Brats club logo

Bredhurst Receiving and Transmitting Society

ILC title

Construction Part 2

Practical 9.


Making a 40W 50 Dummy Load for use on HF

Disclaimer If you build this project, as the course writers and publishers have neither control over its construction nor use, the responsibility for ensuring that it is suitable for your needs rest solely with you and the course writers and publishers cannot be held responsible for damage to equipment connected to the unit howsoever caused.

Q. Why is this dummy load only suitable for HF ?

The reason is because at the VHF frequencies (and above) the lead lengths cause an inductive load to appear (in addition to the resistive load)  and does not give a 1:1 SWR but in fact at 145MHz gives an SWR of 1:2.5 ! So use this dummy load on HF frequencies only !!

This project from the outset might seem so simple as to be ignored, but it involves

  • locating parts

  • do some "metal bashing" (drilling , cutting, filing etc to metal)

  • soldering

  • testing

and so it encompasses all the items that makes it a valid project for the Intermediate Course practical and you end up with a very useful test item. Total time taken to make 3 1/2 hours from start to finish including time to take photos.

Anticipated time for students 2 hours if the tin is already drilled and the connector and solder tags fitted.

The tin shown is not the only one that would make a good dummy load enclosure but it the one that was in the cupboards and emptied of treacle and washed out dried and made ready for use in the project.

You can see that the lid has been drilled to take the aerial chassis socket. Care is needed when drilling that the holes are made in the right place, and are neither too small nor too big, but just right. A small round file can be made to ease out the centre hole if you do not have a drill large enough.

When drilling remember all the safety tips given in this regarding the use of the drill, file and especially holding the piece that is being worked upon.

Here you can see the 4 x 6ba screws three of which have their 6ba solder tags in place the forth shows you that the solder tag in simply bent up at an angle.

The chassis socket and the solder tags have now been attached to the under side of the lid in preparation for soldering up the dummy load.

When you reach this part do make sure that the screws are nicely tight as tightening then up later might be difficult!!!

By this stage you have had to do "metal bashing", bending of the solder tags and fitting the chassis socket to the lid ALL without injury to yourself. If this is the first kind of construction that has included "metal bashing" then well done - GOOD metal bashing is perhaps even more difficult to achieve than good soldering !!!

Now form a piece of solid copper wire into a circle around the solder tags and solder to the outside of the tags with good solder joints.

Then with another piece of solid copper wire solder it to the centre of the chassis connector, then form the wire into a smaller circle than around the solder tags and about 30mm from them and solder the end together. You should now have the project as shown in the picture.

Now make you should make your first continuity check that from the copper wire attached to the solder tags to ensure that there is a complete circuit to the outer part of the chassis socket. If not then find out why not an correct.

Then make the second continuity check that from the smaller circle of wire that there is NO CIRCUIT to the larger circle of wire. It there is then find out why and resolve because this means that there is a sort between the centre of the chassis connector and the outer part of the connector.

Now with both of those check satisfactory check that the smaller circle of wire has a circuit to the centre of the chassis socket.

With all of the check completed to your satisfaction you can start to assemble the resistors.

Have you thought why you need 20 1k resistors of 2W rating to make this 50 dummy load ?

The resistors are being used in parallel thus the resultant resistance value is the value of one of the resistors divided by the number of resistor. 1000 / 20 = 50 and the rating of the unit is the number of resistor multiplied by the rating of one of the resistor 20 x 2 = 40 so 40 watts.

Now those of you with eagle eyes will have noticed that the colour code of the resistors used is Brown Green Red. Why has this value of resistor been used - well because a wrong order was made and it was a case of use what was available - so how many resistor will be needed to still make a 50 Dummy load?

Note the the resistors are crowded together. This is merely to ensure that they will fit into the container. The minimum distance one to another should be twice the diameter of the component - so even those on the outside will get warm !!

So have you worked out the new number of resistors?

The value of of one resistor -  1500 divided by the (Number of resistors) = 50

So 1500 / 50 = number of resistors = 30

So what is the new power rating ?

30 x 2 = 60W but that is the maximum limit if the resistors were well spaced and the Dummy Load was running for a long time. However fill the tin with fresh cooking oil and leave a small gap at the top and put the assembly into the tin and push down and you would from the original calculation a Dummy load that for short periods of a few seconds would comfortably handle 50W the power limit for the Intermediate Licence. If you left it in use for a long time the resistors would eventually make the oil hot and that could be dangerous as there is no vent hole to let out and internal pressure.

So be careful how you use the dummy load. No responsibility can be accepted for accidents howsoever they are caused as you have control of the situation.

The dummy load as shown above should be able to cope for an infinite period of time at 10W and that is the power at which it will be tested.

The soldering on such a project needs to be "butch" and for certain no dry joints else the health of the rig is at risk and with the 30 resistors used there are 60 joint to the resistors, 4 more on the solder tags and at least 2 to make up the centre support link making a total of 66.

So let no one tell you that this is an easy project but it is a simple project.

The measured resistance of this dummy load with a digital meter was 50.4 which is well within tolerance of the components.

Component list

Resistors

Capacitors

Semiconductors

Value

Required

Value

Required

Type

Required

1k 2W carbon

20

None

None

or 1k5 2W

30

Other components

If is essential that the resistors have no inductive component and thus must be made of carbon

Aerial connector chassis socket (female) preferable with screw fixing holes and not a chassis socket which is secured by a single ring nut.

4 6ba screws and 6 ba nuts

4 6ba solder tags

Suitable container

two lengths of copper wire


Back one page

brats copyright logo