RF Probe and Watt Meter
Circuit : Andy Collinson
Email  :

This RF probe can be used at High Frequency (HF) or Ultra High Frequency (UHF) on both 50 and 75 ohm coaxial cables. In addition the RF voltage can be measured under load or no-load conditions which allows the circuit to double as an RF Watt meter. The RF probe can be used for oscillators and small transistors for powers up to 2 Watts.

Circuit Notes
The circuit is a simple half wave rectifier. In this circuit it works at radio frequencies (RF) and converts any RF signal to a DC voltage, in addition S1, allows a resistive load to be switched in or out of circuit. S1 is a single pole, double throw switch with a Centre off position. The centre position is no load, and left and right positions1 are for 50 and 75 ohm measurements. First, a small section on measuring RF voltage, current and power, then I'll describe how to use this simple test instrument.

Measuring RF Voltage
Digital and analogue multi meters can already measure AC voltages so why can't they be used at radio frequencies? The reason is that they can only measure with accuracy a limited frequency range. My Maplin meter measures frequencies up to 400Hz with 1% accuracy, and up to 20KHz at 4%. This also requires that the waveform is a sine wave. At frequencies above 20KHz, accuracy is not reliable.

To measure radio frequencies (RF) a simple diode detector circuit is all that's needed. The detector in this probe is an OA91 germanium diode, but any germanium diode will work. Germanium diodes have a low forward voltage drop (about 0.2V) and are preferred to silicon diodes which have a higher (0.6 - 0.7V) voltage drop. The diode rectifies the RF signal and converts it to a DC voltage, which can be read by a multimeter with good accuracy; the 1nF capacitor is there to smooth the rectified DC signal presented to the meter.

RF Power, Voltage and Current
When measuring any AC or RF signal, the currents and voltages are only in phase if the load is purely resistive. All transmitters are tested with a dummy load which are resistive. This simplifies the calculations and the pie chart for Ohms's Law at AC can now be used.

Typical RF Voltages
For example, a 1 watt transmitter delivers an average power of 1 watt into a 50-ohm resistive dummy load. Transmitter power is measured in RMS or root-mean-square. As power, P = V2/R, then re-arranging, V(rms) = sqrt(P x R). Power is also found from P = I2R and re-arranging in terms of current, I(rms) = √ (P / R)  Peak values are simply 1.414 x the RMS values.

So for a 1 W transmitter V(rms) = √ ( 1 x 50)  = 7.071 Volts. and current, I(rms) = √ ( 1 / 50)  = 0.141 Amps.

Power OutputAC Volts RMSAC Amps RMSAC Volts Peak AC Amps Peak
2 W10 V0.20 A14.4 V0.283 A
1 W7.07 V0.141 A10.0 V0.200A
0.5 W5.0 V0.100 A7.07 V0.141 A
0.2 W3.16 V0.0632 A4.47 V0.0894 A
0.1 W2.24 V0.0447 A3.17 V0.0632 A

RF Probe Functions
S1 allows a 50 or 75 ohm resistive load to be switched in and out of circuit. This allows the probe to read loaded and no-load voltages. However as the load has a fixed resistance (50 or 75 ohm) then power delivered to the load can also be worked out. Finally because the probe has a fixed resistance and can measure loaded and no-load voltages then it is possible to measure output impedance of a transmitter, see also Measuring Input and Output Impedance may also be of assistance. The RF probe has four functions:

1) Unloaded Transmitter Voltage
In all cases, connect the RF probe between the circuit under test and the meter. The circuit under test could be a transmitter, RF oscillator or other signal source. As the OA91 diode and 10n capacitor are a half wave rectifier, the RF value measured will be a peak value. As V(RMS) = V(peak) / √ 2  then:
V(RMS) = Vpeak = 0.7071 x Vpeak
To measure unloaded RMS transmitter voltage switch S1 to off and multiply the meter reading by 0.7071.

2) Loaded Transmitter Voltage
To measure a transmitter voltage under load switch S1 to either 50 or 75 ohm position. Normally this will be 50ohm, but for Band II ( 87.5MHz - 108MHz) 75 ohm impedance should be used.

To measure loaded RMS transmitter voltage switch S1 to either 50 or 75 ohm and multiply the meter reading by 0.7071.

3) Measuring Output Impedance
To measure the output impedance of an unknown circuit or transmitter you first need to take two readings, one unloaded and then a reading under load at either 50 or 75 ohms. The output impedance can be found from the following equation:
Z = R ( VNL - VL)
   Z = output impedance of circuit in ohms
   R = resistance of probe ( depending on S1 this is either 50 or 75 ohm)
   VNL voltage RMS reading with S1 in centre position (no-load)
   VL voltage RMS reading under load

4) Measuring Output Power
The output power in Watts can also be calculated. Output power is the loaded (RMS) output voltage squared divided by transmitter impedance:
P = VL2
   Z = output impedance of circuit in ohms
   VL voltage RMS reading under load

Output Power and SWR
The output power as measured by the probe will not be exactly the same as the radiated power by the antenna. This is because there are losses in the antenna system and the Standing Wave Ratio (SWR). When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the antenna cable to the antenna. Energy not transferred to the antenna is reflected back towards the transmitter. It is the interaction of these reflected waves with forward waves which causes standing wave patterns. An SWR meter can be used to measure the SWR ratio in order to obtain the best match between antenna and the feedline.

Important Note About Resistors
The components in the circuit are all readily available, however there is one Important consideration. The resistors used Must be carbon type and not wirewound types. The reason is that wirewound resistors contain inductance due to the coiled wire, this is not normally important except at very high frequencies, as in this circuit.

PCB or Veroboard Layout
A circuit this small with very few components is hardly worth the trouble of producing a PCB. However because of its small size it took me about 14 minutes, to draw the schematic and produce the PCB in Kicad. The 3D rendered components are all created by Renie S Marquet, more in the simulation section.

PCB 3D view

Enlarged Component Side

Actual Size copper track view.

If you are thinking of using this PCB layout first printout the actual size copper track view on paper, then you can match up the components to see if they fit the pads. This is the same for any PCB program. It does not matter if its open source or the program cost several thousand pounds, the components that you use must fit the footprints on the PCB board. As sizes of components vary wildly then this is a problem for all PCB layouts.

1 As drawn in the schematic.
Circuit Exchange International Return to TestGear https://www.cxi1.co.uk