Chelmsford Amateur Radio Society Advanced Course Measurements Part-4 - PowerPoint PPT Presentation

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Chelmsford Amateur Radio Society Advanced Course Measurements Part-4

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Some Oscilloscopes have 50 Ohm input setting. It will not handle much power. ... 50 Ohm centre conductor. R must match coupling lines ... – PowerPoint PPT presentation

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Title: Chelmsford Amateur Radio Society Advanced Course Measurements Part-4


1
Chelmsford Amateur Radio Society Advanced
CourseMeasurementsPart-4 RF Power
2
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

3
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

4
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

5
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

6
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

7
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

8
Current Transformer SWR
  • Typical arrangement of many in-line SWR meters

9
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
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