#18: Making Accurate RF Measurements (Basics 10)

[@wb7ond]

I recently purchased a DG8SAQ VNA and have learned a lot from these videos. Especially importing the files in SimSmith. You are right, moving the reference plane using the SimSmith components is really neat. The skill of AE6TY's coding is incredible, not to mention the both of you guys' in-depth knowledge of rf engineering. I am at the point of just being able to see matching Smith Charts in the simulation and the VNA. I learned Smith chart back in the late 1960's at an Air Force Calibration School at LAFB, using a compass and straight edge, drawing constant SWR circles, I would have never guessed that I would be interested in it again after all these years... Thanks for these informative videos, and the time you have invested, they are so very packed with information...

[@w0qe]

Glad you enjoyed the videos.

[@hoonlee4232]

I watched a lot of your videos I want to buy the SMA (OPEN SHORT LOAD) you made Is it possible?

[@w0qe]

Sorry but I don't sell an OSL set but as I showed in the video they are easy to make. Give it a try and even if it isn't perfect it will still be useful. Larry

[@galileo_rs]

How good in terms of RL are those home made 50Ω terminators, could you maybe upload some calibration files of those so that others can compare against yours? A bit off topic but do you have any thoughts on how good the cheap "VNA" kits like the Rigexpert AA-30.ZERO are?

[@w0qe]

Dragan, Calibrating an HP VNA with my HP/Agilent cal kits and then measuring the home made O/S/L loads show that they are quite good up thru 500MHz. The worst one is the "open" due to the fringing capacitance. My big VNA corrects for these effects and also then changes the reference plane to remove the length in the calibration standards. You can download the manuals for VNA's such as the 8753 family and this is explained. I am not familiar with any of the cheap VNA cal. kits and I don't think any of the small VNA's do any type of compensation for things like fringing capacitance and for the physical size of the calibration standards. You can take care of this sometimes in S/W by adding "line extension" if the feature is available or just do it in SimSmith by adding a small length of transmission line with a negative length to move the reference plane. Larry

[@galileo_rs]

Hi Larry, thanks for your response. The cheaper "VNA" have calibration routines but what those do is uncertain. They can move the measurement plane but the rest is unknown. Most have a simple architecture like this one: rigexpert.com/products/antenna-analyzers/aa-35-zoom/how-it-works/ and the manuals do not list any figures for accuracy etc. I lack the knowledge to decide if something like this would be even barely sufficient. Used professional VNAs are practically non-existent on the local market (small country) and even when one shows up it is in the order of several thousand € so out of "reach". Could you tell me what were the approximate RL figures, in the HF range, for those 50Ω terminators that you built?I'm currently playing with a bridge design by g3ynh and if I understood his work he is getting a RL of ~ 80db in the HF range which seems unrealistic. He did use a comerciall terminator but for some reason the graphs of RL for typical commercial terminators are surprisingly hard to find, hence the reason why I ask. Thanks Dragan

[@w0qe]

Dragan, Take a look at: www.w0qe.com/Measuring_High_Z_with_VNA.html and www.w0qe.com/Measuring_RF_Impedances.html where I tried to understand exactly what you are asking. Let me know if there 2 links do not answer what you are looking for. I agree that a RL of 80dB (SWR=1.0002:1) is very unreasonable. Larry