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G4AXX

High Power Attenuator



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Specifications
Attenuation 30 dB
Impedance 50 Ohms
Frequency Range 1 to 50 MHz
SWR < 1.5:1
Size 150mm(H) 120mm(W) 295mm(D)
Weight 4.8 kg


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

The schematic.

The SO239 input on the left, BNC output on the right.

End view showing the fan.

Close-up of the resistors.

The power socket at the bottom right is for the fan (12VDC 1A).

Close-up of input circuit.

The heatsink is made from two finned extrusions bolted together to form a 120 x 120mm box section.

The insertion loss measured using a N2PK VNA using Exeter software.

The input SWR from 1 to 50MHz. The input SWR rises with increasing frequency to 1.5:1 at 50MHz.

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.







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