Here is the list of parts, sources and costs.
Projects under $50
150w Dummy Load
What is in that mysterious Can? A paint can
with an M connector sprouting from it, that magically
took RF and protected your finals from certain
destruction. What could possibly be in a gallon can that
works on all bands? Well I quit being a dummy and
opened an MFJ Dummy load. To my amazement was a
twist of resistors, oil, and a black abyss. So first thing is
there is oil. What kind I am not sure but I knew it wasn’t
extra virgin. Any oil that will not go rancid should do.
Next I noticed a rats nest of resistors. Those resistors
were small and plenty wired in parallel, so I guessed
they are high resistance and low wattage. I then started
researching other projects and realized that a gallon
size was overkill and most of us do not need 1kW of
Dummy when 100W continuous and 150 peak would
do. The bulk of the parts, the can, the oil and the
connector are inexpensive. I knew what would make or
break the project was finding the right source of
resistors.
Shopping list
Source
Part Cost
Part
(3) NH050150R0FC02 Mouser
Shipping
$ 5.89*
$ 8.00
$ 1.57
$ 0.89
$ 3.89
$ 24.02
*If bought in more than 10 count.
$
$
$
$ 8.00
1 Quart Oil
1 Quart Paint Can
(1) SO-239
Pep Boys
HD
Amazon
You will also need some 18AWG wire, solder
and screws to mount the SO-239’s to the top of the can.
You will also need some liquid E-tape to seal the
connector from oil wicking up the connector.
The finished project looks something like
this.
I started off with (2) 100ohm 100w resistors
that I found on the internet. The description said noninductive, very important in this application. When I
installed them the SWR was through the roof; I have
been had by the internet, not the first time. I decided to
skin this a different way and went to Mouser for the
parts. There I found what I was looking for 150ohm 50w
resistors. Three in parallel would make for a 150w peak
dummy load at 50ohms.
The thing about Mouser is it always pays to buy
in volume. These things have a pretty steep price break,
in this case from $12 to $5.89 a piece if you buy 10 or
more at once. I recommend you find 3-4 other people
and go in on making these as the price break is an
artificial cost that can be avoided.
From the photo you will notice I used a copper
braid split into 3 even pieces. One thing that is
important is that all the wire be the same length, type,
and size. This allows the power to be evenly spread
across all three resistors. The return route is not as
clean of a path but is equal lengths of wire. In the end it
will be in a can, so function trumps form. I tried to
spread the resistors out as much as possible to increase
cooling.
Places to splurge on this project would be to
include terminals to measure voltage as transmitted.
This is useful when ascertaining how much power a rig
can push. Also for a cleaner install you can use mineral
oil. It is less messy but more costly than motor oil. A
quart of mineral oil is $8.98 at CVS.
This project is one that is easy on the skills, is a
quick to build, and is straight forward to make work. A
Dummy Load is something we all need in our shacks and
at a price that many 30-60w versions cost is a great
project to tackle. Have fun, be safe and don’t be a
dummy, make your load.
Regards
K3LLC