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Resistive Coupler


Resistive Coupler may be a useful tool  for various HF, VHF or UHF measurements. 

To get 50 Ohms of Input Impedance at the port, it is necessary to terminate rest of ports with real 50 Ohms. The resistors value should be 16.66 Ohm. I used 0.5W Panasonic thick film resistor 32.2 Ohm, 1%, two in stack parallel in every section. Insertion loss between ports is 6 dB.

Resistor size is 1210 (metric 3225), voltage rating 200V. Farnell part number is  232-7494. 

PCB is mounted to the Hammond case 1590LLB  (50x50x 25mm). Distrelec part number is 30 02 71.

Next pictures show basic measurement results. During SWR (RL) measurement two of ports were terminated with 50 Ohms (SMA), third one was connected to the VNA. SWR in the frequency range 1 - 60 MHz is better than 1.025. Return Loss (respective) varies from -38dB to -42dB. Results are quite good even if the resistance value is not very accurate (16.6 Ohm instead of calculated 16.66...Ohm).

Attenuation between ports should be -6dB. Insertion Loss picture shows values very close to 6dB of attenuation between ports A-B. Port C was terminated with 50 Ohms.

The following picture shows serial impedance components of load resistors. Resistor 1 was used as a calibration resistor, resistor 2 had lower real value (49.5 Ohm). This uncertainty was reflected in SWR and Return Loss measurement results.

What happend if one of the ports will not be terminated? Assume that port C will remain free (nothing connected to it).  Next pictures show basic measurement results.  Port B was terminated with 50 Ohms, port A was connected to the VNA. 

Measuring results are total different compared with the proper ports termination.

Conclusion:  For the correct operation all of ports must be properly terminated !

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