I am trying to calculate isolation of an relay in ohm. Can it be approached as calculation of resistance of solid materials?

$$ R_{air} = ρ_{air} * \frac{d_{gap}}{A_{contact}} $$

Let's say resistivity of air equals 3.3e^15 Ωm and gap is 1mm between contacts with area of 1mm^2. As a result we get 3.3e^12. Can this be an approximate value to real isolation value?

Thank you.

Additional information: frequency range is from DC to 5GHz. Load voltage Vp is 24 and current is 20mA.

  • \$\begingroup\$ It can be an approximate value yes. \$\endgroup\$
    – Andy aka
    Nov 15, 2020 at 15:54
  • \$\begingroup\$ Depends. "Isolation" may refer to isolation at DC, or a range of RF frequencies in general, or at a specific RF frequency. Your answer is good at DC. \$\endgroup\$
    – user16324
    Nov 15, 2020 at 17:06
  • \$\begingroup\$ Thanks for the comments. @BrianDrummond what should be the strategy for RF frequencies? Could you give a hint? \$\endgroup\$
    – Reactionic
    Nov 15, 2020 at 18:10
  • \$\begingroup\$ I only commented to avoid the trap of thinking a DC solution applied at high frequencies and finding out later, perhaps expensively, it doesn't. As a first step, I'd add that missing information to the question including the frequency range, power level, and characteristic impedance you're interested in. \$\endgroup\$
    – user16324
    Nov 15, 2020 at 18:29
  • \$\begingroup\$ I added some more details. \$\endgroup\$
    – Reactionic
    Nov 15, 2020 at 18:45

1 Answer 1


A better model of the "isolation" is to view the air_gap as part of voltage_divider.

The other part of the voltage divider is a parallel RC: the total capacitance on the node susceptible to interference, PLUS the resistance of that node.

The resistance defines the corner frequency of the builtin HIgh Pass filter, which causes ZERO RESPONSE AT DC.

At high frequencies, above the HPF corner frequency, your isolation is accurately computed as the ratio of the 2 capacitances.


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