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4.6 Do-it-Yourself Light Bulb Test
Any time that a transmitter is used, it must be outputting into a
load. A load is anything that the output power can be pumped into.
If the transmitter is operated without any sort of load connected,
the final amplifier stage could become severely damaged. You
should never test a transmitter on the air for the first time, if you
are unsure about how to operate it, and if you are unsure whether it
is working properly. You could create harmful interference to other
stations.
To test transmitters without actually operating into an antenna,
dummy loads were created. A dummy load is a load that will
dissipate the energy from the transmitter instead of emanating it
into the ionosphere. Nearly all commercial dummy loads are large
oil-filled cans. These dummy loads change the transmitted energy
into heat, which is absorbed by the oil. Because different
transmitters output different amounts of power, different sizes of
dummy loads must be used. Dummy loads for typical amateur
powers (<500 watts) are relatively inexpensive and are readily
available.
Unfortunately, when you use a can-type dummy load, you can’t
see “what’s happening” with your transmitter. In this case, you can
use a light-bulb dummy load to test your transmitter. Here, the
light bulb is directly connected to the output of the transmitter and
it dissipates the RF energy as light. The light bulb dummy load is
more useful than the oil-can type because you can guess how much
power is being output, you can see the voice modulate the SSB
(the light will flicker with your voice peaks), and you can tune the
transmitter for maximum output (if the transmitter is an older
model that requires tuning).
Before building or using the light-bulb dummy load, remember
that these models typically don’t dissipate the transmitter’s output
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