On 8/13/20 6:04 PM, AB6BT wrote:
What are you using to measure the load resistance?
This is a nice example of the rabbit hole (or prairie dog village) one can go down..
Is the DC resistance the RF resistance? Maybe, maybe not. Generally, though, DC resistance is less than AC resistance.
What's the temperature coefficient of
a) the load
b) the ohmmeter
What is the lead resistance on the ohm meter (or was it done with a 4 terminal Kelvin measurement)? First hit on google brings up Klein test leads that are 41" long, but they don't say what gauge the wires are. let's say they're something like AWG 20 - that's 0.01 ohms/ft or about 0.07-0.08 ohms total - although usually, the meter is calibrated to read zero with the leads shorted.
Looking up something like a venerable Fluke 113 - the accuracy specification is 0.9% + 2 counts with a resolution of 0.1 ohms
1% of 50 ohms is 49.5-50.5 ohms
This is not meant to beat up on Glen, but it points up the challenges in making accurate measurements - you have to worry about "everything" - just because the NanoVNA reads down to -100dB doesn't mean that it's *accurate* at that kind of level. I spent a while about 20 years ago at work trying to accurately measure a 100 dB attenuator to 0.1dB uncertainty - it's an ordeal.
Fortunately, most of us are making "relative" measurements - tuning a filter, checking pass band attenuation, looking for a good match on an antenna.
The people who worry about getting a 40dB match (1% voltage) are people running a LOT of power (S11 of -30dB from 200kW is 200W reflected) or doing precision calibration of things. Mismatch uncertainty becomes the dominant error source in precision RF power measurement - I built a 13.402 GHz precision noise source for amplitude calibrations, and it had lapped and pinned waveguide flanges, with an mate/remate uncertainty (measured) of, I think, 0.0001 in the reflection coefficient (that's -80dB) - because we knew it's noise temperature was about 8400K, +/- <2.7K (i.e. 0.1%)
These two papers (especially the second one, which has more details) describes the kind of painstaking effort it takes to drive uncertainties below 1%
And this is where the $100k VNA with the $20k cal kit earns its keep - when the engineering cost on the thing you're measuring is $500k.
Ultimately, be happy with your 51 ohm load!
At 05:52 PM 8/13/2020, you wrote:
The 50 OHM load (SMA-Male) that came with my nanoVNA-H4 measures 51.13+ ohms.? Not a good start for calibration.? Where is a good source for GOOD loads that are accurate?
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Glen Jenkins, WB4KTF, Austin, TX
