Dummy Loads – What use are they?

A dummy load (in amateur radio) is a resistive termination load for a radio frequency source.  It usually consists of resistors made of carbon or similar materials that can absorb energy.

The dummy load is used to absorb the radiation from the transmitter so that the power output can be tuned,  or the frequency or audio can be checked and adjusted,  or so you can monitor your signal without interfering with other stations by broadcasting “on the air”.

If you are operating a hand held radio on two meters and its antenna is a little rubber duckie or flex antenna on the top of the radio, you probably don’t need a dummy load.  However, once you transmit, the chances are you are going to interfere with other stations.  How do you adjust frequency,  listen to your signal, check audio or peak power out, without interfering with others?  You disconnect the antenna and connect the transmitter/transceiver to a dummy load.

It doesn’t matter what ham band or power level you operate at,  the problem of interfering with other stations is resolved by testing with a dummy load attached instead of an antenna.

The ideal dummy load should measure 50 ohms (or 70 ohms) resistance regardless of frequency.  Carbon resistors meet this need.  A light bulb will work , but its internal resistance changes as the filament heats up.  In the “old days” we soon discovered that light bulbs do radiate over short distances!

How do we get rid of energy?  Energy can be turned into light, heat and/or sound. Today’s dummy loads absorb energy by heat.  If you have a dummy load,  you must know its power limits!  A five watt rated dummy load can handle 5 watts for a short time.  Always watch your dummy load for excessive heat!  Larger rated dummy loads depend on large cooling fins or are immersed in oil for heat dissipation.  Any of these can be overheated.  Always allow a cooling down period between ” transmitter on”  intervals. 

The Pastime Projects Dummy Load Kit uses 4 internal resistors of 2 watts rating each.  This means they should handle 2 times 4 or 8 watts power.  So they will handle 5 watts pretty easily.  Again, just watch the case for heat, and pause your “transmitter on”  time when necessary to allow the dummy load to cool down a bit.  This dummy load kit can be connected to the output of our 6V6 “Remember When” kit to prevent radiation while testing the transmitter.  As long as the power output is within ratings,  the transmitter can be tuned for maximum output and other tests made using the dummy load.  Another use is to check for the second harmonic by watching the “S” meter of a nearby receiver while the receiver monitors the signal on the primary and secondary (harmonic) frequencies.  The harmonic frequency – for instance the 40 meter signal when operating on 80 meters – should be much weaker that the fundamental frequency output. 

The dummy load resistance is chosen to be same as your antenna impedance.  That means, your transmitter will be happy if it is “looking at” a 50 ohm dummy load or a 50 ohm antenna.  In other words,  you can switch from dummy load to on-the-air antenna without adjustment so long as they both are resonant at 50 ohms.. More later…. 73 Glenn  

pastimeprojects(at)yahoo(dot)com.

 

 

 

 

 

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