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Chelmsford Amateur Radio Society Advanced Course Measurements Part-4 – RF Power

Chelmsford Amateur Radio Society Advanced Course Measurements Part-4 – RF Power. 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

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Chelmsford Amateur Radio Society Advanced Course Measurements Part-4 – RF Power

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  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. Scope T-Piece Power in o o o o 50 Ohm Load 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 • 10:1 probes can be used if the voltage on the load is high • But accuracy of 10:1 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. RF Input R1 10R R2 10R R3 10R D1 R4 AA119 10R To Meter C3 R6 R5 10k 1nF 10R 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. RF Input To Antenna R R C2 C1 D1 D2 1nF 1nF M1 M2 Forward Power Reverse Power 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. RF Input T1 To Antenna 1T:12T C1 2.2pF VC1 R1 R2 D1 D2 220R 220R R3 M1 RV1 2.2k RV2 M2 C3 C2 1nF 1nF Forward Power Reverse Power 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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