Hello people...

I'm trying to test a circuit I found on the internet to measure antennas VSWR parameter. And I'm trying to simulate it with LTSpice.

The problem is that I can't get values close enough to theory as I should.

I'll attach the .asc file below and for instance, by theory I should get a VSWR = 2 for a purely resistive load of 25Ω and a VSWR = 1.5 for a purely resistive load of 75Ω. In LTSpice simulation I'm getting about VSWR = 1.4778 for the 25Ω and VSWR = 1.1331 for 75Ω load!

In LTSpice I'm using the following formula to evalute the VSWR:

VSWR = (Vfrw + Vref) / (Vfrw - Vref)
where Vfrw is the voltage of the incident wave and Vref is the voltage of the reflected voltage.

In ARRL book and others they make it even simpler saying that for a Load greater than the transmission line characteristic impedance, VSWR = RLoad / Z_0 and for a load smaller than the transmission line characteristic impedance, VSWR = Z_0 / RLoad, where Z_0 is the transmission line characteristic impedance.

I've also uploaded a xls file that contains the theoretic calcs for both versions of the VSWR formulas from ARRL book and from LTSpice way.

Can anyone help me figuring out why? The circuit I'm using as a base for this is here.

Thanks
PsySc0rpi0n

 

Hi

Suggestions..

Make sure that the signal inputs and output measurements all represent the same values. Should they all be RMS values?
If the input sinewave should be 1V RMS for example, multiply the value by 1.414141. You can check the RMS value by ctl-(left-clk) on the waveform label.

To calculate the VSWR, you can do something like this:

.meas Vfrw RMS V(Vforward)
.meas Vref RMS V(Vreflected)
.meas VSWR = (Vfrw + Vref) / (Vfrw - Vref)

View the error log for the results.

This is just an example.
There is some info in the LTspice help. Search for "measure"

eT

 

Hi

Suggestions..

Make sure that the signal inputs and output measurements all represent the same values. Should they all be RMS values?
If the input sinewave should be 1V RMS for example, multiply the value by 1.414141. You can check the RMS value by ctl-(left-clk) on the waveform label.

To calculate the VSWR, you can do something like this:

.meas Vfrw RMS V(Vforward)
.meas Vref RMS V(Vreflected)
.meas VSWR = (Vfrw + Vref) / (Vfrw - Vref)

View the error log for the results.

This is just an example.
There is some info in the LTspice help. Search for "measure"

eT

I might have found out the problem.

I'm using the .meas command directly using the final formula. No need for the intermediate .meas commands, I guess.

The problem is the same as in real hardware. Input signal was just too small. Also frequency should be higher. I tried 10V and 27Mhz and results are way better!

 

Great! Glad it’s working to your expectations..

eT