Sunday, June 29, 2014


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100ohm power resistors for use in Wilkinson power splitters are often available at low cost. A pair of these resistors will make an excellent dummy load. I recently acquired a power film resistor which has four 50ohm resistors mounted on a large copper heatsink flange. As all of the resistors are connected to the mounting flange at one end, it was not possible to connect the resistors in a series/parallel combination to make 50ohm. The solution was to connect the four resistors in parallel to make a 12.5ohm load and use a 4:1 transmission line transformer to provide a 50ohm match. The transformer consists of two turns of RG58 on a Maplin N90AB ferrite core. This gives a perfect 1:1 SWR from 1.8 - 30MHz. The finished circuit is in Figure 6. Note that the DC voltage from the diode is only half the peak-input voltage because of the 2:1 voltage ratio of the transformer. I haven't been able to make much of an impression on this load during testing. Several minutes of SSB speech at 500W PEP will bring it from room temperature to lukewarm. I'm sure my transmitter would catch fire long before this load gets hot!

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