So your saying that on the Open test one VNA saying LogMag= 0.01db and the other VNA is LogMag= -86.5db, and on the Short test one VNA having LogMag= 0.00db and the other -80.0db, "VNA readings are acceptable and both are working okay"?
tnx
George
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On 3/2/2022 9:19 PM, VK2CZ DAVID wrote:
Bob is entirely correct. All VNA's use directional couplers, with the
'uncertainty' around the directivity often confusing measurements. The
perfect analog is measuring say a 9v battery with two voltmeters and being
confused when one reads 9.000002v and the other 8.999933v when the
voltmeter accuracy is only good to +/- 100uV..
Your VNA readings are acceptable and both are working okay..
On Thu, 3 Mar 2022, 14:07 montanaaardvark,<boblombardi@...> wrote:
On Wed, Mar 2, 2022 at 08:13 PM, WD0AKZ wrote:
#1 VNA w/Load Calibration Load: SWR= 1.001 LogMag= -66.3db X= 5mOhm Z=
50.045
#2 VNA w/Load Calibration Load: SWR= 1.000 LogMag= -87.0db X= 10.0mOhm Z=
49.998
What I¡¯m concerned about is the differences between the LogMag
(ReturnLoss)
and Z (Impedance) on the Open and Short tests. I would also think that a
-21db
difference in LogMag between #1 and #2 on the Load test is a bit
excessive, or
is this just considered ¡°Close enough for Government Work¡± (old saying,
somewhat military speak)?
The way I read this you used the exact same cal standards on both
NanoVNAs, right? Standard practice for calibration in the labs I've worked
in was to clean both connectors with denatured alcohol and use a calibrated
(7 inch*lb) torque wrench for tightening the sets. For every measurement.
I'm going to assume you didn't do this and just put them together by hand.
The way I look at this is to turn the return loss values from dB into
magnitude by doing 10^(dB/10). I get the values:
(-66.3 dB) = 234 *10^-9
(-87.0 dB) = 2.00 *10^-9
I don't have much of a feel for how much to expect out of the H4 since I
just bought mine, but while that's a big relative difference (234 to 2),
it's between two rather small numbers. How many bits difference is that at
the ADC? I have no idea. Look at the VSWRs: 1.001:1 vs. 1.000:1. I can't
think of anything I've ever done where a return loss difference like that
mattered.
Hope that's useful,
73,
___________
Bob - W4ATM - 35 miles south of the Kennedy Space Center
Retired RF Engineer now playing with all the hobbies I never had the time
for.
