500w PLUS DUMMY LOADDescriptionThis article describes how to construct a 500 watt dummy load (artificial antenna) from wire wound resistors and parts form your junk box. The whole project will cost around AU$50.Why wire wound resistors?In 1999 I started doing some study for the Unrestricted Amateur Operators Certificate of Proficiency, now called Advanced or AOCP(A). During that time I needed a simple but effective dummy load that could handle in excess of 500 watts key down. I Googled everything I could find on the topic and I came to the conclusion that the loads used either: -
"They are additive so you get 20 watts" I said. "And what happens to the resistance when it is in parallel?" Ron Asked. "The more resistors in parallel the lower the resistance." I replied. "Right, and what happens when you put inductance in parallel?" he said. "The more inductance in parallel, the lower the inductance." I said. "Ahh Grass hopper..." I thought; this would be a good experiment, if nothing else I would get some soldering practice and a bunch of wire wound resistors for the junk box. Parts List and ToolsThe whole project should cost around AU$50, assuming some items are already in your junk box.Parts list: -
Safety FirstHow easy is it to get and eye injury form flying hot molten solder or a cut finger that could get septic and fall off after a seemingly minor cut or abrasion?OK, maybe I've gone too far with the finger falling off, but you get the idea. Safety glasses, long pants, long sleeves and closed shoes (Not "El-Cheapo Safety Boots.") are a must. Any electrical items, like the cable to the soldering iron or extension cord should be in good condition and tested by a licenced electrician. Tell someone what you are doing, so they can check on you periodically (with a cold drink preferably) if you get too quiet. ConstructionBegin by drilling a 16mm hole in the end cap. This will be for the SO-239 connector. Position the connector so it is near to the edge of the end cap, this will leave enough room for the 6.5mm vent holes above it should you include the fan option. If you are not including the fan then just locate the connector centrally in the end cap.File or cut a flat on to one side of the PVC flange, this will stop the dummy load form rolling of your bench. Once this is done you can secure the 100mm computer fan to the PVC flange. It is better to pressurize the unit with air than to try and draw the air out. Leave enough wire on the fan to connect it to your 12 volt power source. I connected mine via a switch so that it could be turned on and off as required. Lay the brass welding rod down so that they are parallel to each other and solder the resistors to it. Remember to keep an air gap of at least 5mm between each resistor. You might like to solder some locating tabs on to the brass welding rod to suit your PVC tube while you are at it. NOTE: You MUST keep the resistor leads as short as possible. Once you have bent the resistor assembly to suit your PVC tube, you have to solder a link from each end back on to itself to prevent stray inductance. Sounds strange but believe me if you don't do this the dummy load will not provide a good match to your transmitter. That’s it, assemble the parts, and check the dummy load with a RX noise bridge, antenna analyzer or SWR meter and multi-meter. It should show very close to 1.3:1 and 50 ohms. ConclusionThis article has described how to construct a 500 watt dummy load (artificial antenna) using wire wound resistors. Without a fan, the load will handle 500 watts Key down for 1 minute. With the fan option, the dummy load will handle substantially more than this.When I made the original dummy load, I used 54 off 2.7K 5w wire wound resistors giving me 270 watss capability. With the fan turned on it has handled 400 watts key down for 15 seconds and has lasted me nearly 10 years at time of writing (2009). The load that I built has been tested up to the 10m band, and has a resistance of 50.2 ohms, giving an indicated SWR of 1.2:1. I have not measured the inductance of one of the wire wound resistors, but it would be negligible, particularly when you get so many in parallel. When I built the lower power version the whole project cost me around AU$13, as I had all of the gear to build it, except the resistors. The argument has always been that you can not use wire wound resistors for a dummy load. Though this may be true for say 6m and up, for H.F. there is no problem. I hope that you have enjoyed this article, and, if anything, it has given you some ideas so that you can build your own "artificial antenna". BCNU Adam VK4IMBrisbane, Queensland. AUSTRALIA |
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