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

#### Didier Juges, July 28, 1999

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

### Example:

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

### Reference:

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