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MICROWAVE MEASUREMENTS
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.
Table of Contents for Electronics Warfare and Radar Engineering Handbook
Introduction |
Abbreviations | Decibel | Duty
Cycle | Doppler Shift | Radar Horizon / Line
of Sight | Propagation Time / Resolution | Modulation
| Transforms / Wavelets | Antenna Introduction
/ Basics | Polarization | Radiation Patterns |
Frequency / Phase Effects of Antennas |
Antenna Near Field | Radiation Hazards |
Power Density | One-Way Radar Equation / RF Propagation
| Two-Way Radar Equation (Monostatic) |
Alternate Two-Way Radar Equation |
Two-Way Radar Equation (Bistatic) |
Jamming to Signal (J/S) Ratio - Constant Power [Saturated] Jamming
| Support Jamming | Radar Cross Section (RCS) |
Emission Control (EMCON) | RF Atmospheric
Absorption / Ducting | Receiver Sensitivity / Noise |
Receiver Types and Characteristics |
General Radar Display Types |
IFF - Identification - Friend or Foe | Receiver
Tests | Signal Sorting Methods and Direction Finding |
Voltage Standing Wave Ratio (VSWR) / Reflection Coefficient / Return
Loss / Mismatch Loss | Microwave Coaxial Connectors |
Power Dividers/Combiner and Directional Couplers |
Attenuators / Filters / DC Blocks |
Terminations / Dummy Loads | Circulators
and Diplexers | Mixers and Frequency Discriminators |
Detectors | Microwave Measurements |
Microwave Waveguides and Coaxial Cable |
Electro-Optics | Laser Safety |
Mach Number and Airspeed vs. Altitude Mach Number |
EMP/ Aircraft Dimensions | Data Busses | RS-232 Interface
| RS-422 Balanced Voltage Interface | RS-485 Interface |
IEEE-488 Interface Bus (HP-IB/GP-IB) | MIL-STD-1553 &
1773 Data Bus |
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