Coaxial line loss problem

a. The higher the frequency of the signal the stronger its skin effect, the more concentrated in the metal surface transmission, its transmission cross-sectional area is smaller, so its impedance is larger, the loss is larger, in order to reduce the loss, the RF line using precious metals (high conductivity, precise manufacturing process, etc.).

b. Coaxial line loss is divided into dielectric loss and metal conductor loss, which is mainly dielectric loss, generally requires a low relative permittivity, dielectric loss angle factor is small, so that the attenuation is small . The medium requires a consistent structure to ensure uniform impedance, the higher the frequency, the more difficult it is to maintain a consistent continuous impedance, reflection loss will also be larger.

      1. Dielectric loss: When the frequency is very high, due to the dispersion of the medium, the dielectric constant is a function of frequency. The root cause or charged particles with alternating electric field has different changes. Dielectric coefficient with frequency change should have had a maximum value, but because the coaxial line insulation is a highly non-polar material, from low frequency to high frequency dielectric coefficient dispersion is very weak.

       2. Conductor loss: Strictly speaking, conductor loss can actually be divided into two parts: heat loss and electromagnetic leakage due to incomplete shielding, the same shielding rate for different frequencies of electromagnetic waves shielding effect is not the same, the shielding effect of high frequency is not as good as low frequency (of course, this is not the main part of the loss).

c. skin depth δ = 1/πfuσ; cross-sectional area of the transmitted current s = π[(r+δ)²-r²]; transmission resistance R = 1/σs.

  Conclusion: ----- the thicker the wire, the larger the cross-sectional area, the lower the transmission resistance.

         ----- the greater the conductivity, the smaller the skinning depth, the smaller the cross-sectional area, increasing the transmission resistance; the greater the conductivity, reducing the transmission resistance; from both considerations, the latter is dominant, so the greater the conductivity, the smaller the resistance.

         ----- the higher the frequency, the smaller the skinning depth, the greater the transmission resistance.

d. The thinner and longer the coaxial cable, the greater the loss, and the higher the signal frequency, the greater the loss.