RF Power meter / dummy load

By Guy, de ON6MU

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 more then diode is used). 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.

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 meters you buy in the shop) power meter!

Calibration

Is done with a good (commercial or already calibrated) SWR/RF power meter 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 is sufficient.

Schematic fig1

Parts list

• alu box (or plastic box painted inside with graphite) of 100mm X 100mm X 50mm

• 1 female PL 259 chassis (SO239)

• Analog Meter (as sensitive as possible and calibrate the scale with a good powermeter)

• C1,C3 = 330pF

• C2,C4 = 47nF high quality

• C5 = 100nF

• C6 = 10uF/6v tantal

• D1 = 1N1448 (or for higher powerlevels use 2 or 3 more diodes in serie)

• D4 = 1N1004 (protects the meter for all voltage higher then 0.6 volts)

• S1 = 3 pos. switch (or more if you want more power scales)

• L Dr = 500uH or 1M Ohm carbon resistor 1 watt covered with 0,2mm Cul 3 times (or more) turned over the length of the resistor

• R1 = 18k

• R2 = 100k variable resistor

• R3 = 250k variable resistor

• R4 = 1M variable resistor

• Rd = 50 Ohm Dummy load of at least 20 watt (see text)*

Specifications

• precision power meter capable of measuring power scales of 500mW...100W (depending on components used, see text)*

• frequency range: entire HF band 1Mc...30Mc (50Mc should is possible but the precision tolerance will drop)

• switchable scale ranges (in this schematic 0.5W, 5W, 50Watts)

• can be used as dummy load also HI

Inside the powermeter

Dummy load

Is build out of 21 carbon resistors 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 and solder the 21th 1K resistor where the two parts come together. See fig1 and 2. Do not use inductive type of resistors! Always use carbon based resistors. 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 carbon resistors.

fig.2.

Don't forget to check these out:
.ON6MU Homebrew projects
.Radioamateur related projects

.ON6MU Ham mods
.Modifications of transceivers


73"

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