SG-231 MANUAL
SGC Inc. SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
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resolution at the lower frequencies where antenna systems are inherently narrow band.
Usually, only one or two memory positions are needed to provide satisfactory coverage
at higher frequency bands.
6.7 Do-It-Yourself Light-Bulb Dummy Load
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. The
problem is that 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 (under 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 as well as an oil-can dummy load. The
result is that RF will "leak" out; I have heard a few stories of amateurs who were heard
around town while operating their transmitters into a light-bulb dummy load. If you
use this system, make sure that you test the equipment on a clear, harmless frequency
(NEVER test with the transmitter set on an emergency frequency, such as 2182 KHz).
SGC recommends that you build the light-bulb dummy load with the following parts
(although I have made one with an old light fixture and a makeshift version with just
alligator clip leads and a light bulb):
* AC socket to cable with a PL-259 connector (for transceiver)
* AC socket to cable with alligator clips (needed with coupler)
* Light bulb to AC adapter
* 75 to 125 watt light bulb, 120 to 220 VAC
* 100 watt radio transceiver