The AIM4170 and AIMuhf can be used to measure the filter bandpass when the filter output port is terminated in 50 ohms.
Here are two pictures showing the same filter measured with one port and with two ports. The VNA2180 picture called "15M_BPF_portB.gif" shows the data for Port A (s11) and Port B (s21) on the same graph. On the AIM's, s11 is called "RHO" and this data is related to SWR and Return Loss. Note the RHO scale is not logarithmic on the AIM graph.
Hi Bill, Calibrate Port B with a through cable connected from A to B. This will provide the 0 dB reference level for measurements. This cable(s) should be the same used to connect the filter that you want to measure. For the HF band, the length isn't too critical. For the UHF range, the length is very critical and the cables should be as short as possible.
A single cable can be connected from A to B or two cables can be used with a coupling between them. This coupling takes the place of the filter for calibration purposes.
After calibration, connect the filter from port A to port B. Port A is the output of the VNAuhf and it provides the test signal. Port B is like a receiver with a 50 ohm input impedance.
Use the Setup menu to select the parameters to plot. For a start, plot only S21 on Port B. Than will provide the most interesting data without cluttering up the display. S21 is the ratio of the filter output to the filter input in dB.
Select Port B and then click Scan to see the filter response. To get started, try scanning an attenuator. This will provide a known level for S21 so you can get a feel for how it works.
On this page there is a graph showing the response of a notch filter and a drawing showing how the filter is constructed. w5big.com/vnauhf.htm
Port B has a 50 ohm input impedance, so you don't need an external termination for filters that are designed to work with 50 ohm loads.