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Using a waveguide-to-coax adapter as an EMC antenna

It is current practice to use a waveguide to coax adapter feeding a spectrum analyzer to check microwave equipment for RF leakage. While this is useful to pinpoint the origin of a leak if one is suspected, it is more difficult to use the readings to ascertain compliance to regulatory requirements such as MIL-STD-461 or Part 15 of FCC Rules.

To convert a power reading in dBm into a field strength in dBµV/m, the antenna factor of the device used must be computed.

The antenna factor is the term used in EMC testing to convert a voltage or power level fed by an antenna to an EMI analyzer into the field strength units (usually dBµV/m) of the electromagnetic field producing that voltage or power.

In a 50 ohm system, the antenna factor (expressed in dB(m-1)) of an antenna of absolute gain G (expressed in dB) at a wavelength L (expressed in meters) is:

    AF[dB(m-1)] = 19.8 - 20*log(L[m]) - 20*log(G[dB])

To convert a voltage reading V (expressed in dB(µV)) into an electric field E (expressed in dB(µV/m),) use the following formula:

    E[dB(µV/m)] = V[dB(µV)] + AF[dB(m-1)]

To convert a power reading P across a 50 ohm impedance (expressed in dBm) into an electric field E (expressed in dBµV/M,) use the following formula:

    E[dBµV/M] = P[dBm] + AF[dB(m-1)] + 107


Conventional wisdom has it that within its operating bandwidth, an open WR-90 waveguide adapter has a gain of approximately 7dB. Its antenna factor at 7.5 GHz (4.0 cm wavelength) is:

    AF(dB(m-1)) = 19.8 - 20*log(0.04) - 20*log(7) = 30.9 dB

If we measure -70 dBm on the receiver/spectrum analyzer, the E field is:

    E = -70 + 30.9 + 107 = +67.9 dBµV/m


John Osburn, EMC Antenna Parameters and Their Relationships, ITEM 1996