Measurement Procedures
Calculate your estimated power losses before attempting to perform a measurement. The ideal input to a
measurement device is in the 0 to 10 dBm (1 to 10 mW) range.
Linearity Check
To verify that a spectrum measurement is accurate and signals are not due to mixing inside the receiver, a linearity
check should be performed, i.e. externally insert a 10 dB attenuator - if measurements are in the linear region of the receiver,
all measurements will decrease by 10 dB. If the measurements decrease by less than 10 dB , the receiver is saturated. If
the measurements disappear, you are at the noise floor.
Half-Power or 3 dB Measurement Point
To verify the half power point of a pulse width measurement on an oscilloscope, externally insert a 3 dB attenuator
in the measurement line, and the level that the peak power decreases to is the 3 dB measurement point (Note: you cannot
just divide the peak voltage by one-half on the vertical scale of the oscilloscope).
VSWR Effect on Measurement
Try to measure VSWR (or reflection coefficient) at the antenna terminals. Measuring VSWR of an antenna through
it's transmission line can result in errors. Transmission lines should be measured for insertion loss not VSWR.
High Power Pulsed Transmitter Measurements
When making power measurements on a high power pulsed transmitter using a typical 40 dB directional coupler,
an additional attenuator may be required in the power meter takeoff line, or the power sensor may be burnt out.
For example, assume we have a 1 megawatt transmitter, with PRF = 430 pps, and PW = 13 Fs. Further assume
we use a 40 dB directional coupler to tap off for the power measurements. The power at the tap would be:
10 log(Pp) - 10 log(DC) - Coupler reduction =
10 log(109mW) - 10 log(13x10-6)(430) - 40 dB =
90 dBm - 22.5 dB - 40 dB = 27.5 dBm (too high for a power meter)
Adding a 20 dB static attenuator to the power meter input would give us a value of 7.5 dBm or 5.6 mW, a good
level for the power meter.
High Power Measurements With Small Devices
When testing in the presence of a high power radar, it is normally necessary to measure the actual field intensity.
The technique shown in Figure 4, in Section 6-7, may not be practical if the measurement device must be small. An
alternate approach is the use of a rectangular waveguide below its cutoff frequency. In this manner, the "antenna"
waveguide provides sufficient attenuation to the frequency being measured so it can be coupled directly to the measurement
device or further attenuated by a low power attenuator. The attenuation of the waveguide must be accurately measured since
attenuation varies significantly with frequency.
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