I have often wondered whether my FA-VA5 can accurately measure a UHF (70cm band) bandpass filter I made recently.
Well, I tried it out today and the answer is: yes, it can.?
While a filter is clearly a 2-port device with input and output, there is a way to measure at least the passband VSWR. The latter is a good indication of proper tuning of the filter.
The idea of a filter is to deliver a filtered output with only the desired frequency spectrum of interest.
Take for example a simple USB-type SDR and connect it to a good outdoor antenna. The poor SDR may get quite overwhelmed by the strong RF signals at its input. Most antennas have little or no real selectivity. (One exception is perhaps the magnetic loop antenna). So, the filter cures that, and often the difference is quite remarkable.
Back to the measurement. The filter under test was terminated at its output with a good 50 Ohm termination. In this arrangement, the input impedance should be close to 50 Ohms over the entire filter passband, meaning low VSWR of course.?
Outside the passband, the VSWR deteriorates sharply because all unwanted signals outside the pass band shall be reflected back and not delivered to the output.?
So, by terminating the filter output with 50 Ohms, we "convert" the 2-port filter to a 1-port device and that is what the FA-VA5 can handle.
The results shown in the images are quite impressive and not far from measurements with a big VNA boat-anchor machine.
In cases where a filter is deployed somewhere and taking it out is a problem, the FA-VA5 can sure be used to check the tuning status. Perhaps a curing re-tune can also be performed.
Any comments or questions - pls reply or send me an e-mail.
73
Dieter, VK3FFB
Dieter VK3FFB
