1. Chelmsford Amateur Radio Society Advanced Course Measurements Part-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
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. 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
AA119 R6
10k C3
1nF R1
10R R2 R3 R4 R5
RF Input
To Meter
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 D1 C1
1nF
RF Input
To Antenna C2 M1 M2 R D2
Forward Power
Reverse Power

8. Current Transformer SWR
Typical arrangement of many in-line SWR meters R3
2.2k T1
1T:12T D1 C2
1nF
RF Input
To Antenna C3 M1 M2 R1
220R D2
Forward Power
Reverse Power
RV1
RV2 C1
2.2pF
VC1 R2

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