Transcript Measurements-4 - RF Power

Chelmsford Amateur Radio Society
Advanced Course
Measurements
Part-4 – RF Power
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
1
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
2
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
Power in
T-Piece
Scope
oooo
50 Ohm
Load
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
3
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
4
Dummy Load with Detector
RF Input
R1
10R
R2
10R
•
•
•
•
•
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
R3
10R
R4
10R
R5
10R
Chelmsford Amateur Radio Society
Advanced Licence Course
D1
AA119
C3
1nF
R6
10k
Anthony Martin M1FDE
To Meter
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
5
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
6
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
RF Input
To Antenna
R
D1
C2
1nF
C1
1nF
M1
Forward Power
Chelmsford Amateur Radio Society
Advanced Licence Course
R
D2
M2
Reverse Power
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
7
Current Transformer SWR
• Typical arrangement of many in-line SWR meters
T1
1T:12T
RF Input
C1
2.2pF
VC1
M1
D1
R1
220R
R2
220R
R3
2.2k
RV1
Forward Power
Chelmsford Amateur Radio Society
Advanced Licence Course
D2
RV2
C3
1nF
C2
1nF
To Antenna
M2
Reverse Power
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
8
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Anthony Martin M1FDE
Slide Set 24: v1.0, 23-Oct-2004
(4) Measurements – RF Power
9