10 Watt 50 ohm 40 dB Attenuator

This 10 Watt 50 ohm Terminator (Dummy Load) provides 40 dB Attenuation for frequency and power measurements up to 200 MHz

by Dr. Carol F. Milazzo, KP4MD (posted 10 April 2015)
E-mail: [email protected]

Introduction

Design and Construction

Parameter evaluation

Conclusion

References

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Figure 1.  Schematic Diagram
Figure 2.  1/2 watt 50 ohm termination and 40 dB attenuator.
Figure 3. 10 watt 50 ohm load and 40 dB attenuator. Two five watt 100 ohm metal oxide film resistors in parallel across the attenuator input. This physical layout minimized the inductive reactance on the input measured up to 200 MHz.
Figure 4.  VNA measuring SWR & Impedance
Figure 5.  Vector Network Analyzer measuring Attenuation

INTRODUCTION

This 50 ohm impedance terminator & attenuator (dummy load) provides a non-reactive load (VSWR <1.03:1 from 0-200 MHz) and 40 dB attenuation for frequency and power measurements of radio frequency sources up to 10 watts (40 dBm).  This provides the appropriate signal level for low level measurement devices such as a frequency meter, oscilloscope or spectrum analyzer.  The attenuator is constructed with low cost available components and fits conveniently inside an Altoids tin box.

DESIGN AND CONSTRUCTION

Parts list:

• J1, J2 - Two single hole chassis mount BNC female connectors,
• R1, R2 - Two 100 ohm 5% 5 watt metal oxide film resistors,
• R3-R6 - Four 620 ohm 2% ½ watt metal film resistors,
• R7, R8 - Two 100 ohm 2% ½ watt metal film resistors,
• All resistors should be non-inductive (not wirewound), thermally stable (not carbon composition) and of appropriate power rating,
• and an Altoids tin.

The 40 dB Attenuator Circuit.  The 40 dB attenuator (shown in Figure 1 excluding R1 and R2) consists of a 100:1 voltage divider comprised of 2480 ohms (R3 through R6) in series with 50 ohms (R7 and R8 in parallel). When the 50 ohm input impedance of the measuring device is attached to output jack J2 in parallel across R7 and R8, the output resistance becomes 25 ohms, that is ~1/100 of total 2505 ohm resistance that is presented to the input signal at J1.  The 100-fold voltage attenuation yields a 10⁴-fold or 40 dB power attenuation at the output jack J2.  A full 10 watts (40 dBm) applied to a 50 ohm load at input jack J1 will produce 22.4 volts across and ~9 mA through the voltage divider (each 620 ohm resistor will dissipate about 0.05 watts, well within its 0.5 watt maximum rating), and the voltage at output jack J2 will be 224 mV, equivalent to 1 mW (0 dBm).

First configuration: External 1/2 watt 50 ohm Terminator on BNC Tee Connector with 40 dB Attenuator (Figure 2).  The 50 ohm ½ watt termination shown attached to the BNC tee connector in Figure 2 provides the proper impedance load for a signal source up to 0.5 watt (27 dBm) applied to the input jack. The internal lead length and stray capacitance of the BNC Tee connector are expected to introduce a reactive load at higher frequencies.  With the 50 ohm termination removed, the BNC tee connector may be left connected in line between a transmitter and a 50 ohm antenna for on the air power and frequency measurements.  Power measurements taken in this manner will be accurate only when the antenna presents a non-reactive 50 ohm load.

Second configuration: Internal 10 watt 50 ohm Terminator with 40 dB Attenuator (Figure 3).  The BNC tee connector and 50 ohm termination were removed from input jack J1 and the 10 watt 50 ohm internal load consisting of two 100 ohm 5 watt metal oxide film resistors (R1 and R2) were soldered in parallel from the input jack J1 to ground.  This configuration serves for accurate measurement of transmitter power up to 10 watts (40 dBm) into the internal non-reactive 50 ohm load but does not permit inline measurements between the transmitter and an antenna.

PARAMETER EVALUATION

For comparison of both configurations, the input impedance and attenuation parameters of each were evaluated over 1-200 MHz using a miniVNA Pro vector network analyzer² (Figures 4 through 13) and the observed values for 1.9, 28, 50 and 144 MHz were charted in Table 1.

For either configuration, the attenuation varied between 40 dB and 40.2 dB up to 50 MHz, and approached 41.5 dB at 200 MHz.  The internal 10 watt 50 ohm load presented the least reactance with an SWR from 1.015:1 to 1.025:1 between 0.1 and 200 MHz, while the attenuator with the external 50 ohm termination and BNC tee connector presented a higher input SWR from 1.04:1 to 1.17:1 over that range.

CONCLUSION