guide for beginners

[tomlayne1]

I recently purchased the AIM 4170d, works great. Now I'm having to learn about smith charts to really understand matching antenna a bit better. Not that it is a problem, but since I don't understand all the terms used just yet on Zmag and phase and what is better or worse when getting the SWR down to what I have come to expect from previous use of a MFJ259...
Rho mag, I could use a bit more understanding here.

the Equivalent circuit information on right side of chart, it equals where ever my vertical line is as it move across the chart, ok fine, not sure what to do that that information?

SWR certainly clear enough, %reflected power certainly clear and output of course,

hoping to find a dummies guide to get me moving a bit faster in my understanding.

might someone have a good clean sweep of a 40 meter vertical with a VNA? I just built one and it plays well.

I added my file in case some one wants to look.

Thanks in advance.
tom
KB5VWZ / XE3 / aka LA1BRA

Attachments:

first scan with inductor in and out.scn (21.2 KB)

first scan with inductor in and out.csv (34.31 KB)

[Bob]

Hi Tom,

First the AIM measures the magnitude and phase of both the voltage and the current flowing into the circuit under test.

Then:  Zmag = Mag_Voltage/Mag_Current
          Phase = Phase_Voltage - Phase_Current

The cal data is used to correct for imperfections in the raw data. That's transparent to the user.

All the other parameters can now be calculated:

Rseries = Zmag*cos(phase)
Xseries = Zmag*sin(phase)

Zmeasured = Rseries + j*Xseries // This is the series equivalent circuit.
// The parallel circuit is calculated using these Rs,Xs values.
// Both the series and parallel circuits 
// have the same Zmag and Phase.

Zs = measured series load impedance = Rs + jXs

Zref is the reference impedance used for calculating SWR. It does not have to be 50 ohms. For example, if you're
working with a dipole, you can use 75 ohms. It can be any value, even a complex number.
If you change Zref, you do not have to calibrate again.

rho = (Zs - Zref)/(Zs + Zref) -- equation for complex reflection coefficient, rho (or gamma)

SWR = (1 + magnitude(rho))/(1 - magnitude(rho))

ReturnLoss = -20*LOG(rho)

To get a feel for measurement data, start off with some loads that have known parameters. First scan the cal resistor. Then scan a 1K ohm resistor. Why does Zmag of the 1K resistor decrease as the freq goes up? This is supposed to be a 1K resistor ??  Look at the values of Rp and Xp.  The AIM is a very educational instrument.

Put a capacitor of a few hundred pf in parallel with a resistor of a few hundred ohms. As you move the cursor, you can see that Rs changes but Rp stays about the same.

Take a piece of coax 10 or 20 feet long. Put a resistor of around 200 ohms at the far end. Watch the measured impedance change with freq. Compare the results to a computer program like Transmission Line Details ( AC6LA.com )
AC6LA.com has a lot of information on Smith charts and transmission lines.

Inductors can show some surprising effects. They are much more complicated that generally expected.  Measure some inductors to check their series resonant frequency.  The value of inductance can vary a lot, even in the ham HF bands.

You can find more info on complex numbers in the appendix of the AIM manual. For doing complex arithmetic, get the program called SpeqMath at SpeqMath.com. This program (or an equivalent program) is very helpful for analyzing RLC circuits.

--73/Bob

[tomlayne1]

Thanks Bob.
I'll be back in Texas in a few weeks, I need some supplies for the radio station down here, I will gather up some cap's and was thinking about building a little test fixture to mount different components to do exactly what you suggest.

73/tom

[smokchsr]

Tom,

It sounds like you are going to do some broadcast work. In that world and typically in the HAM world the main thing you are looking at is the Rs Resistance Series and Xs. You've likely heard the phrase "50-J0", which is typically what we are wanting the transmitter to see at it's output. 50-J0 would indicate 50 Ohms Series Resistance and 0 Ohms Series Reactance. So Xs & Rs are your prime concerns with the line on the center frequency of the radio station. Once you get those to 50 & 0 respectively the VSWR should be down to the zero line and the Return Loss should be somewhere lower than -30db.

In the Broadcast world when we are loading with a continuous high power signal into the antenna we do have to be very picky about getting those numbers very close.