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High Power Attenuator
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Specifications
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This dummy load/attenuator was built for testing linear amplifiers. It provides nominally 30dB of attenuation, so an input of, for example 1kW, is reduced to 1W output.
The resistors used are all 5% tolerance, so once built, the insertion loss on each HF band was measured. The results are shown below.
| Band | Attenuation |
| 1.8 | 30.24 dB |
| 3.5 | 30.24 dB |
| 5.0 | 30.23 dB |
| 7.0 | 30.21 dB |
| 10.1 | 30.17 dB |
| 14.0 | 30.13 dB |
| 18.1 | 30.08 dB |
| 21.0 | 30.02 dB |
| 24.9 | 29.94 dB |
| 28.5 | 29.82 dB |
Click on a thumbnail below to see a larger picture
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The schematic. |
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The SO239 input on the left, BNC output on the right. |
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End view showing the fan. |
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Close-up of the resistors. |
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The power socket at the bottom right is for the fan (12VDC 1A). |
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Close-up of input circuit. |
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The heatsink is made from two finned extrusions bolted together to form a 120 x 120mm box section. |
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The insertion loss measured using a N2PK VNA using Exeter software. |
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The input SWR from 1 to 50MHz. The input SWR rises with increasing frequency to 1.5:1 at 50MHz. |
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The input impedance from 1 to 50MHz plotted on a Smith chart. The impedance is 50 Ohms at LF, the capacitance between the 500W resistor and the heatsink affects the impedance as the frequency rises. |