I sold low-power FM
transmitters in a former job, and one of the things I learned is that
most customers lacked a suitable dummy load for testing. In new LPFM
or translator applications, you can save a lot of time if you have a
way to confirm proper operation.
Fig. 1: Shown are the
parts to construct your 50-watt dummy load.
Remembering the days of
the Heathkit “Can-tenna,” which mounted the necessary load
components in a paint can, I contacted RW contributor Buc Fitch, who
frequently describes useful modifications and upgrades in our pages.
Buc recently developed a dummy load of his own that mounts in an
empty quart paint can (the kind of can you can buy at Lowes or Home
Fig. 1 shows the needed
parts. Visit your hardware drawer for brass bolts, washers and nuts.
You’ll also need 15 dual-hole solder lugs for the resistors, using
five for each of three layers, with two resistors on each lug. A
total of three single-hole lugs will also be needed. These tie the
center pin and ground connector leads to the resistor stack. (Head to
radioworld.com/links for a URL for solder lugs from Digi-Key.)
You’ll need 30 dummy load resistors — 1.5k-ohm, metal film,
2-watt resistors soldered in parallel. You choose the RF connector;
type N or UHF female are the most common and flexible. You’ll need
a small 3/8-inch rubber grommet, to serve as a vent, as well as about
two feet of #18 tinned solid wire.
Sort your parts and you’re
ready to go. Here are the Steps:
1. Take a long, 4-40
brass bolt and slip five dual position solder lugs onto the bolt
moving them up against the bolt head. Thread a 4-40 hex nut onto the
bolt and spin it down until the five solder lugs are snug.
Fig. 2: Solder the
resistors to the lugs to form a circle.
2. Position the five
lugs into a “star” configuration, with each lug offset in an
approximately 72-degree azimuth. Refer to Fig. 2 to see what this
3. Holding the lugs
roughly in position, tighten down the nut. Select two of the thirty
1.5k-ohm, 2-watt resistors, confirm that the leads are clean by
scraping each resistor leg with a razor blade knife until they shine.
Cut off one end to a 1-inch length, and 3/4-inch length on the
4. Mechanically attach
the two resistors through the lugholes by bending the short end at 90
degrees. The lead with the 3/4-inch length should be attached to the
far hole and the 1-inch lead to the solder hole nearest the bolthole.
5. Position angularly
as indicated in the photos and solder the resistors to the lug.
6. Bend the uncut end
in imitation of the assembly in Fig. 2, such that a common circular
“ring” will be created when all 10 of these resistors are
7. Solder the tip of
the 1-inch cut resistor to the 3/4-inch cut resistor.
8. Repeat this
procedure on the next lug in clockwise rotation and continue through
the next three lugs until all 10 resistors are installed.
Fig. 3: View of
Important: Even with
the best effort, sometimes there is insufficient lead length to make
a perfect common circle. Use small pieces of the bus wire to complete
a uniform, even and professional-looking circle.
9. Thread a 4-40 hex
nut onto the bolts, and spin down to just about 3/4-inch from the
10. Slip five dual
position solder lugs onto the bolt, moving them to the end against
the nut threaded in Step 9.
11. Thread a 4-40 hex
nut onto the bolt and spin down, until the five solder lugs are snug.
12. Once again,
position the five lugs into a “star” configuration with each lug
offset in an approximate 72-degree azimuth pattern.
13. Rotate this lug
cluster so that it is positioned with the lugs between the first set
and, holding the lugs roughly in position, tighten down the nut.
14. Repeat Steps 5
through 11 above on this set of lugs.
15. Thread a 4-40 hex
nut onto the bolt and spin down to just about 1-1/2 inch from the
16. Slip five dual
position solder lugs onto the screw moving them to the end against
the nut threaded on in Step 12.
17. Thread a 4-40 hex
nut onto the bolt and spin down, until the five solder lugs are snug.
18. Once again,
position the five lugs into a “star” configuration.
19. Rotate this lug
cluster such that it is positioned with the lugs in-line with the
first set and holding the lugs in position, tighten down the nut.
20. Repeat Steps 3
through 8 above for this set of lugs.
21. Choose a place on
the outer circle nearest the bolt head between the resistors and
attach a length of bus wire about five inches long.
22. Route the bus wire
on the outside of the middle resistor group circle and inside the
final circle common section.
23. Solder the three
connection points on the three circles. See pictures for routing
24. Repeat Steps 21
through 23 on the opposite side of the common circles.
25. At the very end of
the bolt, thread a 4-40 hex nut onto the bolt and spin down to just
about 1/4-inch from the bolt end.
26. Slip a single
solder hole lug onto the bolt.
27. Thread a 4-40 hex
nut onto the bolt and tighten.
28. Measure the
diameter of the center assembly of the chassis-mounted female N
Fig. 4: The finished
29. Select a drill bit
and drill a hole into the center of the lid to accommodate the
Important: The can
is very thin sheet metal; for optimal results, mark the hole using a
small drill bit such as 1/8-inch. Using this first hole as a guide,
progressively and carefully enlarge the hole until the hole is the
30. Insert the
connector and mark at least two of the four holes. If only two are
used they should be diametrically opposite.
31. Drill the holes for
4-40 bolts (approximately 1/8 inch in diameter).
32. Mark the top for
the “breather hole,” which should be eventually 3/8-inch. This
hole should be approximately 1 inch from the lid outer edge.
33. Drill out the hole
for the rubber breather grommet progressively, as outlined above, for
34. Mount the RF
connector using a 4-40 nut/washer/and bolt. Place a solder lug under
the two diametrically offset bolt positions mentioned above.
35. Bend down the
solder lugs under the connector.
36. Bend the one
single position lug at the end of the bolt into a half circle such
that the open lug hole will mate with the wire end of the N
connector, position the bus wires such that they are in line with the
connector solder lugs and solder.
37. At this point the
crenellated resistor assembly is attached to the connector at a
single point, so carefully thread the two common bus wires through
the connector lugs and solder in imitation of the pictures.
38. Cut off excess bus
wires. The unit is complete.
39. Distilled water may
be used as a coolant for testing and measurement but empty the dummy
after each use to limit rust and deterioration. Mineral oil is a good
permanent coolant; just keep the load upright so no coolant escapes
the breather vent.
40. Enjoy your dummy,
but always make certain that there is coolant in the can, the lid is
on tight and the breather unclogged.
Buc measured this load
flat out to beyond the 160 MHz RPU band, with a power handling
capacity of 50 watts. The parts for the load cost about $20.
Thanks, Buc, for a
to Workbench. You’ll help your fellow engineers, and qualify for
SBE recertification credit. Send Workbench tips to
Fax to (603) 472-4944.
Author John Bisset
has spent 44 years in the broadcasting industry and is still
learning. He handles West Coast sales for the Telos Alliance. He is
SBE Certified and is a past recipient of the SBE’s Educator of the