has lower loss per unit of length  
C: Foam dielectric coaxial cable has higher velocity factor  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

To the point: Foam dielectric uses a mix of plastic and air, which lowers the effective dielectric constant and loss, but also lowers dielectric strength. So foam coax typically has lower loss, higher velocity factor, and lower maximum safe voltage than otherwise-identical solid-dielectric coax. Therefore, all these choices are correct.

1) Why the correct answer (D) is correct
- Lower safe maximum operating voltage (A): Air voids in foam reduce dielectric strength and promote corona/partial discharge, so breakdown occurs at lower voltage than in solid polyethylene.
- Lower loss per unit length (B): Foam has lower effective dielectric constant and loss tangent, reducing dielectric loss component of attenuation.
- Higher velocity factor (C): Lower effective permittivity means waves travel faster in the cable.

2) Why the other choices (A, B, C) are “incorrect” as answers
- Each of A, B, and C is true, but each lists only one difference. Since more than one difference applies, the most complete choice is D.

3) Useful formulas and rules of thumb
- Velocity factor:
  $$VF \approx \frac{1}{\sqrt{\varepsilon_{\text{eff}}}}$$
  Typical values:
  - Solid PE: $\varepsilon_r \approx 2.25 \Rightarrow VF \approx 0.66$
  - Foam PE: $\varepsilon_{\text{eff}} \approx 1.3\text{–}1.6 \Rightarrow VF \approx 0.80\text{–}0.85$
- Dielectric loss (part of total attenuation):
  $$\alpha_d \propto f \cdot \tan\delta \cdot \varepsilon_{\text{eff}}$$
  Foam reduces $\varepsilon_{\text{eff}}$ and typically the effective loss tangent, so $\alpha_d$ is lower.
- Coax breakdown voltage scales with dielectric strength:
  $$E_{\max} = \frac{V}{a \ln(b/a)} \quad\Rightarrow\quad V_{\max} \propto E_{\text{break}}$$
  where $a$ is inner conductor radius, $b$ is inner radius of the shield. Foam lowers $E_{\text{break}}$ vs solid dielectric.
- Rule of thumb:
  - Foam coax: lower loss, higher VF, but lower voltage rating than the solid version of the same size and impedance.