pa0nhc RF watt meter
Preliminary development data.

(C) The use, copy and modification of all info on this site is only permitted for non-commercial purposes, and
thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

Correct indication with 100W output at 7MHz without any adjustment.

This sensitive active analog thru - line meter system indicates real rms RF power at an external 50 Ohms load.
It has a wide measuring range : 57 dB, from - 4 dBm to +53 dBm (0.4 mW to 200 W).

s1 s2 Min. measurable Max. readable
-20 -20

0,41 mW = -4 dBm

2,1 mW = +13 dBm
0 -20   1,98 W = +33 dBm
-20 0   2,1 W = +33 dBm
0 0   198 W = + 53 dBm

The expected useable frequency range is 160 kHz to 30MHz.
Easily extendable down to 16kHz or even lower if needed.
Power supply is 4 V to 5V, from 3x AAA battery at 3 to 8 mA load, with at least 25 hours battery life.
As long as the stated components are used, is the circuit free of adjustments, and works immediately. Inaccuracy due to the use of standard component values is estimated to be less than +- 1dB.

An easy to implement dBm meter scale is given below.

All resistors and capacitors are low cost, low inductance SMD 1206 types. The PCB is designed with longer soldering islands for easier heating, easier solder flow, and less component heating. The miniature toggle switches are chosen for their small size and low cost.

REM : The switches need to have 4.14 mm pin pitch.
            If a meter is used with different internal resistance, R4,5,6 have to be adapted for full scale deflection at 3.3Vdc and 0.33Vdc IC output,

Download the PDF partslist HERE

Download the schematic PDF HERE


This table can help you designing a meter scale for other moving coil meters.

IC1 pin1 [V] RFpower [W] dBm mA
3,22 200 +53 0,976
2.28 100 +50 0,691
1,61 50 +47 0,488
1,14 25 +44 0,345
0,805 12.5 +41 0,244
0,57 6,25 +38 0,173
0,405 3,125 +35 0,123
0,285 1,6 +32 0,086

Download this dBm scale for a
Voltcraft 60mm wide 1mA meter

Print this PDF at 100% size.
Cut the scale along the 57.1mm wide rectangle.
Glue it on the backside of the original scale.

Download this switch panel

This instrument measures the effective RF voltage on the internal 50 Ohms transmission line. One BNC bus is connected to the RF source, the other to a 50 Ohms load.

IC1 is an accurate and sensitive wide band 2.5 GHz "rms detector" SMD chip, and has a conversion gain of 7.5 Vdc out, per 1 Vrms in. It is capable of detecting as little as -30dBm or 7mVrms with only -1dB error. Between 0 and 100 MHz is has a constant input impedance of 225 Ohms // 1pF.

With s1 in position "0", the total impedance on the inpout of IC1 is : 22.3 Ohms. The input attenuation is then : (5,023 + 0,0223) kOhm / 0,0223 kOhm= 226,2x.
With s1 in poisition "-20", the input attenuation is : (5,023 + 0,225) kOhm / 0,225 kOhm = 23,32x. Resulting in 20dB higher sensitivity. 

The total resistance of R5//R6 is 3.083 kOhms.
With s2 in position R5//R6 = "0", and full scale deflection of mete M, the output of IC1 is : (210 + 3083) Ohm x 0,001 A = 3,293 Vdc, rounded off to 3,3Vdc.
With s2 in position  R4 = "-20", and full scale deflection of mete M, the output of IC1 is :
0,001 A x (210 + 120) Ohm = 0,33Vdc. Resulting in 20dB higher sensitivity.

The measurable power range is therefore +53dBm - (-4dBm) = 57dB (!!).

To prevent influence from low frequency signals, coupling capacitor C2 causes the sensitivity to roll off 6db / octave from abt. 160 kHz down.

        Extending the frequency range down :
By soldering an extra SMD capacitor of 100 nF on top of (in parallel to) C2, the low frequency range can be extended down to abt. 16 kHz (-45 dB @ 50 Hz).
By soldering an extra SMD capacitor of 1uF on top of (in parallel to) C2, the low frequency range can be extended down to abt. 1.6 kHz (-25 dB @ 50 Hz).
By soldering an extra ceramic SMD capacitor of 10uF on top of (in parallel to) C2, the low frequency range can be extended down to abt. 160 Hz (-5 dB @ 50 Hz).

Drilling data for a Hammond 1554EGY ABS box.
The LED hole could be drilled into the top, and the switch holes with 6.2 mm, for easier installation of the PCB.




PCB top, seen from the switces.

The plastic stud at the front inside,
near the switch holes,
has to be removed.
REM: the 5mm hole near it is for the LED.

        The housing.
The PCB is designed to fit in a Hammond 1554EGY ABS box between two BNC bushes .
        See the picture >>
The moving coil meter is situated in the center of the front.
The switches will come in the top. 

        See the drilling sketches.  
The sketches contain two different hole sizes for two different types BNC bushes.
The PCB fits both. You can use a flange or nut type.

        It helps to shift the PCB easier with the LED and switches into their holes when :
- the LED hole is drilled into the TOP of the box
- the switch holes are drilled to 6.2mm
- and counter sink them at the inside of the box.

        You have to remove a plastic screw facility near the middle switch hole, as it could hinder the insertion of the PCB.
- Using a 9.5 mm drill in a table top drill stand, and the plastic box in a vice, carefully remove the round part of the screw facility by drilling it flat. See photo >>.
- Then break away the two remaining little plastic wings.
- Using the drill as a fraise, flatten the internal surface.
- Flatten it further using sanding paper.

Solder IC1 as last, and take anti-static measures.

Solder all electronic components onto the PCB.
See to it that each component is centered onto its soldering islands, and no component is overlapping its insulation slit. 

        I found the following work strategy practical :
- First pre-tin the components soldering islands on the PCB.
- Remove excess solder using a de-soldering wick. Remove the wick before the solder hardens

For each component, one soldering island will be used to pre-fix the component before definitive soldering. This island (*) should contain a bit more solder.

- Place a component in correct position. Without supplying extra solder, fix it by quickly flowing a little solder from the solder island to the component
- Check the correct position and correct it if needed.
- Apply solder to and flow the other contact.
- As last, apply a little solder to the first (*) contact to flow the solder there.

- All resistors R1//R2 can be pre-fixed to the 50 Ohm transmission line.
- Then all other sides soldered.
- Then the first sides soldered.
Good solder flow helps cooling the components when RF power is connected.

IC1 can be placed and pre-fixed by soldering pin4 first. Check correct position using a magnifying glass. 
Then use a hot iron (350C) and a short, medium sized, solder tip. Tin the tip.
- QUICKLY overflow pins 1-3 with solder.
- Let cool down well.
- QUICKLY overflow pins 4-6 with solder.
- Let cool down well.
- QUICLY remove excess solder from pins 1-3 using fresh de-solder wick. 
- Let cool down well.
- QUICLY remove excess solder from pins 4-6 using fresh de-solder wick. 
- Let cool down well.
Inspect solder quality at IC1 using a magnifying glass.

        Use the three holes in the top of the box as lining-up guides for the switches :
- Place the switches upside-down into the switch holes at the top of the box.
- Shift the PCB with the non-component TOP side over the switch pins.
- Solder the switch pins at the PCB component-side.
- Solder the LED, battery cable and meter wires.
- Glue the wires near their solder points onto the PCB, to prevent breaking later.

Shift the PCB into the box, with the switches and the LED sliding into their holes. Leave the lower nuts of the switches loose..

Install both BNC bushes, with each two ground lugs facing downwards to the bottom grounding surfaces at the PCB..

Let the PCB bottom surface rest onto the BNC central pins.
Solder the BNC pins onto the bottom "50 ohms line" surfaces.

Bend the BNC ground lugs so they make contact to the "ground" surfaces at the bottom copper side of the PCB. 
At both BNC bushes, solder the lugs to two different bottom PCB ground surfaces.
TIP : If the lugs are to short, use de-solder wick as a flat copper wire to connect the grounding surfaces to the BNC bushes.

Adjust and tighten the nuts of the switches. 

Install meter M, and connect it. Do not fit springs if screwing the nuts gives problems.

        Meter scale.
        Prevent steel particles to enter the meter internal space (strong magnet inside). Clean the work surface.

Print the meter scale below. Cut it along the thin rectangle lines.
Remove the meter lid, and unscrew the meter scale. 
Glue the new rms meter scale to the back of the meter scale plate.
Install the meter scale plate and close the lid.

Insert batteries.
Stick some plastic foam into the lid, to gently clamp the battery holder in place when the box is closed.
Close the box.

Set SW1 and SW2 both in un-sensitive position "0".

Connect a 50 Ohms dummy load and 100W RF power at 3.65 MHz.

The meter needle should point out +50 dBm.