UHF Dummy Load

A homebrew UHFDummy Load

A homebrew dummy load is usually a combination of several parallel carbon resistors, in order to be able a final resistance of 50 Ohms. The number of resistor depend on power of dummy. Actually a dummy for HF is easy to construct, but the same Dummy has problem into UHF (430-440 MHZ), it appears an unexpected high SWR ratio, so in practice this type of Dummy is unavailing for any measurment.

The reason of the unwanted high SWR ratio, its the parasitic capacitance of resistors. FIG.1 shows a carbon resistor. The two "contact hats" on the opposite sides of ceramic body of resistor, they acts as two plates of a small capacitor ! Additional the capacity value of each resistor "multiply" by the number of total resistors ! Catastrofy !

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FIG. 1

If we want to cancel this unwanted capacity action, the solution is very simple. Just by using a small coil, in practice that is enough to minimize the SWR ratio to 1:1. Fig. 2 shows the diagram of a UHF Dummy Load.

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FIG. 2

The "R" value is 560 Ohms / 2 W. I've used 11 resistors in parallel for a final value of about 50 (50.9) Ohms. The total Power is 11x2 = 22 Watts, but in practice this Dummy works up to 100 W for a few seconds without problems. This power its enough for any measurment, the most commercial UHF transceivers have an output Power between 5 and 35 Watts.
FIG.3 below shows the practical construction of Dummy (back-top & side view). I have used a chassis N-Type female connector. The "L" is 4 turns of 0,8 mm wire on 3 mm diameter, length of coil about 15 mm (air-coil, without tube). If you have not the optimum SWR, you can increase or decrease the length of coil to minimize the SWR. Finally, keep in mind this Dummy with this coil is just for UHF. (For VHF appears SWR 2:1 but I suposse it will works with a larger coil.)

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FIG. 2

That's all folks !
Have fun with Homebrewing !

Makis SV1BSX

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