RF Powermeter / dummy load
RF HF powermeter dummyload
By Guy, de ON6MU
rev1.2

About the power meter / dummy load

A 50 Ohm dummyload is an essential part for any radioamateur as is a powermeter. The prices of such relative simple equipment is expensive, but not for us handy Hams HI. All you need is a metal box (or plastic box painted inside with graphite or other conducting/RF-shielding capable material), a few resistors and basic components (which can be salvaged from old radio's, switching power supplies etc..) and a analog meter. I used a Radio-shack meter, but any (sensitive) meter can be used. It's all a matter of calibrating your meter correctly, which is easy if you can lend a good commercial RF power meter.

This RF-power meter combined with build-in dummy load is made to measure power levels starting from a few milliwatt up to 50 watts (or more if suitable components are used and an avalanche sinterglass diode). IT has 3 scale readings: 0.5 watt, 5 watt and 50 watts. Again, you can extend the scales easily. The power meter is ideal for measuring QRP levels and by adding an BNC connector you'll have an easy oscilloscope measuring point.

In short, an easy and cheap project to build yourself. Even a beginner in HAM homebrewing can make his own fair (if not better then most you buy in the shop) power meter!

Calibration

Is done with a good (commercial or already calibrated) SWR/RF powermeter capable of measuring HF power levels from 5 (or less) to 50 watts and has a frequency range that covers the entire HF-band. You also need a transceiver which you set in series with the meter: TRX -> COMMERCIAL RF METER -> ON6MU RF METER. Set all potentiometers (R2,R3,R4) to maximum resistance. Choose one of the scales (0.5, 5 or 50 watts) to start with. Other power levels/scales with the same step (X1 X10 X100) will have the same indication multiplied. So if you choose scale 2 being 5 watt and calibrate at least 5 power levels of your transceiver it should be ok for the other scale selections. Set R3 for full scale at 5 watt and work your way down. One calibration for all power level settings respectively is sufficient.

You can of course use an oscilloscope or RF voltmeter to calibrate the RF powermeter.
online: https://www.pasternack.com/t-calculator-power-conv.aspx

volt
Examples:
1W = 30dBm = 10V = 20Vpp)
100mW = 20dBm = 3.16V = 6324mVpp
50mW = 16.99dBm = 2.23V = 4472 mVpp
5mW = 6,99dBm = 0,7V = 1,4mVpp
U = Uv *2 = SQR(Uv)

Schematic fig1

RF power meter and dummy load schematic diagram

Parts list

Specifications

Note rev1.2*: sometimes the diodes broke when measuring 50watts. To measure power levels up to 100 watts or more a better diode (BYW55) has been used that is suitable for high voltages (Standard Avalanche Sinterglass Diode).

Inside the powermeter

Dummy load

Is build out of 20 carbon resistors (or non inductive metalfilm) of 1K and 1Watt all parallel. I used two 15mm X 50mm print boards and soldered two times 10 resistors on each side. Solder the two parts on top of each other. See fig1 and 2. Do not use inductive type of resistors! Always use carbon based resistors or non inductive metalfilm ones. This dummy load is able to dissipate 21 watts continues and no problem to handle a 10 second peek of 50 watts. Long enough to measure the power. Be sure not to transmit high power > 21 watt for a long time as this will burn out your dummy load! IF you need the dummy load to handle more power then you could use 45 2k2 1 watt resistors which doubles the amount of power (and peek power). Of course you can use 1 k resistors of a higher power rating as long as they are non-inductive resistors.

fig.2.

 

2nd example:
RE-M3RFP Microwatt RF Powermeter

 

Parts list


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