On 3/2/22 8:52 PM, WD0AKZ via groups.io wrote:
I'm sorry, I don't understand "reflection coefficient".
The reflection coefficient is the voltage "reflected" back from the unit under test (it's voltage, not power, but it's similar to Ref/Fwd on a directional watt meter) - shorts and opens are "perfect mismatch" so they reflect all the power.? A load absorbs all the power so nothing is reflected (i.e. reflection coefficient is 0)
20 *log10(abs(reflection coefficient) is what's displayed as magnitude of S11.? So short and open, which have a reflection coefficient of -1 and 1, will have S11 of close to 0dB, load should have a reflection coefficient of 0
I did now Reset Calibration and looked at the LogMag results if that what you were asking:
#1 VNA O= -3db? S- -1db? L= -40db
#2 VNA O= -86db? S= 85DB? l= -88db
VNA #1 looks about right. O and S are close to 0dB, and Load is "very small".
VNA #2 has something wrong. A short with +85 dB S11 is odd.
So the next question is whether it's a computation problem, or is the hardware broken.? You're giving values at some spot frequency - is the value pretty much the same across the band for all 3: S,O, and L?? If you saw -80dB for all three, I'd say "the receiver is broken" - it's not seeing any signal with the short or open. Or, the reference receiver is fouled up.? The measurement is "reflected/reference"? so you could get a tiny number if the reflected is very small, or if the reference is very big.
A broken connector (open) would be one cause of no reflected power.? Sometimes, the jack in the middle of a SMA gets bent out of shape and the center pin of the mating plug doesn't make contact. I've also seen things like paint or plastic stuck in the jack.
One thing you could do is calibrate your VNA#1, and connect the two CH0 together (with the VNA2 powered off) - the impedance looking into the CH0 should be fairly close to 50 ohms. It's just a resistive bridge.? If it's not, then that's something to investigate.? You could also do a Thru measurement - even without calibration, you should see something around 0 dB (+/- a few dB). The other thing to do is see if there is a signal coming *out* of CH0 when you're making a measurement - if you have a power meter, an RF voltmeter, or a spectrum analyzer, that's something you can check.
