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Radio Room
Building A Dummy Load
Building and adjusting radio transmitting equipment means testing, tinkering, and experimentation.
However, radio frequency spectrum-space is limited, shared, and regulated. Therefore, it's an
important matter both of courtesy and of law that you don't clutter the airwaves with electronic noise.
What do you do with RF energy you want to get rid of? Dump it into a dummy load. The dummy load
can
take a transmitter's signal and turn it into heat without allowing any part of it to radiate into space.
You can buy dummy loads easily enough. However, I had some heat sinks and some other odds
and ends and decided to fabricate my own. Below are images showing how things came together.
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This is an image of some aluminum heat sinks I had
scrapped out of a couple of large computer UPS's. In
their original application, the manufacturer used them to
dissipate heat produced by some large SCR's.
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Neither heat sink was, in my estimation, large enough
by itself. I decided to link the two together. This image
shows an aluminum bar, about six inches in length, that
was used to couple the two heat sinks together.
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Here we can see the heat sinks coupled together. The
"legs" on the heat sinks are plastic stand-offs. I didn't
put those there, they came with the heat sinks. I
decided to leave them on.
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I knew that any wiring associated with the dummy load
should be enclosed. After all, I don't want any RF
energy escaping. This meant that I needed some lid
plates to cover the top and bottom of the assembly.
These are two chunks of extruded aluminum that were
salvaged from the case of an old test instrument I had
taken apart.
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This is the photo of good fortune. The lid plates (above)
feature tabs that hook inward. The heat sinks each
sport a pair of fins with hooks that point outward. By
wonderful coincidence, the two features interlock,
allowing me to slide the lids into place.
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These aluminum plates are used to seal of up the ends
of the dummy load assembly. Notice that one end
plate has been punched to accept a coaxial connector.
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Here, the end plate with connector has been
mounted. Notice also the 6-32 threaded studs that will
be used to mount a pair of power resistors. Also, note
the two ground lugs.
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This is an outside view of the end plate with its
connector. I should point out that I used lock washers in
conjunction with all screws. This was to assure good
electrical contact everywhere.
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This image shows the power resistors bolted into place,
and the wiring complete. I used two 25 ohm, 100 watt
resistors (Mouser part number 684-MP9100-25). These
resistors were chosen because of their high power and
low inductance. Together, they form a 50 ohm load
capable of dissipating 200 watts. I used silicone grease
to assure good thermal contact with the heat sinks.
Also, the round objects you see are small disks of
fiberglass circuit board material, used as washers. I was
concerned that if I tried to tighten a nut directly against
the resistor housing, I might crack it.
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Here's a final shot of the dummy load just before I slid
the top lid into place. Once set, I screwed on the
right-hand end plate, and the dummy load was
complete. A quick check with my digital multimeter
shows 50.3 ohms resistance. |
Document Revision 1,
xx/xx/2003
Document Revision 2, 04/22/2007
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