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Thursday, 5 September 2013

So what can you do with this bridge?


Now I've got what I think is a reasonable Return Loss Bridge - what can I do with it?

Lets take a very simple example, we have already noted previously that you calculate the Return Loss by:


Open = Peak to Peak voltage with no connection to the "?" port
DUT = Peak to Peak voltage with the Device Under Test connected to the "?" port

So if we were measuring the output with an oscilloscope we would use that equation to calculate the difference in dBm. However, as I have a Spectrum Analyser that reads directly in dBm, in this example I'll cut some corners.

So, initially I have the Spectrum Analyser and the RLB set up like this:

To start with I have the "?" Open and my Spectrum Analyser, at my selected test frequency of 7.1 MHz sees this:

So at 7.1 MHz we have a signal from the Tracking Generator of -31.67dBm. Now, lets couple up the Carolina Windom I made here:

to the "?" port. Now the Spectrum Analyser can see this:

So now, at 7.1 MHz we have a signal strength of -42.74 dBm - that's a difference of 11.07 dBm.

If we now use this equation to calculate the VSWR:

and plugging in the results:

we can calculate that the VSWR is 1.78.

Now, what does my newfangled Antenna Analyser thingamabob think the VSWR is at 7.1 MHz

the RLB works, doesn't it?

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