Friday, December 14, 2012

Hermes as a VNA: Reflection

Next task was to look at filter return loss. In addition to hermes one needs a reflection bridge. This has a port for RF input, a port for reflected output and a port to connect the device under test. Looking in the RF goodies box I found a commercial Wiltron 80NF50 5MHz to 2GHz SWR bridge which would be ideal. To try it out a filter in the 0.005-2Ghz HF bands was needed so looking in the MW2I contest parts box I found one of the two W3NQN bandpass filter box that was built on the PCB from 5B4AGN and decided to test the 15m filter
To make sure it still worked i first configured the system for transmission measuring 3 to 30MHz as described yesterday. The results were as follows:-



Which showed it still worked. Next the system was configured for return loss. The Hermes transmit output was connected through the 6dB attenuator to the bridge RF input. The Reflected RF output from the bridge was connected to the receiver input of Hermes. The device under test port was left open circuit. The software mode was changed to reflection chart , It was time to calibrate.
For return loss a calibrated open circuit, short circuit and 50 ohm load are needed. I had purchased a suitable set from SDR-Kits with SMA male connectors so these were used, being attached as prompted by the software.
The filter box, terminated in a 50 ohm load  was then connected to the device under test port and the following measurement produced:-


This showed that the filter had around 20dB return loss over the whole of 15m. To expand the vertical scale I clicked the "auto range" button under "display":-
 To further expand the horizontal axis the start and stop frequency boxes under the plot were changed to 15 and 25MHz:-

This showed how good the return loss was. Obviously it is still working fine.
All results (measurements AND calibrations) can be saved to disc to allow later use.

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