Simple Dummy Load Power Meter

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This Dummy Load Power Meter is very useful for RF power measurements in HF and VHF frequency bands. It reads up to 5W and presents a load resistance of 50Ω, enabling it to be used as an indicated dummy load. Three ranges are provided using one scale with each range having a full scale deflection ten times the next lower range; Full scale corresponds to power of 50mW. 0.5W and 5W, for each range, respectively. Useful indications can be obtained down to 1mW.


CONSTRUCTION

The most essential part of the circuit is the input attenuator series resistor network of R1, R2 and R3. These resistors should be non-inductive - wire wound types are not suitable. R1 is best achieved with two 68Ω, 2W resistors in parallel; R2 with 18Ω in parallel with 27Ω (both 0.5W) and R3 can be two 10Ω, 0.5W resistors in parallel. Keep all leads as short as possible particularly around the input and components R1, R2, R3, SI, D1, C1, C2. Use a physically small switch for S1. It is recommended to mount everything in a metal box. The metallic enclosure will add style and will keeps the RF where it should be!

Simple Dummy Load Power Meter

The ground connections should also be short. You can use any DC micro or milliamp-meter with suitable resistors R4 and R5 in series to read about 2 volts at full scale. The internal resistance of the meter should be more than 2ΚΩ. The diode D1 should have low forward voltage drop which means it has to be either a Schottky type or germanium. In the prototype, we use a BAT85 Schottky diode.


CALIBRATION

For accurate readings, you should calibrate the instrument. For the calibration, you will need a stable variable low voltage DC source capable of giving up to 50mA into the 50Ω load. Read the applied DC voltage to the power meter using a digital voltmeter. Adjust the voltage to the values in the table. You can then mark the power meter scale, either in dBm (power relative to 1mW-more commonly known as 0dbm) or in mW. Calibration is required only for the most sensitive range. 

 

Table 1: Calibration Data

Input DC Volts | Power (mW) | Power (dbm)
0.316 1 0.0
0.447 2 3.0
0.548 3 4.8
0.707 5 7.0
0.837 7 8.5
1.000 10 10.0
1.410 20 13.0
1.730 30 14.8
2.000 40 16.0
2.240 50 17.0

Table 1 is derived from the formula: Vdc = √(2RP), where R is 50Ω and P equals power.

 
DUMMY LOADS

What is a dummy load? A dummy load is a non radiated substitute for an antenna. There are several reasons to use a dummy load instead of an antenna, during RF tests. First of all it is illegal in most countries to radiate a signal when testing transmitters if it interferes with other users in the channel. Another reason is that it is just plain rude to cause interference in a channel just because you want to test your transmitter. Rather than pressing the push to talk button and interfering with other stations, you could silently key the power into a dummy load.

There is also a very good technical reason for using a dummy load to test a transmitter: antennas cannot be relied upon to provide the constant and consistent test load that is necessary to make sense out of transmitter tests and adjustments. The measurements that you make may not match the specifications given in the transmitter’s manual; even through there is nothing wrong.      

 

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