Chelmsford Amateur Radio Society Advanced Course Measurements Part-4

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Chelmsford Amateur Radio Society Advanced
CourseMeasurementsPart-4 RF Power
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RF Power Meters

• RF Power can be measured using
• Thermal instruments
• AC ammeters and a load
• AC voltmeters and a load
• In-line directional-coupler instruments
• Oscilloscopes
• The range of power meters can be extended using
  attenuators
• Make sure components are adequately rated for the
  power
• If mismatches are introduced, your instrument
  will be inaccurate

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Measuring across a Load

• Some Oscilloscopes have 50 Ohm input setting.
• It will not handle much power.
• Use an external attenuator.
• If there is no 50 Ohm setting, use a 50 Ohm
  terminator and a T-piece
• 101 probes can be used if the voltage on the
  load is high
• But accuracy of 101 probes is poor at high
  frequency

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Measuring RF Amplitude

• For lower frequencies, pk-pk volts can be
  measured directly across a load R.
• Depends how fast your scope is.
• The amplitude is half the pk-pk value
• The RMS voltage is amplitude x 0.707
• Calculate power P V 2 / R
• Transmitters must be operated in continuous CW
  mode or with an SSB tone

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Dummy Load with Detector

• Resistors present a 50 Ohm load
• Divides voltage by 5
• Diode detects peak of RF
• RMS volts can be calculated
• Power in 50 Ohm load can be calculated

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SWR Measurement

• SWR can be determined from detectors of forward
  and reverse power.
• SWR can be determined by comparing samples of
  voltage and current.
• SWR is displayed using meters with calibrated
  (non-linear) scales

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Coupled Lines SWR Meter

• 50 Ohm centre conductor
• R must match coupling lines
• Mechanical construction, but effective at UHF and
  above
• Maximum coupling with lines ¼ wave long

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Current Transformer SWR

• Typical arrangement of many in-line SWR meters

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Measuring PEP Amplitude

• This is the two-tone-test
• PEP is Peak Envelope Power
• The amplitude is half the pk-pk value
• The RMS voltage is amplitude x 0.707
• Calculate power P V 2 / R
• Reduced stress on Tx designed for SSB voice