1 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power Chelmsford Amateur Radio Society Advanced Course Measurements Part-4 – RF Power

2 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – 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 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power 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 Scope 50 Ohm Load T-Piece Power in o o o o

4 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power 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 Calculate power: P = V 2 / R Transmitters must be operated in continuous CW mode or with an SSB tone

5 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power 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 D1 AA119 R6 10k C3 1nF R1 10R R2 10R R3 10R R4 10R R5 10R RF Input To Meter

6 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power 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 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF 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 D1 C1 1nF RF Input To Antenna C2 1nF M1M2 RR D2 Forward PowerReverse Power

8 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power Current Transformer SWR Typical arrangement of many in-line SWR meters

9 Chelmsford Amateur Radio Society Advanced Licence Course Anthony Martin M1FDE Slide Set 24: v1.0, 23-Oct-2004 (4) Measurements – RF Power 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 Calculate power: P = V 2 / R Reduced stress on Tx designed for SSB voice