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20 dB 100 Watt RF attenuator.

On this page the description, and some measurements of the 20 dB 100 Watt RF attenuator I made.

Figure 1:  the attenuator.
The heat sink measures 200 x 190 x 45 mm.

Figure 2:  two of these 10 dB flange mount chip attenuators are used. (datasheet_ATC_attenuator.pdf)
They can handle a maximum input power of 100 Watt, at a flange temperature of 100°C.




Figure 3:  the interior of the 20 dB attenuator.
Input and output connector are N type sockets from Radiall, part number R161.404.000 (datasheet_Radiall_R161404000.pdf)
The two connectors are interchangeable in function (input or output).
From some aluminium profiles, an enclosure is made for the components, the aluminium bottom plate is removed for this picture.



Figure 4:  detail of the mounting of the chip attenuators.
The PCB board has two rectangular holes for the attenuators.
The copper foils below the nuts are both soldered to the PCB board, and (at the back side of the foil) to the flange of the attenuators.
A thin layer of heat sink compound is applied between the heat sink and the flange of the chip attenuators.



Figure 5:  detail of the coax cable soldered to the N Socket.
The coax cable has Teflon inner insulation
 

Measurement of attenuation:

First the attenuation is measured, with a spectrum analyser with tracking generator.



Figure 6:  attenuation between 0 and 2100 MHz (this is the maximum span of the spectrum analyser).



Figure 7:  attenuation between 0 and 200 MHz.

 

Measurement of return loss.

Next, the return loss is measured at the two N connectors.


Figure 8:  measuring the return loss of the attenuator, with my homemade return loss bridge

Figure 9:  return loss at connector 1, between 0 and 2100 MHz.
Trace A (yellow) is with connector 2 not terminated.
Trace B is with connector 2 terminated with 50 Ω.

Figure 10:  return loss at connector 2, between 0 and 2100 MHz.
Trace A (yellow) is with connector 1 not terminated.
Trace B is with connector 1 terminated with 50 Ω.

Figure 11:  return loss at connector 2, between 0 and 200 MHz.
Trace A (yellow) is with connector 1 not terminated.
Trace B is with connector 1 terminated with 50 Ω.

Measuring at connector 1, gives almost the same result in this 0 - 200 MHz frequency range.
 

Thermal measurement.



Figure 12:  here the attenuator is connected to a DC power supply.
The power supply was turned to it's maximum output voltage of 65 Volt.
This results in about 84 Watt power dissipation in the attenuator.
The output of the attenuator was terminated with a 50 Ω terminator in this test.


Figure 13:  thermal image of the attenuator after 1 hour at 84 Watt power dissipation.
The heat sink is 69 °C, measured with the marker on a piece of black tape on the heat sink.
The rest of the heat sink is shiny aluminium, which reflects in this image the temperature of the room (about 20 °C).
The hottest point (white colour) is 100 °C, measured on one of the resistor chips.

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