Mismatched terminations

Click here to go to our main page on microwave terminations

Click here to learn about VSWR

Click here to learn about power handing of mismatched terminations.

Click here to learn about phase-variable mismatched terminations

New for November 2020.  Let's kick around some ideas for creating mismatched terminations. Then we can talk about how to make phase variable mismatched terminations on this page.

Shorted attenuator

Here's something you can do easily with lab equipment: use a fixed attenuator, and a short circuit. Try not to use one of the cal standards for the short, that will get you kicked out of the lab.

Here's a Microwave Office schematic, where we varied the attenuator from 1 to 5 dB in 1 dB steps

Shorted Attenuator

You can use a little math to calculate the VSWR of such a circuit, we did it in Excel below. The formula for converting return loss to VSWR can be found in a graphic on our VSWR calculator.

Shorted attenuator VSWR Excel plot

Or you could use Microwave Office to arrive at the same results, and plot them over frequency:

Shorted attenuator VSWR MWO plot

Multiple, parallel matched terminations

Here's two ways to make broad-band 5:1 mismatched terminations from matched loads. You will have to make a circuit board in each case. You can use chip terminations, or add connectors and screw on five loads.  If you want a high-power mismatch, use diamond resistors, or high-power coaxial loads.

In idea 1, the input splits into five fifty-ohm lines. The length of the lines, and their losses, are unimportant. The six-way junction will be the hard part of the design, trying to get it to remain impedance matched over a wide bandwidth will take some EM work.


Broadband mismatched termination Idea 1

Here's another idea that gets rid of the need for a six-way junction. Here we grouped five terminations into a three-way and two-way circuit, then combine them. Transmission line routing the three-way must have impedance of 16.7 ohms (50/3), and the line routing the two-way must have impedance of 25 ohms. Once again, the lengths do not matter.

Broadband mismatched termination Idea 2

Now, click here to learn about power handling of mismatched loads.


Author : Unknown Editor