Dummy loads.

What are they, and what are they for?

A dummy load takes the place of an antenna. It takes the transmitter power, and converts it to heat.

Transmitters that are unable to output the power they have generated tend to suffer damage. When transmitter output stages used valves, they would withstand a “bad load” situation for a short while, and possibly even withstand a “flashover” inside the valve. So the situation was tolerable, if unwise.

When transistor transmitters first came along, they would not withstand a bad load, and were easily damaged. Circuitry was devised to prevent damage, and this was done by effectively building a VSWR “meter” into the transmitter, but instead of the voltage it produced driving a meter, it was used to feed back to an earlier part of the transmitter circuit, reducing the gain, and thus the output power. This resulted in a transmitter output stage operating at a power that would not cause damage, even with no load. But with your £1500 radio, would you intentionally prove the point?

Either way a transmitter not connected to a decent “load” is not operating as designed, and could have undesirable characteristics. A decent antenna will present a correct load to a transmitter, but will radiate a signal. If you wish to test transmitter power, and make adjustments, then you will not want to radiate a signal, and this is where a “dummy load” becomes useful.

Antenna basics

An antenna is a piece of wire. (Mechanical considerations may require a stiff tube or similar, but for electrical purposes, it is still a piece of wire). A piece of wire, whether straight or wound into a coil possesses inductance, and as part of an antenna system, may exhibit capacitance. Whether inductance, or capacitance, these are “reactive” properties. A reactance does not convert energy to heat. It will store that energy, to return it on the next half cycle of the radio frequency energy that is fed to it. If an antenna is perfectly resonant, (and having the correct “radiation resistance”) the signal will all be radiated. If that antenna is just a little too long, it will be “inductive”, and if too short, “capacitive”. Some of the applied energy will then be returned to the transmitter. The higher the frequency in use, the more the effect of a few unwanted millimetres of wire.

Figure 1

Dummy load considerations

The dummy load is also subject to these constraints. A wire wound resistor is not just a bad choice for a dummy load, it would be completely ineffective. Carbon resistors or similar give much better results, but even here, the lead lengths are critical for correct performance, with VHF operation being problematic, and UHF operation impractical.

A “decent” dummy load needs to be very small, with exceptionally short leads, but at the same time, it needs to be able to handle the power it has to convert to heat. Fortunately, there are “termination resistors” available that meet the requirements. When you consider that a standard resistor maybe 25mm long and 5mm in diameter can handle a watt or so, then figure 1 illustrates a resistor that is 25mm long, 10mm wide, and can handle 250 watts, at frequencies up to 3 Ghz! Easily available resistors start at 50 watts, and go to 250 watts. The load on the left can handle 250 watts, while the other one is a 150 watt unit.

I have a 49 year old HF transceiver whose output impedance can be adjusted anywhere between 40 and 120 ohms. But all my other radios are designed for a 50 ohm output impedance. Ensure you obtain the appropriate value resistor. (All mine are 50 ohms).You can see that it looks like a power transistor for higher frequencies, in that it has a “foot” with securing holes to attach it to a heatsink, but it only has one “tab”, the other connection being the metal foot. If you just get a piece of co-ax and connect it to the tab as short as you can, and earth the co-ax to a copper plate that the resistor is fitted to, keeping the leads as short as possible, you will probably get a dummy load that works very well to way over 500Mhz, and for most of us, that will be just fine. See figure 2.

Figure 2

Be careful

The first caveat… The “tab” is very fragile indeed. These termination resistors are intended to connect to a “stripline” printed circuit board. Making such a PCB, although not difficult, still leaves you with a transition from co-axial (from a socket) to the PCB, so you may just as well dispense with that, and with due care, connect the inner of the co-ax directly to the tab. This will be the LAST part of the construction.

Parts required

Termination resistors are freely available on ebay, at a reasonable cost. You will also need co-ax, maybe 150mm of very thin stiff wire, a small piece of braid, a connector of some sort, (DO NOT use an SO239 or PL259. BNC or “N”types are a must!)a heatsink, two screws and nuts to secure everything to the heatsink, (but see the “options” section for the required length), and possibly a fan. A small piece of copper plate between 0.5 and 1mm thick will also be needed. Finally, heatsink compound is required.

Construction

The construction is fairly straightforward, remove about 25mm of the co-ax outer jacket, push the braid back slightly, then strip off 3mm of inner insulator. Bring the co-ax braid forward again, so that it covers up to the end of the inner insulator. If necessary, trim off a very small amount of braid to allow the last 3mm of inner to be exposed. Use the very thin wire to bind around the outside of the braid to keep it in place. Solder this with a large, very hot iron, (so that you can do the job quickly).

Prepare the copper plate by marking and drilling it, and drill the heatsink. The idea here is to fix the termination resistor to the copper plate and the heatsink. As I had taps, I tapped the heatsink, but through holes with nuts and screws will be equally good.

Make sure that all holes are “deburred”, and that the copper plate is flat, with no rough edges. If these points are not adhered to, the power handling of the project will be compromised.

Some of my dummy loads have used “flying leads”. That is to say the co-ax connects to the resistor/copper plate, and the other end has a plug. It is then necessary to secure the co-ax somewhere on the heatsink to avoid straining the joint to the resistor. The completed project shown here has an integral socket at the end of the heatsink, and was more fiddly to assemble. More elegant perhaps, but more work required.

Assembly

First, make sure everything fits, all the holes line up etc. Screw the resistor to the copper plate, offer the co-ax up to the copper plate, and align the centre to the resistor tab. (DO NOT SOLDER TO THE TAB YET!). Solder the co-ax outer to the copper plate. (Make provision for the fact that it is going to get very hot…).

When cool, remove the screws and the resistor. Apply heatsink compound on the underside of the resistor, and also to the underside of the copper plate. Assemble to the heatsink and tighten. Attach the plug, provide strain relief for the co-ax by clamping it to the heatsink (if using a flying lead).

When assembling, the LAST thing to solder is the co-ax centre to the tab. This must be done after everything is aligned and attached, as mentioned earlier, the tabs are fragile. A powerful soldering iron with a small bit is helpful, because although it is a small joint, if you have all the heatsink arrangements correct, the heat will be taken away from the joint.

Testing

First thing to test is to measure the resistance as seen at the plug. This should be very close to 50 ohms.

With a 70cm FM transmitter, and a VSWR meter that can work correctly at that frequency, connect the radio to the meter and the meter to the dummy load, and set the radio to LOW power. Check VSWR, which should be at, or almost zero. (1:1).

Raise the power if this is successful, and check at medium power, say 20-30 watts. Transmit for one minute if the VSWR is OK, then see if the heatsink is starting to warm. If it is, OK, if not, check the termination resistor, and if that is getting hot, something is amiss with the heatsinking.

If everything is OK, transmit at full power, (but not more than the rating of the resistor you obtained). Check the heatsink and resistor after 20 seconds. Remember that the radio may not appreciate full power for extended periods, my HF radio is 10 seconds only at full power carrier. A hot heatsink is a very good sign. A “too hot to touch” heatsink is a sign to take a break!

What if you don’t have a 70cm radio? Do it at 2 metres, where you should have absolutely no VSWR at all. If you get a 70cm radio later on, you can always check it then…

Options

It is unlikely you would need to fit the extra piece of braid, and even less likely you would need to fan cool the heatsink. The choice is yours.

Figure 3

Finally, figure 3 shows how (as much as possible) the “co-axial connection” aspect is maintained where the co-ax inner joins the tab. The extra braid that passes over the top actually reduced the 900Mhz VSWR from 1.4 to less than 1.2. That braid gave a capacitance to a piece of co-ax inner about 3mm long. And at 900 Mhz, it made a difference. Perhaps you can see why a bunch of resistors hanging off the back of an SO239 just does not “cut it”…

To fit the braid over the joint) will require dismounting the resistor. Slacken the screws but do not remove them. (Longer screws may temporarily be required, exchange them one at a time, it is vital to avoid strain on the resistor tab). A small piece of fibreglass PCB or similar could then be slid underneath the copper plate. Gently tighten the screws. This arrangement will allow you to solder the braid; if the copper is still attached to the heatsink, soldering will be nigh on impossible, even with a 100 watt iron. I know….. Solder the braid onto the top of the co-ax, and also the copper plate, ensuring that you do not short out the centre of the co-ax to earth. Proceed with the main testing sequence.

Slacken the fixing screws, remove the material you placed under the copper, and ensuring sufficient heatsink compound remains, retighten the screws and run through the testing section.

Dummy load uses

You use a dummy load to test a radio. If you have a very old transmitter, you can use it to tune the radio to produce maximum output into a 50 ohm load. (Like my old transmitter). I could tune for maximum output at 120 ohms impedance, but no-one makes a VSWR meter for 120 ohms, and I don’t have a 120 ohm dummy load…..

You can check that the power of your radio is what it should be. With a decent dummy load and a decent power meter, you will get an accurate reading, but if there is any VSWR on the antenna system, you will never be able to get an accurate power measurement. (For two reasons, the transmitter may be “cutting back” to protect itself, and the power meter cannot accurately measure in the presence of a VSWR).

I have a mobile DMR radio, permanently on a dummy load. It works into a “hotspot” in the shack. No need to radiate all over Kent when I need to go 1 metre….

Alternative dummy load designs.

Should you buy a 100 metre roll of very cheap, thin co-ax, and put a plug on one end, and NOTHING (or a complete short circuit) on the other end, you will get acceptable performance as a dummy load for 70cms and higher. If you have that coil of co-ax, and the dummy load under discussion connected at the far end, it will be good for pretty much any frequency you could throw at it. If you uncoil the co-ax and run it up the garden, it will have even more power capability.

Conclusion

Easy to make, and you do need one. A used one of similar performance would set you back over £50. This should work out less that £20, if you had to buy all new parts. I used an old 25 watt CB “boots” heatsink I have had for close on 40 years….

73 de Stan, G4EGH.

Word document of this project is available for download, please use the link below.

Dummy loadsDownload

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