Dummyload

By Cor van Rij

By law a radio amateur is required to have in almost every country to have a so-called nonradiating load for the transmitters output. This dummy load is an absolute must-have for measurement and test purposes. Aswell it is the way to make sure you keep your neighbours happy by not disturbing their TV receptions with unintended interference...

The described dummyload has the ability to absorb 10 watt of RF power continuously and 20 watt for several minutes. This might not seem like a lot (and truefully it isn't) but for QRP work it is more then enough. Especially considering that there are lots of people for achieved the "1000 miles per Watt" award and contacted more then 100 countries with just 1 Watt of RF power...

The dummy load is build up of 20 resisters of 1K? @ 0.5Watt, which gives the standard impedance of 50 ohm. The main task of the dummyload is to absorb the transmitter RF output power and to reflect a minimum of power back to the resistor. To make sure that inductance is kept to a minimum, the wires of all resistors should be kept as short as possible. For the same reason make sure NOT to buy wirewound resistors!

The dummyload is put into an empty applesyrup can. Make sure to cleanse it properly and leave no food residues. After cleaning let the can dry throuroughly. Still air, is a bad thermal conductor. However, drilling holes in the case is not an option since we don't want the dummy-load to radiate. Therefor the can will be filled with a mass that is a good thermal conductor, in our case sand. The sand is used as a thermal conductor between the resistors and the metal case, thus effectively enlarging the surface area and improving the colling properties.

After the can is dried, apply two layers of lacquer to the can so it gets a nice finish. It doesn't add anything to the performance of the dummy but just because it is a homebrew device doesn't mean it cannot have a nice finish.
Drill a hole in the lid for the BNC connector. Lid and tin should have good electrical contact so don't paint the inside of the lid and the leave the very top of te can clean. Actually it might be a good idea to sand it and remove any lacquer and reveal the pure metal.

To prevent any unwanted inductance, all wires should be kept as short as possible. To make the assembly easy I used two small PCBs with 9x9 holes. Keeping 1 hole spacing between the holes used to insert the resistor this gives 5x5=25 places to put the resistors. I kept the center one free (for the wiring coming from the BNC connector as well as the 4 holes in the corners.

This is the final results after playing around with the iron for 30minutes. The solder pattern on the top PCB is made in a snowflake kind of shape. To keep wires short
You will notice that the solder pattern on the bottom PCB is like a horseshoe. This is to keep the center hole free. From the BNC's center connection a small wire is coming through the center hole in the bottom PCB to the center hole in top PCB. Theground connection of the connector is soldered to the horseshoe on the bottom, caprice?

I used sand to fill the can and to be the thermal conductor between the resistors and the can. Some people use oil (as do most professional made dummyloads). However, this can get very messy if it starts to leak. I just took some sand from the beach and let it dry thoroughly in the oven. I heated it twice at 200慢. In between I reshuffeld the sand a bit to make sure it was heated evenly. I am not sure how critical it is but I figured I'll use the sand immediately after cooling down before it had the possibility of soaking some humidity from the air.

Niw, it's time for teh final ssembly. Gently press the lid with the resistor/PCB-sandwich in the sand. Turning the lid slowly in a screwlike motion helped a lot.

A quick test revealed that the resistance is close to what is should be, 0.6% deviation. OK at DC... (Unfortunately, I didn't have an impedance brdige to measure it at higher frequencies.)

Final result and yes... actually I am quit proud of it :-)
Next step(s)? Finish my new powersupply and build a transmitter to get it hot. Oh yes, and get my amateur license (hopefully due in December).

Have fun
Cor

	The dummyload is put into an empty applesyrup can. Make sure to cleanse it properly and leave no food residues. After cleaning let the can dry throuroughly. Still air, is a bad thermal conductor. However, drilling holes in the case is not an option since we don't want the dummy-load to radiate. Therefor the can will be filled with a mass that is a good thermal conductor, in our case sand. The sand is used as a thermal conductor between the resistors and the metal case, thus effectively enlarging the surface area and improving the colling properties.
	After the can is dried, apply two layers of lacquer to the can so it gets a nice finish. It doesn't add anything to the performance of the dummy but just because it is a homebrew device doesn't mean it cannot have a nice finish. Drill a hole in the lid for the BNC connector. Lid and tin should have good electrical contact so don't paint the inside of the lid and the leave the very top of te can clean. Actually it might be a good idea to sand it and remove any lacquer and reveal the pure metal.
	To prevent any unwanted inductance, all wires should be kept as short as possible. To make the assembly easy I used two small PCBs with 9x9 holes. Keeping 1 hole spacing between the holes used to insert the resistor this gives 5x5=25 places to put the resistors. I kept the center one free (for the wiring coming from the BNC connector as well as the 4 holes in the corners.
	This is the final results after playing around with the iron for 30minutes. The solder pattern on the top PCB is made in a snowflake kind of shape. To keep wires short You will notice that the solder pattern on the bottom PCB is like a horseshoe. This is to keep the center hole free. From the BNC's center connection a small wire is coming through the center hole in the bottom PCB to the center hole in top PCB. Theground connection of the connector is soldered to the horseshoe on the bottom, caprice?
	I used sand to fill the can and to be the thermal conductor between the resistors and the can. Some people use oil (as do most professional made dummyloads). However, this can get very messy if it starts to leak. I just took some sand from the beach and let it dry thoroughly in the oven. I heated it twice at 200慢. In between I reshuffeld the sand a bit to make sure it was heated evenly. I am not sure how critical it is but I figured I'll use the sand immediately after cooling down before it had the possibility of soaking some humidity from the air.
	Niw, it's time for teh final ssembly. Gently press the lid with the resistor/PCB-sandwich in the sand. Turning the lid slowly in a screwlike motion helped a lot.
	A quick test revealed that the resistance is close to what is should be, 0.6% deviation. OK at DC... (Unfortunately, I didn't have an impedance brdige to measure it at higher frequencies.)
	Final result and yes... actually I am quit proud of it :-) Next step(s)? Finish my new powersupply and build a transmitter to get it hot. Oh yes, and get my amateur license (hopefully due in December).